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FR0106667
V annual report 2000
NEUTRONS FOR SCIENCE PLEASE BE AWARE THAT ALL OF THE MISSING PAGES IN THIS DOCUMENT WERE ORIGINALLY BLANK Please note that the authors of the article The elementary jump in intermetallic alloys with CsCl-structure (p. 46) should be as follows
M. Kaisermayr, B. Sepiol {University of Vienna) J. Combet (ILL 85 Universite Strasbourg) B. Frick (ILL) G. Vogl (HMI Berlin) annual re port 2000
The Institut Max von Laue-Paul Langevin
The Institut Laue-Langevin (ILL) is an international research centre
using neutrons to probe the microscopic structure and dynamics of a
broad range of materials. The combination of the world's most powerful
neutron source with dedicated instrumentation enables the study of a
wide variety of scientific questions. Problems in materials science,
chemistry, biology, solid-state physics and nuclear physics are investigated.
For these diverse studies, the Institute offers its experimental
facilities (some 30 instruments) to scientists world-wide via a peer-review
system; confidential industrial experiments are also welcomed.
NEUTRONS FOR SCIENCE n lsl January 2000 the ILL launched the Millennium I Programme. This ambitious programme will involve the 0.continuous renewal of almost all ILL instruments and infra- structure over the years to come. To celebrate the New Millennium and the launch of the Millennium Programme the Directors organised a Millennium party on 21 January. All ILL people and their families were invited. After the buffet everyone danced to the sounds of the two ILL groups, the Groupe Chanson' and the ILL rockers2.
1 Barbara Standkc (vocals), Gerard Grandene (electrical and electro-mechanical workshop, syn- thesizer), Claude Yoccoz (experimental hall and vacuum laboratory, drums). Froltagc Cheese (Charles Dewhurst, instrument scientist on D22), Aphra Denmark (Andrew Wildes, instrument scientist on Dl 7), Rock Mcl.oonie (Ross Stewart, instrument scientist on D7) and Trouser McCoff (Trefor Roberts, polarised :iHe project).
Dirk Dubbers, the ILL director, refusing to sing along with his secretary, Barbara Standke, and choosing to give an after din- ner speech instead.
Barbara Standke singing one of her old favourite songs The Greatest Rockin' Rollin' Band. (We are the champions).
Colin Carlile, the British director, head of the projects and Claude Campos, electronics service, and Bob Pratt, reactor crew, techniques division, asking Eddy Lelievre's daughter for a dance. improvise a blues. contents
Director's report 6 v\'MW Scientific 1—6 highlights 9
Q magnetism 10
Q chemistry & structure 32
(-• liquids & glasses 40
O materials science 44
Q biology , 50
O soft matter 58
Q fundamental & nuclear physics 64
Q modelling & theory 68
Millennium programme and new developments
Q millennium programme 79
Q technical developments 86
(-• instrument rebuild , 91
O new experimental techniques 95
Workshops 103
Experimental programme 109
Facts and figures 117
.\*J«f Publications 121 Director's
The year 2000 was a good one for the ILL. Our Millennium Renewal ner has announced its intention to make successive increases over the next Programme got off to a prompt start on the Is' of January 2000, and the few years in its contribution to ILL, as well as further investment on a tar- programme's first five instruments (D3, IN20, VIVALDI, the Strain [mager, geted basis in ILL-approved projects. This gives ILL the chance to start and the SANS detectors) have proceeded apace over the course of the year. work immediately on other parts of its ambitious Millennium Programme. At the same time, six other instrument projects have been successfully On a sadder note, in the course of 2000, Professors Louis Neel and Heinz completed, four of them (D4, D16, HI 13, D17) on schedule and the remai- Maier-Leibnitz passed away - two of ILL's greatest pioneers whose memo- ning two (D20, IN4) once unforeseen technical challenges had been over- ry will be honoured by us all. come. Users have welcomed all these new instruments with great enthu- In the past year the ILL directors have been particularly occupied with the siasm. IN5 and IN8, the final two instruments in this series, are on the way, question of the directions to be taken by science at ILL in the future - we for delivery as planned in the course of 2001. know from experience that if the 'product' is right- and for ILL this means As far as future projects are concerned, the Millennium Programme research of the highest quality - the rest will follow. In the following pages involves not only many more excellent propositions for instrument renwal, we present a selection of the work carried out at the Institute over the last but also plans for an overhaul of ILL's infrastructure, including the renewal year, and we would like to thank all those who have helped to make such of all the old neutron guides. With a recently new reactor, strong new infra- fine results possible. There were also other important challenges to be met structure, a suite of instruments comprehensively renewed and a rejuve- in the year - the securing of the fuel cycle, the introduction of a 35-hour nated workforce, the Institute could not be better prepared to face the future. week, and the resolution of intricate financial questions, to name a few- The Millennium Programme has been presented and discussed, both inter- and these have been dealt with in their usual masterly fashion by ILL's nally in ILL's committees, and on a national and international level in a technical teams. We would like to take this opportunity to thank them for number of scientific fora; this discussion is now being extended to involve their contributions. all former and potential users with a stake in the question. It is our duty to ensure that ILL maintains its rightful place in what is a rapidly evolving All in all ILL has the wind in its sails. There are all the indications that field. It is therefore essential that ILL's strengths, and in particular the over the decades to come the Institute will confirm its position in the advantages of very strong continuous neutron beams, be brought fully to forefront of neutron science. bear upon those areas of research most likely to bear fruit. We must also now investigate the potential for widening the scope of neutron Dirk Dubbers technology, and we are lending all our efforts in support of the project to construct the ESS, a pulsed spallation source in Europe. At the end of 2000 more good news came in for the Institute: the UK part-
Directeur
L'an 2000 a ete une bonne annee pour 1'ILL. Des le lcr janvier 2000, le pro- frastructure de TILL, y compris de tous les anciens guides de neutrons. Avec gramme de renouvellement de TILL, le 'Programme Millennium', a ete un reacteur pour ainsi dire neuf, une infrastructure saine, un pare instru- lance. La realisation des cinq premiers instruments de ce programme (D3, mental entierement renove ainsi qu'une equipe fortement rajeunie, I'lnstitut IN20, VIVALDI, le scanner de contrainte et les detecteurs DNPA) est en est parfaitement equipe pour affronter l'avenir. bonne voie. Parallelement, six projets instrumentaux lances au cours des Le 'Programme Millennium' a ete presente et discute au sein des comites de annees precedentes ont ete acheves avec succes (D4, D16, HI 13, D17, 1'ILL ainsi que dans differents colloques scientifiques au niveau national et D20, IN4), les quatre premiers dans les delais prevus, les deux derniers international, les discussions se poursuivent maintenant sur une base plus avec un certain retard en raison de problemes techniques inattendus. Ces large au sein de la communaute des utilisateurs actuels et potentiels de neu- instruments ont ete accueillis avec enthousiasme par les utilisateurs de trons. Dans un environnement qui evolue rapidement, il est de notre devoir de 1'ILL. Les deux derniers instruments de cette serie (IN5, IN8) seront dis- donner a 1'ILL la place qui lui revient. Pour cela, il est essentiel de mettre en ponibles, comme prevu, en 2001. valeur les points forts de 1'ILL, en particulier les avantages des faisceaux de Pour les annees a venir, le 'Programme Millennium1 comprend de nom- neutrons continus et tres intenses dans des domaines de recherche particu- breuses autres propositions d'excellente qualite pour le renouvellement des lierement prometteurs. En outre, il faut chercher a diversifier les applications instruments, mais egalement pour un renouvellement en profondeur de l'in- possibles de la neutronique. Simultanement, 1'ILL soutient fermement la pro-
Director's 6 report position de construire la source à spallation puisée européenne ESS. de l'ILL au cours de l'année 2000. Nous remercions tous ceux qui ont per- La fin 2000 a également été marquée par une autre bonne nouvelle pour mis d'obtenir ces résultats scientifiques remarquables. D'autres défis l'Institut. L'Associé britannique a décidé d'augmenter progressivement sa importants ont été relevés, il a fallu, par exemple, assurer le cycle du com- contribution à l'ILL au cours des prochaines années. En outre, la Grande- bustible, mettre en œuvre la semaine de 35 heures et résoudre des pro- Bretagne a l'intention d'investir de manière ciblée dans des projets de TILL blèmes de technique financière. Comme d'habitude, ces problèmes ont été déjà approuvés. L'ILL sera ainsi en mesure de s'attaquer immédiatement maîtrisés par les spécialistes de l'ILL avec une grande efficacité. Nous vou- à d'autres parties de son ambitieux 'Programme Millennium'. lons également profiter de cette occasion pour les remercier. En 2000, deux des grands pionniers de TILL, les professeurs Louis Néel et Dans l'ensemble, FILL a le vent en poupe et tout laisse à penser que Heinz Maier-Leibnitz, ont disparu. Nous honorerons toujours leur mémoire. l'Institut pourra maintenir au cours des prochaines décennies son rôle pré- Au cours de l'année écoulée, la Direction de l'ILL s'est plus particulière- dominant dans le domaine de la recherche neutronique. ment penchée sur la question de l'orientation future de la science à l'ILL. En effet, comme le montre l'expérience, lorsque le 'produit' est bon, c'est- à-dire lorsque la science est de grande qualité, tout le reste suit. Dans les Dirk Dubbers pages suivantes de ce rapport, nous présentons une sélection de l'activité
des Direktors
Das Jahr 2000 war ein gutes Jahr für das ILL. Pünktlich mit dem 1. Januar Gegen Ende des Jahres 2000 gab es eine weitere gute Nachricht für das 2000 wurde das ILL Millennium-Erneuerungsprogramm angefahren. Die Institut: Der britische Partner hat beschlossen, seine Beteiligung am ILL in ersten fünf Instrumente aus diesem Programm (D3, IN20, VIVALDI, Strain den kommenden Jahren stufenweise anzuheben; zusätzlich will Scanner, SANS-Detektoren) sind im Laufe des Jahres gut vorangekommen. Großbritannien gezielt in bereits genehmigte ILL-Projekte investieren. Dies Parallel dazu wurden sechs Instrumentprojekte aus vergangenen Jahren wird es dem ILL erlauben, weitere Teile des umfangreichen Millennium- erfolgreich abgeschlossen (D4, D16, H113, D17, D20, IN4), die ersten vier Programms sofort in Angriff zu nehmen. innerhalb des vorgesehenen Zeitplans, die beiden letzten erst nach Über- Im Jahre 2000 sind zwei der großen Pioniere des ILL, die Professoren Louis windung unvorhergesehener technischer Schwierigkeiten; alle fertigen Néel und Heinz Maier-Leibnitz für immer von uns gegangen. Wir werden ihr Instrumente sind von den Nutzern mit großer Begeisterung aufgenommen Andenken stets in hohen Ehren halten. worden. Die letzten zwei Instrumente aus dieser Serie (IN5, IN8) werden im Auch im vergangenen Jahr hat sich das ILL-Direktorium vorrangig mit Jahr 2001 fristgemäß fertiggestellt. Fragen der zukünftigen Wissenschaft am ILL befasst, denn die Erfahrung Für die kommenden Jahre umfaßt das Millennium-Programm nicht nur lehrt: Wenn das "Produkt" stimmt, das heißt, wenn die Wissenschaft von viele weitere sehr gute Vorschläge zur Instrumenterneuerung, sondern auch höchster Qualität ist, dann folgt alles andere nach. Auf den folgenden Seiten Vorschläge zur grundlegenden Erneuerung der Infrastruktur des ILL, ein- dieses Berichts stellen wir eine kleine Auswahl aus der ILL-Produktion des schließlich aller älteren Neutronenleiter. Mit einem fast neuen Reaktor, vergangenen Jahres vor. Wir danken allen, die diese schönen wissenschaft- einer gesunden Infrastruktur, einem rundum erneuerten Instrumentenpark lichen Ergebnisse möglich gemacht haben. Andere wichtige sowie einer stark verjüngten Mannschaft ist das Institut dann bestens für Herausforderungen, wie z.B. die Sicherung des Brennstoffzyklus, die die Zukunft gerüstet. Einführung der 35-Stundenwoche, oder die Lösung finanztechnischer Nachdem das Millennium-Programm in den Ausschüssen des ILL sowie in Fragen wurden von den zuständigen Fachleuten des ILL in gewohnt her- verschiedenen nationalen und internationalen wissenschaftlichen Gremien vorragender Weise gemeistert. Auch ihnen wollen wir an dieser Stelle vorgestellt und diskutiert wurde, wird nun die Diskussion auf breiterer besonders danken. Basis mit allen interessierten bisherigen und zukünftigen Neutronennutzern Insgesamt befindet sich das ILL in gutem Fahrwasser und alles deutet weitergeführt. Unsere Aufgabe ist, das ILL in einem sich schnell ändernden darauf hin, daß das Institut auch in den kommenden Jahrzehnten seine füh- Umfeld bestmöglich zu plazieren. Dazu müssen die Stärken des ILL insbe- rende Stellung in den Neutronenwissenschaften wird behaupten können. sondere im Bereich kontinuierlicher Neutronenstrahlen in zukunftsträchti- gen Gebieten der Forschung optimal zur Geltung gebracht werden. Ferner streben wir eine Diversifizierung der möglichen Neutronenanwendungen Dirk Dubbers an. Gleichzeitig unterstützt das ILL nach Kräften den Vorschlag zum Bau einer gepulsten europäischen Spallationsquelle ESS.
Director's report Christian Vettier (right), the French director, Head of the Science The French Acaderm of Science held its annual regional meeting in Division, shows the experimental hall to the Slovakian Ambassador, Grenoble, in Novembei 2000 As a part of the three-dav e\ent they Vladimir Valach, who visited the ILL in March. visited Grenoble's two major scientific facilities, ILL and ESRF. In the photo, Ekkehardt Bauer, Head of the Reactor Division, guides the members of the Academy around the reactor.
A living atom, made of red protons, blue neutrons and yellow La Fete de la Science - in October 2000 - helps the general public electrons, spins around during Les ateliers des neutrons, organised to find out more about scientific research and technology. by Anne Le Noel, Communication Officer at the ILL.
Scientific highlights Scientific highlights
The year 2000 has been another highly productive period for the scientific program- me of the ILL. The scientific highlights presented in this section of the Annual Report illustrate the tremendous range of high quality science achieved at the ILL. This is reflected in the large number of exciting results being presented, ranging from quan- tum critical points to orbital ordering and to the role of orbital moments in magnetic fluctuations. Magnetism as a field of research is evolving very rapidly: new materials are discovered and new problems are contemplated such as magnetic excitations in thin films, quantum phase transitions and correlated fermions. ILL users take advan- tage of other complementary techniques, such as the synchrotron x-rays produced by our neighbour, the ESRF, in order to reach a better understanding of their scientific problems. This is particularly true in the case of electronic properties of solids and chemistry. The unique characteristics of neutrons - to penetrate deeper into mate- rials, to detect the different isotopes of the same element and to probe the dynamics of atoms and molecules over a wide energy range - allow the investigations of concrete problems such as: conformations of hydrogen bonds; diffusive modes in liquids, intermetallic alloys and ionic conductors; and the structure of polymers and micro emulsions. Furthermore, noteworthy developments have been achieved in bio- logy. The neutron methods coupled to the judicious labelling of the macromolecular assembly provide a powerful investigation tool allowing important observations. Examples include the examination of the rearrangement of proteins and the role played by thermal fluctuations in proteins stability. At another extreme end of the length scale probed at ILL, it is recognised that neutrons are an ideal tool to inves- tigate fundamental interactions such as the gravitational field and to induce nuclear reactions giving indications about the structure and the excited states of nuclei.
The activity of the ILL Theory Group has also contributed to the reputation of the ILL and over the years has become a famous centre of attraction thanks to the leader- ship of Philippe Nozieres. Last year, when he decided to retire, the ILL and the ESRF celebrated together the fruitful collaboration with him by hosting a meeting during which his many friends could present the most recent developments in theoretical models for strongly correlated fermions. The ILL wishes to express the deepest thanks to Philippe Nozieres for such a productive and rewarding partnership.
The ILL is very grateful to the many collaborators who have eagerly submitted their highlights to the present report. We wish to thank most warmly all of you who have contributed to this year's version and, more especially, those whose contributions could not be included. Scientific highlights 0 magnetism
Origin of the 3D ferromagnetic ordering in the free radical Nit(SMe)Ph
O S. PLLLET, M. SOUHASSOU, C. LECOMTE (UNIVERSITE NANCY 1)
0 Y. PONTILLON (ILL)
0 A. CANESCH1, D. GATTESCHI (UNIVERSITY OF FLORENCE)
0 E. RESSOUCHE, J. SCHWEIZER (CEA, GRENOBLE)
A combined study of the magnetic properties of a nitronyl nitroxide free radical exhibiting 3D ferromagnetism, by experimental spin and electron density has been performed using non polarised and polarised neutron and X-ray diffraction. In this family of compounds, the short contacts between radicals are thought to explain the macroscopic magnetic behaviour. Charge transfer from the nitroxide fragments to the methyl-thio group has been evidenced. The role of (b) (c) intermolecular contacts is also high- lighted to explain the long range Figure 1: View of the Nit(SMe)Ph radical (a) and short intermolecular contacts (b, c). magnetic ordering involving three hydrogen bonds and one nitroxide- nitroxide short contact. the distribution of the whole electrons of the involves different intermolecular contacts, system. These informations are of utmost as shown on Fig. lb and lc. First, a network importance to understand well the magnetic of intermolecular hydrogen bonds between coupling mechanisms at the microscopic the two oxygen atoms of the nitroxide groups Since the discovery of the first purely organic level. In a second step, this should help in ferromagnet in 1991 [1], the |3 form of the p- and methyl fragments, links the radicals in designing new molecular systems exhibiting nitrophenyl nitronyl nitroxide Nit(p-NO2)Ph the crystallographic a and c directions. The more efficient magnetic interactions (higher (Tc = 0.6 K), these series of organic free radi- O-N-C-N-0 groups of each radical are stack- Tc), if one could rationalise the length and cals have been extensively studied with the ed in a nearly perpendicular way in the a orientation of the intermolecular contacts in hope to find molecular magnets with high cri- direction (4.8229(1) A). the solid state. tical temperatures. The long range magnetic The behaviour of one radical in the crystal, ordering observed in this kind of compounds, thus showing the effects of the magnetic cou- which requires co-operative effects, is driven The 2-(4-methylthiophenyl)-4,4,5,5-tetrame- plings on this radical, has been investigated principally by the nature of the radical (i.e. thyl-imidazoline-l-oxyl-3-oxide [Nit(SMe)Ph] in two different ways using the complemen- subsituents) and the crystal packing (short exhibits bulk ferromagnetism with a low tarity between polarised neutron diffraction intermolecular contacts). The techniques Curie temperature (Tc = 0.2 K) [2,3], This with spin density modelling and high resolu- used in this context are polarised neutron radical carries a single delocalised unpaired tion X-ray diffraction with electron density diffraction which gives direct access to the electron (S = 1/2) which is the basis for its modelling and topological analysis. It must be distribution of unpaired electrons in the solid magnetic properties (Fig. la). noted that the models (multipolar model, see and X-ray diffraction which gives access to The packing of the radicals in the cell for example Goppens 1997, [4]) used to
Scientific 10 highlights reconstruct the spin and electron density are charge transfer between these two parts of regions using its topoligical properties. In similar, enabling a direct comparison of the the molecule. The main motivation for stu- order to obtain the real positions of the results. The polarised neutron measurement dying accurately the intermolecular regions hydrogen atoms and the elements of their - done at the diffractometer DN2 (CEA, were these observations on the behaviour of anisotropic thermal displacement tensor, Grenoble) - demonstrates that the unpaired one Nit(SMe)Ph radical (spin and charge neutron diffraction measurements were per- electron is mainly located in a it* molecular densities), reflecting the intermolecular formed on D9 and D15. This poin t is of major magnetic orbital but also partially delocali- interactions. importance to model the electron density in sed on several parts of the radical. the hydrogen bond regions by the so-called Significant spin density is also observed on The intermolecular interactions involve three (X-(X+N)) [8] multipolar refinement (a com- the carbon of the methyl-thio group and on hydrogen bonds and one short N-O-O-N bination of diffraction data collected with one methyl-carbon of the nitronyl ring, invol- contact for which the study in the context of classical neutron and X-ray). In the intermo- ved in the intermolecular contacts (Fig. lb, magnetic couplings, is of great interest. For lecular regions, an electron density of :i lc); this gives evidence for the intermolecu- this purpose, the electron density distribu- ~ 0.05 eA is observed (Fig. 3), only in the lar magnetic interactions. tion was analysed in the hydrogen bond hydrogen bond paths. This network of hydro- gen bonds in the solid could induce the long On the other hand, high resolution X-ray range magnetic ordering observed by magne- diffraction measurements provide accurate tic measurements [1,2] at much lower knowledge of the electron density distribu- temperature. tion in this compound and therefore give a clear picture of the interatomic links, com- This work has confirmed that the combination pleting the previous spin density (only unpai- of neutron diffraction (polarised and non- red electrons) point of view [7]. The delocali- polarised) and high resolution X-ray diffrac- sed system spanned to the sulphur atom is tion is a perfect technique to study magnetic demonstrated by the X-ray study : bond interactions at the microscopic scale in the lengths and electron density in all bonds field of molecular magnetism including ato- involved in the delocalisation scheme are in C6 mic, interatomic and intermolecular beha- a medium range between those of a pure viours. single and a pure double bonds. The parame- terisation of the multipolar model used to reconstruct the experimental electron densi- ty shows small negative net charges (-0.16 e) on each nitroxide fragment which is surpri- sing for such electronegative atoms, whereas Figure 2: Projection of the spin density, the methyl-thio group is highly negative as obtained by wave function modelling and projected into the ONCNO plane (-0.59 e). This can be interpreted in terms of (units are U.BA'2).
REFERENCES O [I] P. TUREK, K. NOZAWA, D. SHIOMI, K. AWAGA, Y. MARUYAMA AND M. KINOSHITA, CHEM. PHYS. LETT. 180 (1991) 327 0 [2] A. CANESCHI, F. FERRARO, D. GATTESCHI, A. LE LIRZIN AND E. RENTSCHLER, INORS CHEM. ACTA 235 (1995) 159 O[3] A. CANESCHI, F. FERRARO, D. GATTESCHI, A. LE LIRZIN, M. NOVAK, E. RENTSCHLER AND R. SESSOLI, ADV. MATER. 7 (1995) 476 o [4] P. COPPENS, X-RAY CHARGE DENSITIES AND CHEMICAL BONDING, IUCR, OXFORD UNIVERSITY PRESS (1997) c [5] Y. PONTILLON, A. CANESCHI, D. GATTESCHI, A. GRAND, E. RESSOUCHE, R. SESSOLI AND J. SCHWEIZER, CHEM. EUR. J. 5 (1999)
3616 o [6] Y. PONTILLON, A. CANESCHI, D. GATTESCHI, E. RESSOUCHE, J. SCHWEIZER AND R. SESSOLI, PHYSICA B (1999) 51 0 [7] S. P1LLET, M. SOUHASSOU, Y. PONTILLON,
3 A. CANESCHI, D. GATTESCHI AND C. LECOMTE, NEW J. CHEM. Figure 3: Electron density as 3D representation, isocontour at 0.02 eA level showing one of the intermolecular (2001 SPECIAL ISSUE) IN PRESS 0 [8] E. ESP1NOSA, contacts by accumulation of electron density in the intermolecular region. Only the hydrogen atom involved in C. LECOMTE, E. MOLINS, S. VEINTEMILLAS, A. COUSSON AND the hydrogen bond is represented and only O-N-C-N-O--H-C-S fragment is included in electron density repre- W. PAULUS, ACTA CRYST. B52 (1996) 519 sentation for clarity.
Scientific 11 highlights Quantum magnetic phase transition: a top-down approach
N. CAVADINI, W. HENGGELER, A. FURRER application. Comprehensive experimental pure academic interest. Quantum critical (LNS, ETH ZURICH & PSI VILLINGEN) studies often relied on the detailed microsco- behaviour, related to the existence of compe- o K. KRAMER, H.U. GUDEL pic data accessible by neutron scattering. ting ground states in strongly correlated (UNIVERSITY OF BERNE) Recent years have witnessed the growing electron systems, is for instance believed to O H. MUTKA, A. WILDES (ILL) interest in a different mechanism leading to have deep implications for the yet unsolved symmetry breaking: quantum phase transi- issue of high-Tc superconducting materials [1,3], U P. VORDERWISCH (HMI BERLIN) tions occurring at "T=0". At a quantum criti- Again, magnetic insulators provide us with cal point - controlled by an external parame- appealing model systems, which represent ter like doping, pressure or magnetic field - the complexity of true many-body quantum the ground state changes from order to disor- correlations with well-understood spin Quantum magnetism exhibits unique der due to zero-point fluctuations of quantum interactions. It is along these lines that we collective features, bearing no classi- nature [1], Statics and dynamics at "T=0" started the microscopic investigation of the cal analogies. The unconventional are deeply interconnected, as expressed by unconventional S=l/2 antiferromagnets magnetic excitations realised in S =1/2 the upper critical dimension d<4-z separa- KCUCI3 and TIGUGI3. Both compounds share copper insulators KC11CI3 and TIC11CI3 ting non-classical from classical critical the same monoclinic structure, characteri- were investigated by inelastic neutron behaviour, where z is the model specific sed by the stacking of nearly isolated C112CI6 scattering. We identify in TIC11CI3 dynamic critical exponent [2]. The novel plaquettes in double chains along the a axis a promising candidate for the micro- insights to be gained by the study of such (Fig.1). Static magnetic measurements scopic study of a quantum phase unconventional phenomena are by no way of reveal a nonmagnetic singlet ground state transition, driven by zero-point spin fluctuations at the lowest temperature. Such novel studies often remained precluded to neutron scattering due to the lack of suitable samples. Nevertheless, they address some of the most fundamental and debated issues of condensed matter physics.
Temperature-induced phase transitions are ubiquitous phenomena in condensed matter S=O singlet / physics. The crossover from occurrence of disorder to order, dictated by an underlying symmetry breaking, generally reflects the competition between cooperative interac- tions and thermal fluctuations. Classical magnetic systems, in which well-localised spins interact among themselves on the grounds of exchange couplings, provide a rich variety of ordered phases. Below a cha- racteristic critical temperature Tc, collective spin correlations overcome thermally driven fluctuations, and a spontaneous magnetisa- tion sets in. Concepts like order parameters, critical behaviour and universality find ideal realisations in the field of classical magne- tism, which has constituted a privileged tes- Figure 1: Schematic view of the magnetic interactions in the S=l/2 (K/Tl)CuCl3 family, projected on the be plane of the unit cell. Centrosymmetric CU2CI6 plaquettes behave as singlet-triplet dimer units, building a 3-D inter- ting ground for theoretical models of wide acting dimer network. Correlations along the a axis are not shown.
Scientific 12 highlights TIGuGls retain a quantum disordered ground state clown to the lowest temperature. A direct comparison of the neutron spectra fur- ther shows that the network in TlCuClg is stronger than in KGuCla (Fig. 2), resulting in an appreciable energy bandwidth across the leading parameter J. In the presence of an external magnetic field H, the excited triplet state undergoes Zeeman [-x 0 2x] *A [OOx] [xOO] splitting, which eventually overcomes the Y energy gap at the critical field given by (-0.5 0 2) (0 0 1) (0 0 2) (0.5 0 2) g(XBHc ~ A where g denotes the gyromagne- 2 momentum (h 0 I) [r.l.u.] tic factor of the Cu * moment (Fig. 3). For H>HC, zero-point quantum fluctuations are quenched by the cooperative effect of the Figure 2: Observed energy dispersion of the magnetic excitations in TIC11CI3 (circles - experimental data; 3-D exchange interactions, and the gapless dashed lines - model calculations) and KCUCI3 (continuous line - model calculations; data not shown), compared along selected directions of reciprocal space at T~2K. Calculations reflect the model explai- spin system recovers an ordered magnetic ned in the text. slate [7], The quantum phase transition separating order at H>HC from disorder at with a finite energy gap to excited states [4]. magnetisation measurements. However, only H
Scientific highlights 13 The temperature dependence of the magnetisation in Fe6sNi35 invar: evidence contradicting the two state model
0 P.J. BROWN reasonable account of the ground state excited transitions between these two states (ILL AND LOUGHBOROUGH UNIVERSITY) properties of 3d transition metals, the same are supposed to compensate for the lattice is not true for the finite temperature proper- expansion due to the anharmonic components G K-U. NEUMANN, K.R.A. ZlEBECK ties of which the invar effect is one. of the lattice vibrations. Recent spin polarised (LOUGHBOROUGH UNIVERSITY) band structure calculations made by Entel [6] The invar effect disappears above the Curie lend support to this model. They predict the existence of at least two stable states in an We have used polarised neutron dif- temperature, which suggests that the magne- fee iron-nickel alloy, which have different fraction measurements on single tic properties must in some way offset the moments and atomic volumes. Although the crystals of the Fe6sNi35 invar alloy to normal lattice expansion and this premise is thermodynamic consequences of such a determine the magnetisation distribu- the foundation of most explanations given for model are difficult to quantify, it has been tion in the temperature range 100 to the effect. Most theories are based upon suggested that the invar effect arises from the 600 K. We find that the shape of the volume instabilities of the magnetic moments magnetisation distribution remains essentially unchanged throughout the whole temperature range. The frac- tion of magnetic electrons with eg symmetry remains constant at -0.47. This finding is inconsistent with 0. 1 models of the invar effect in which a \ state, or states, with a configuration different from the ground-state and I \ t with slightly higher energy, is thermally 0.0 5 populated. Q. CO \ I o ± \_^ 0 c T* ° en X ^ The invar effect, was discovered more than CO \
Scientific 14 highlights 0.550 in the two states are divided differently be- tween the orbitals with eg and with t2g symmetry. This means that the shape of the I 0.500 associated magnetisation distributions differ. The magnetic moment of the higher energy state is supposed to be mostly due to 0.450 electrons in t2g orbitals, so the proportion of 3.00 CD electrons with this symmetry in the mean dis- tribution should rise with increasing tempera- O 33 ture. Neutron scattering provides a unique 2.00 ° means of probing this distribution. O Tl In a ferromagnet the spatial distribution of CD the periodic magnetic moment can be 1 determined from precise measurements of LL. 1.00 — the magnetic neutron scattering in the Bragg reflections. In order to obtain the magnetisa- tion distribution in Fe65Ni35 we have carried - 0.00 out a series of polarised neutron diffraction 100 200 300 400 500 600 700 experiments. In these experiments the Temperature (K) polarisation dependence of the intensity of Figure 2: Temperature variation of the parameters describing the shape of the magnetisation distribution in the Bragg reflections was used to determine the Fe6sNi35 invar alloy. The blue triangles give the fraction of magnetic electrons in eg orbitals (left hand scale) ratio of magnetic to nuclear scattering and and the red circles the ratio of the Iron to Nickel moments (right hand scale). The red and blue horizontal lines hence to obtain the magnetic scattering are drawn through the corresponding mean values. amplitudes. Measurements were made at temperatures between 100 and 600 K, cho- be applied to later data sets below this limit. significantly between 100 and 600 K. sen to cover the region of anomalous thermal In a second experiment made on D3 at 0.84 A Furthermore the ratio iWuNi is essentially expansion and the paramagnetic regime (without the hot source) we measured flip- constant, showing that there is little variation where the thermal expansion has regained its ping ratios for the same set of reflections at in the radial density either. There is therefore normal lattice behaviour. temperatures of 100, 200, 300, 400, 500 and no significant change in the shape of the time- 600 K in an applied field of 9 T. From these averaged magnetic moment distribution be- tween 100 and 600 K. In particular there is We have made measurements on two pillar data we were able to determine the tempera- no significant thermal population of a low shaped single crystals of Fe6sNi35, elongated ture dependence of the magnetic moment moment, low volume state with a high pro- parallel to (110) with dimensions ~2.5x2.5x5 distribution. portion of electrons in orbitals of t2g symme- mm and 1x1x5 mm; these were cut from a try in this temperature range. We conclude larger crystal grown using the Bridgeman In order to quantify the changes taking place therefore that the two-state model of the with temperature we used a simple model of technique. They were mounted with their long invar effect based on the static interpretation the magnetisation distribution. The model axes parallel to the field direction of a cryo- of band structure calculations can not be consisted of a random arrangement of atoms magnet on the polarised neutron diffractome- valid. Recent neutron inelastic scattering stu- on a fee lattice with 65% having a moment ter D3. In a first experiment, made whilst the dies of Fee5Ni35 [7] suggest that further pro- PFe and scattering like the 3d electrons of iron hot source was in place, we measured the gress in understanding the invar effect may and 35% having moments UM and scattering polarised neutron flipping ratios of all hhl be obtained by considering the dynamic impli- like the 3d electrons of Nickel. The aspherici- reflections with sin 6/1 < 1.0 A-' from both cations of instabilities in the band structure. ty of the distribution about the atomic sites crystals at neutron wavelengths of 0.85,0.71, was modelled by allowing a fraction y of the 3d 0.61 and 0.48 A. We measured under this electrons to have eg and the rest t2g symme- wide range of experimental conditions to try. The parameters UFe, IM, and y of this allow a reliable model for the extinction in the model were fitted to the observed magnetic REFERENCES crystal to be elaborated, since extinction is structure amplitudes at each temperature. 0 [1] CH. E. GU1LLAUME, C.R. ACAD. SCI 10 (1897) 235 the biggest source of error in this type of mea- O [2] P. MOHN, NATURE 400 (1999) 18 C [3] M.V. Very good fits were obtained as can be seen SCHILFGARDE, I. A ABR1KOSOV AND B. JOHANSON, NATURE surement. By comparing the eight sets of data from the graph of the observed and calculated 400 (1999) 46 O [4] J.W. LYNN, N. ROSOV, M. ACET AND we were able to establish an extinction model amplitudes at 400 K shown in Fig. 1. H. BACH, J. APPl. PHVS. 75 (1994) 6069 O [5] R.J. WEISS, which proved reliable up to about 20% of PROC. ROY. SOC. 82 (1963) 281 o [6] P. ENTEL, The variation with temperature of y and the E. HOFFMANN, P. MOHN, K. SCHWARZ V. L. MORUZZI, PHYS. extinction. By using the smaller crystal to ratio upe/UNi is shown in Fig. 2. It is clear from REV. B 47 (1993) 8706 O [7] P. J. BROWN, B. ROESSLI, measure the strong low angle reflections and this figure that the symmetry of the magneti- J.G. SMITH, K-U. NEUMANN AND K. R. A. ZIEBECK J. PHYS. the larger one to measure those at higher sation distribution around the atomic sites, CONDENS. MATTER 8 (1996) 1527 (PAPER ACCEPTED: J.PHYS. CONDENS. MATTER !3_1563 (2001) angle, we were able to keep the corrections to which is characterised by y, does not change IN PRESS)
Scientific 15 highlights Magnetic configurations: complementarity of neutrons and synchrotron X-rays
P. NORMILE, W. G. STIRLING applying an external perturbation such as a Resonant X-ray magnetic scattering (RXMS) (UNIVERSITY OF LIVERPOOL) magnetic field or uniaxial stress, and a series experiments (performed at the synchrotron at of studies [1] in the early 1980's, and subse- Brookhaven National Laboratory, USA) gave D. MANNIX, J. REBIZANT, F. WAST1N, G. H. LANDER (INSTITUTE FOR quent work, has allowed the magnetic struc- important information on the element-specific TRANSURANIUM ELEMENTS, KARLSRUHE) tures of many actinide (5/) compounds with magnetism associated with each actinide the NaCl structure, such as UN, UAs, USb, cation in the solid solution [4], and found ano- F. BOURDAROT, P. BURLET (CEA-GRENOBLE) NpAs, PuSb etc. to be determined. Such malies in the intensities around T'. RXMS has moment configurations have recently been the the advantage over neutron scattering that subject of theory [2, 3] so that, for example, there is an extra term in the cross section that the 3(j structure (Fig. 1) in USb is a gives rise to additional satellites [5] at posi-
To understand the unusual nature of the 5f electrons in actinide systems, we use both neutrons and synchro- tron X-rays to characterise their ground-state configurations. The experiments reported here involve single crystals containing plutonium, an element in which the 5f electrons are close to being localised. Despite the considerable knowledge provided by the synchrotron experiments, a key experiment, allowing a clearer inter- pretation of both the synchrotron and magnetisation results, was performed Figure 1: Illustration of left: single-
Scientific 16 highlights understood, nor were some other features of Figure 2: (a) Temperature dependence of the lq and 2q satellite in the RXMS (measured at the magnetisation and RXMS studies. Brookhaven National Laboratory) experiments on We then turned to neutron experiments on (Puo.7oUo.25)Sb. The initial growth of the 2q satel- D15. This instrument has the capability of lite is due to the presence of the 3q configuration (see Fig. 1), but at T there is a sluggish transition performing experiments with the sample in an to the 1 q state, in which the 2q satellite does not applied magnetic field, and with the normal- exist. There is only a small change at T in the first- beam technique we can measure intensities order satellite (lq) in zero field with either neu- from different components. A key result is trons or resonant photons. (At 40 K the 2q satelli- te is 3-10"1 times weaker than the lq satellite. To shown in Fig. 2(b), in which we show the show their relative T-dependence we have norma- intensities of the three Fourier q*, %, and qz lised their intensities at 40 K) (b) Intensities of the components as a function of temperature with three Fourier %, %, and qz components as a func- tion of temperature with a 1 T field applied paral- a 1 T field applied parallel to the [1, -1, 0] lel to the [1,-1, 0] direction as measured on D15. direction. Just below TN all intensities are The sudden change of intensity at T is interpreted equivalent; this shows that the configuration is as the magnetic symmetry changing from 3q (above T") to 1 q (below T). The relatively poor sta- 3q. However, below T' the component with the tistics are due to the high absorption of 23!)Pu and antiferromagnet moments perpendicular to the resulting small intensity of scattered neutrons. the field rapidly dominates, which is due to the structure changing from 3q to 1<7 at T'.
Fig. 3 shows a schematic (H, T) phase diagram 20 40 60 80 TN ioo 120 of this material. There are at least two inter- Temperature (K) esting features (1).
The critical field Hc = 3.5 T suppresses the 3q structure independent of temperature, and (2) there is a sluggish (entropy driven) transition from 3q to 1 q as the temperature is lowered for all H < He. Determining the magnetic confi- gurations of actinide materials has been found to be a prerequisite for understanding their electronic structure [2,3]. Although the RXMS study gives spectral information not covered here, the results of the D15 neutron experi- ments are fundamental in identifying the ground-state configuration. They allowed a theory of the 3q -> 1 g transition, as well other information from the RXMS (and magnetisa- tion) experiments, to be placed on a much fir- mer base; thus demonstrating, again, the com- plementarity between the synchrotron and neutron techniques. Magnetic Field [T]
Figure 3: Schematic phase diagram for the (Puo.75Uo.25)Sb sample as a function of field (H) and temperature (T). Note that TN (as shown in Fig. 2(b)) is ~ 90 K, and is independent of magnetic field. The plot has been made to show the percentage of the crystal in the Zq magnetic configuration at any 11 and T. Zero 3q implies that the entire sample is in the is 1 q state - see Fig. 1 for the difference between these.
REFERENCES 0 [ 1 ] J. ROSSAT-MIGNOD ET AL., PHYSICA B 102 (1980) 237 O [2] L. M. SANDRATSKI, ADV. IN PHYSICS 47 (1998) 91 0 [3] H. YAMAGAMI, PHYS. REV. B 61 (2000) 6246 o [4] SEE, FOR EXAMPLE, E. LIDSTROM ET AL, PHYS. REV B 61 (2000) 1375; P NORMILE ET AL., TO BE PUBLISHED O [5] C. VETTIER ET AL., SPIE CONF. PROC. 3773 (1999) 351
Scientific highlights 17 Incommensurate antiferromagnetic fluctuations in superconducting YBa2Cu3O6.85
O H. M. R0NNOW, L.-P. REGNAULT (CEA, GRENOBLE) 120 Q=(1.5,0.5,1.7) O C. ULRICH, B. KEIMER Q=(1.8,0.6,L7) 100 (MAX-PLANCK INST1TUT, STUTTGART)
O M. OHL (FZ, JULICH) 80 O P. BOURGES, Y. SlDIS (LLB, SACLAY) I60
Among the tremendous efforts devo- 40 ted to understand high-Tc supercon- ductivity, a special position is held by 20 inelastic neutron scattering studies of 40 50 antiferromagnetic fluctuations within 10 20 30 the superconducting state. If not Energy [meV] directly responsible for the coupling Figure 1: Energy scan through the antiferromagnetic zone center at Q=(l .5,0.5,1.7) (circles) and at a background mechanism, these magnetic fluctua- position 0=(l.8,0.6,1.7) (squares) for an incident polarisation parallel to 0 (spin-flip channel). The energy tions at least provide an important dependence of the background is due to the increased counting time at higher incident energies. The shaded area illustrates the magnetic signal peaking at the resonance position of 41 meV. probe to the nature of the phenome- na. The main feature of the magnetic which form quantum mechanically coherent resonance around 41 meV at the wave vector excitation spectrum in superconduc- pairs. It has been established that also high- (jt.jt) of the antiferromagnetic zone center ting YBa2Cu3O6+x is the presence of a Tc superconductivity is due to pairing of the [2]. Also in La2-xSrxCu04 is there a signal resonance at a characteristic energy charge carriers, but that the coupling mecha- around (jt.jt), but no sharp resonance has around 41 meV and a wave vector nism is not the standard phonon mediated been observed. Instead it was discovered (jT,jt) corresponding to the antiferro- coupling. The main problem in understanding that the intensity is constituted of four magnetic zone center. Recently it was the phenomena is therefore to find the fluctua- incommensurate contributions at (jt±5,it) found by unpolarised neutron scatte- tions that are responsible for the coupling and (jt,ii±6) [3,4], which has been associated ring technique that below the reso- mechanism. One generic feature of the high- to the existence of charge stripes [5]. These nance energy the magnetic signal Tc superconductors is that they all contain results were encouraging in the sense that splits into incommensurate positions CuO2 layers, which in the undoped parent the magnetic excitation spectrum in the (jx±6,jt) and (jt,jt±S). We have applied compounds form antiferromagnetic insulators superconducting state shows interesting neutron polarisation analysis to [1]. This could indicate that the coupling anomalous behavior, but at the same time obtain state of the art data on the mechanism is mediated by magnetic fluctua- the apparent differences between the two incommensurability of the resonance, tions, which indeed have been found to under- systems could indicate that the behavior are from which a precise model for the go profound changes as the materials are peculiarities for each material and not excitation spectrum can be built. doped from antiferromagnetic insulators to universal to high-Tc superconductivity. superconducting metals [2], Recently, a unified picture emerged from the Inelastic neutron scattering measures directly discovery using unpolarised neutron scatte- One of the most important outstanding pro- the magnetic excitation spectrum with the ring that also YBa2Cu3O6.6 develops an blems in condensed matter physics is that of one drawback that the relative weakness of incommensurability below the resonance high temperature superconductivity. In addi- the neutron scattering signal makes the avai- energy, where the signal contains four contri- tion to the potential technological applica- lability of large single crystals a prerequisite butions at (jt±S,;t) and (it,ji±5) [6]. In more tions, it represents a novel quantum state of for such studies. For this reason, experimen- elaborate investigations on YBa2Cu3O6.85the coupled electron systems. In conventional tal investigations have mainly been limited to incommensurate signal has been tracked superconductors the lattice fluctuations YBa2Cu3O6+x and La2-xSrxCu04. In down to 35 meV [7]. Quite interestingly and mediate a coupling between the electrons, YBa2Cu306+x the most prominent feature is a novel, the incommensurate fluctuations,
Scientific 18 highlights well-defined below Tc, were found to almost behavior can more naturally be understood if vanish in the normal state, contrary to LSCO one assumes the presence of two signals. The where the incommensurate peaks persist sharp dispersion of the resonance, whose well above Tc. The unpolarised data could be intensity almost vanishes above Tc, and a interpreted in terms of a downward disper- broad commensurate signal, which is less energy and temperature dependent. These sion relation connecting the resonant mode signals are sketched in figure 3, which also and the incommensurate fluctuations. shows the commensurability at 1.5 K as extracted from fits with the resolution convo- In order to confirm these puzzling results, we luted model. have undertaken a polarized neutron scatte- ring study of YBa2Cu3Oe.85. By separating In summary, we have presented a state of the the magnetic and structural (phononic) scat- Figure 3: The peak locations as a function of energy for respectively the unpolarised (squares) art polarised inelastic neutron scattering terring into respectively the spin flip and the and the polarised (circles) neutron scattering data. experiment to characterise the incommensu- non spin flip channels, polarised neutron As described in the text, the observed signal may be rability around the resonance feature in the constituted of a sharp signal illustrated by the solid scattering has repeatedly demonstrated its magnetic excitation spectrum of the high-Tc power to confirm the detailed behavior of line and a broad commensurate contribution as sketched by the shaded area. superconductor YBa2Cu3O6+x. There are two complex scattering signal. The experiment main theoretical approaches to describe was performed on the thermal three axis this incommensurability. One is based on the spectrometer 1N22 (CRG) at the ILL using respectively. Scans through (1.5,0.5,1.7) at formation of static or slowly fluctuating char- focusing Heusler monochromator and analy- three energies respectively above, at and ge stripes [5] and the other builds on Fermi- ser to obtain a flipping ratio of 17 at a fixed below the resonance energy are shown in surface effects [8,9]. We believe that the 1 3 final neutron wave vector of 4.1 A . A 4 cm Fig. 2, which unambiguously confirm the selection of the most probable model will be sample was mounted with the reciprocal incommensurate splitting below the resonance based on its ability to reproduce the emer- space directions [3,1,0] and [0,0,1] in the energy, while on the other hand the signal ging experimental picture of the incommen- horizontal scattering plane, such that scans remains commensurate and resolution limi- surate to commensurate crossover as a func- could be performed around the (1.5,0.5,1.7) ted at higher energies. With these data at tion of energy and temperature. Even if the position. hand, it is now possible to conclude several magnetic fluctuations should turn out not to definitive facts about the spin fluctuations in be directly responsible the coupling mecha- Figure 1 depicts an energy scan at the anti- nism in high-Tc superconductivity, it is a YBa2CU306.85 ferromagnetic zone center Q=(l.5,0.5,1.7) strong fingerprint of the electron correlations within the system. And any theory aiming to and at a background position Q=(l .8,0.5,1.7) 1) Below the resonance energy, an incom- explain the coupling mechanism should be mensurate signal exists down to 25 consistent with the detailed experimental meV, below which the intensity picture that is currently being established for T=1.5K T=120 K became untractable. the magnetic excitations spectrum. 500 Qk 2) Above the resonance energy the signal stays commensurate and reso- • lution limited up to at least 45 meV. r • -v W- A j • 3) Above To (at 120 K) the signal at Qh 40 meV almost disappears, while 1.3 1.4 1.5 1.6 1.7 the signal at 33.5 meV is only redu- 40meV ced by a factor of 1.6. 4) Based on fits with resolution 1 ,Jtf tr^t— convoluted models for the scatte- 33.5 meV ring, we can add that after a rapid development just below the reso- nance energy, the incommensurabi- REFERENCES ¥ lity saturates around 6=0.09X2K 0 [1] D. VAKNIN, S. K. SINHA ET AL., PHYS. REV. LETT. 58 1 1.5 2 1 1.5 2 (see Fig. 3). (1987) 2802 O [2] J. ROSSAT-MlGNOT, L.-P. REGNAULT ET Q=(Qh,Qh/3,1.7) Q=(Qh,Qh/3,1.7) AL., PHYSICA C 185-189 ( 1991) 86 C [3] J. M. TRANQUADA ET AL., NATURE 375 (1995) 561 0 [4] B. LAKE, G. AEPPLI Figure 2: Constant energy scans through Q=(l.5,0.5,1.7) ET AL., NATURE 4000 (1999) 43 O [5] V. J. EMERY, S. A. as sketched in the upper right corner, which also illustrates It is difficult to imagine how the dis- KlVELSON AND 0. ZACHAR, PHYS. REV. B 56 (1997) 6120 the FWHM of the resolution ellipse. Three energies respecti- persion like feature observed at 1.5 O [6] H. A. MOOK, P. DAI ET AL., NATURE 395 (1998) 580 0 [7] P. BOURGES, Y. SlDIS ET AL., SCIENCE 288 (2000) vely below, at and above the resonance are shown for K can loose its intensity at 40 meV T=1.5 K (left) and the two lower energies are also shown for 1234 0 [8] F. ONUFRIEVA AND P. PFEUTY, COND- T=120 K (right). The scans have been displaced vertically but not at 33.5 meV when tempera- MAT/99O3O97 O [9] Y.J. KAO AND Q. SI AND K. LEVIN, PHYS. and the solid lines are guides to the eye. ture is increased to 120 K. This REV. B 61 (2000) 11898
Scientific 19 highlights Weak antiferromagnetic order of UPt3 studied under extreme conditions
pressure [3] showed a direct N.H. VAN DUK (IR1 DELFT) relation between the size of B. FAK (ISIS, CEA GRENOBLE) 1000 the weak ordered moment = (3/2+q,-l/2-q,0) and the splitting of the super- P. RODIERE, A. HUXLEY, L.P. REGNAULT, UPt conducting transition tempe- J. FLOUQUET (CEA GRENOBLE) ratures ATC = Tc* - Tc. F. YAKHOU (ESRF) By applying a pressure of 3-4 500 kbar, the antiferromagnetic .": M.-T. FERNANDEZ-DIAZ (ILL) c order is fully suppressed and 3 the two superconducting p= 0.37 kbar pile The antiferromagnetic order of the transition temperatures heavy-fermion compound UR3 shows merge. As a consequence, the a remarkably small ordered moment 1000 weak antiferromagnetic order of only 0.02 LIB below TN = 6 K, which is generally considered to act is believed to play a crucial role in the as a symmetry-breaking field unconventional superconducting for the superconductivity. behaviour at low temperatures Recent small-angle neutron-
(Tc = 0.55 K). In order to gain more 500 scattering measurements [4] insight in the nature of the antiferro- of the superconducting flux- magnetic order, we have performed T = 1.6K line lattice in an applied 3 p= 2.54 kbar low-temperature elastic neutron- • T = 10K magnetic field along the c scattering measurements under axis demonstrated the uncon- uniaxial pressures up to 6 kbar and in 0 ventional nature of the super- magnetic fields up to 12 T. The -0.04 -0.02 0 0.02 0.04 conductivity and uniquely uniaxial pressure dependence is q assigned the symmetry of the found to be strongly anisotropic with superconducting gap. a suppression of the ordered moment We studied the weak antifer- for pressure along the hexagonal c Figure 1: Magnetic Bragg peak intensity of UPt3 at 0 = (3/2+^,-1/2,0) as romagnetic order in UPt3 by axis and a domain repopulation for a function of q under an applied pressure of p = 0.37 and 2.54 kbar for p\\c at T= 1.6 K. For comparison, the high temperature scans (T> 7/v) elastic neutron-scattering pressure in the hexagonal plane. Both are included. measurements under applied the size of the ordered moment and pressures up to 6 kbar (p||a the domain population are however The magnetic Bragg peaks are not resolution and p||c) and magnetic fields up to 12 T (B||a hardly affected by applied magnetic limited but show a Lorentzian broadening that and B\\c) on the cold triple-axis spectrometer field up to 12 T along and perpendicu- corresponds to a finite correlation length of IN 14. Pyrolytic graphite (002) planes were lar to the hexagonal axis. the order of \ ~ 250-500 A. used as both monochromator and analyzer and a beryllium filter was placed before the At low temperatures, UPt3 shows unique sample. For the heavy-fermion superconductor UPt3, unconventional superconducting properties. early elastic neutron-scattering measure- The superconducting transition at Tc* = 0.55 The uniaxial stress was applied with a ments [1] revealed the onset of antiferroma- K is followed by a second superconducting hydraulic press via a stainless steel rod and gnetic order below TN = 6 K with an transition at Tc~ = 0.47 K. As a function of a knee-joint placed at low temperature to the unusually small ordered moment of m = magnetic field and temperature, an exotic faces of the sample. This system designed by 0.02 UB /(U atom). The antiferromagnetic superconducting phase diagram [2] is obser- the pressure group of the ILL allows for order has a propagation vector of ved with three different superconducting changing the pressure without heating the k= (1/2,0,0) with the ordered moment along phases that meet at a tetracritical point. pressure cell. The magnetic field was applied the propagation vector in the basal plane of Combined elastic neutron-scattering and spe- by the 12 T vertical cryomagnet from the CEA the hexagonal close-packed crystal structure. cific-heat measurements under hydrostatic Grenoble.
Scientific 20 highlights I I I I . UPt
22 bJD o hydrostatic pressure - (Hayden et al.) 3 4 5 10 12 14 p (kbar) B(D Figure 2: Relative integrated intensity of the magnetic Bragg peaks at 0 = (3/2,-1/2,0) and Figure 3: Relative integrated intensity of the magnetic 0 = (0,1/2,2) as a function of the applied pressure for p\\c and p\\a, respectively. The data Bragg peak as a function of applied magnetic fields up to points for p\\a (solid squares) and pile (solid circles) are normalised to the extrapolated 12 T (B\\a and B\\c). The measured data points (solid zero-pressure value and compared with the earlier measurements under hydrostatic pres- spheres) are normalised to the zero-field value and com- sure (open circles) [3). pared with earlier low-field measurements (open squares) [6]. In Fig. 1, scans through the magnetic Bragg pulation of the three k domains with a up to 12 T. In combination with the finite cor- peak at 0 = (3/2,-1/2,0) along k = (1/2,- constant moment. These results are in good relation length of § « 250-500 A, this sup- 1/2,0) are shown for applied pressures along agreement with specific heat measurements ports a scenario of slowly fluctuating the c axis of p = 0.37 and 2.54 kbar in anti- under uniaxial pressure [5] which show that moments which appear static on the time ferromagnetic state (T> TN) and the para- the splitting in Tc decreases for pile and is scale of cold neutrons. magnetic state (T> TN). Under a pressure of constant for p\\a. This confirms the existence 2.54 kbar the Bragg peak intensity, which is of a direct relation between the size of the proportional to the square of the ordered ordered moment and the splitting of the moment, is strongly suppressed. superconducting transition temperatures + Tc and Tc~. The relative pressure dependence of the inte- grated intensity of the magnetic Bragg peak Similar scans as shown in Fig. 1 have been is shown in Fig. 2 for p\\a and p\\c [5], For performed in magnetic fields up to 12 T for applied pressures along the c axis the B\\a and B\\c [6], The relative integrated REFERENCES magnetic Bragg peak intensity shows a non- intensity of the antiferromagnetic Bragg linear decrease for increasing pressure, 0 [I] G. AEPPLI, E. BUCHER, C. BROHOLM, J.K. KJEMS, peaks is shown in Fig. 3. The observed field J. BAUMANN, AND J. HUFNAGL, PHYS. REV. LETT. 60 (1988) while for applied pressures along the a axis a dependence closely corresponds to earlier 615 O [Z] S. ADENWALLA, S.W. LIN, Q.Z. RAN, Z. ZHAO, significant non-linear increase in the relative low-field measurements and is very weak in J.B. KETTERSON, J.A. SAULS, L. TAILLEFER, D.G. HINKS, integrated intensity is observed. The uniaxial M. LEVY, AND B.K. SARMA, PHYS. REV. LETT. 65 (1990) magnetic fields up to 12 T. For B\\a no signi- 2298 0 [3] S.M. HAYDEN, L. TAILLEFER, C. VETTIER, AND pressure dependence for p\\c strongly ficant domain repopulation is observed. J. FLOUQUET, PHYS. REV. B 46 (1992) 8675 resembles the data from earlier elastic neu- Additional temperature-dependent measure- o [4] A. HUXLEY, P. RODIERE, D. MCK. PAUL, N. VAN DIJK, R. tron-scattering measurements under hydro- CUBITT, AND J. FLOUQUET, NATURE 406 (2000)160-164 ments of the antiferromagnetic Bragg peak o [5] N.H. VAN DIJK, P. RODIERE, F. YAKHOU, static pressure [3] as shown in Fig. 2. The intensity have shown that the ordering tem- M.-T. FERNANDEZ-DIAZ, B. FAK, A. HUXLEY, AND J. strong increase in Bragg peak intensity perature of Iiv=6K is insensitive to applied FLOUQUET, PHYS.REV. B63 (2001) 104426 0 [6] N.H. VAN observed for p\\a therefore suggests a repo- DIJK, B. FAK, L.P. REGNAULT, A. HUXLEY, AND M.-T. pressures up to 6 kbar and magnetic fields FERNANDEZ-DIAZ, PHYS. REV. B 58 (1998) 3186 Scientific highlights 21 Unconventional superconductivity and the flux-line lattice 3 A. HUXLEY, P. RODIERE, J. FLOUQUET a preferred orientation of the FLL. Even in a extended defects can locally perturb the (CEA-GRENOBLE) conventional superconductor small deviations orientation of the lattice (FLL). This effect •") D. McK. PAUL (UNIVERSITY OF WARWICK) from perfectly cylindrical screening currents can however be substantially reduced if all can occur due to the anisotropy of the under- •J N. VAN DUK (DUT, DELFT) the flux lines are made to move, since the lying fermi-surface. For a field applied along defect potentials seen by the moving flux ) R. CUBITT (ILL) a four fold axis this anisotropy can lead to lines are then averaged out. deformed hexagonal lattices and changes of The measurements we report have been their alignment [1]. However for the special made on one of the best known heavy-fer- case we will consider when the field is mion superconductors, UPt3. Since the applied along a hexagonal crystal direction magnetic contrast from the FLL that gives When a magnetic field is applied to a the fermi-surface re-enforces the hexagonal rise to neutron scattering is inversely pro- superconductor the magnetic field structure with a preferred orientation. portional to the square of the electronic mass can penetrate as an array of flux lines. Another potential source of anisotropy in the enhancement, the diffracted signal from the These lines are often ordered to form screening currents comes from the angular FLL in UPt3 is very weak. It is then essential a lattice, the orientation and geometry dependence of the superconducting gap. This to have a combination of a high neutron flux, of which reflects the anisotropy of the effect is expected to be particularly signifi- highly stable apparatus and a high quality underlying interactions between the cant for an unconventional superconductor crystal to detect the scattering. However the lines. Our experiments show that where the locking of the electron phases has superconductivity in UPt3 is highly unusual. these interactions can be strongly modified in an unconventional super- conductor, which in the case of UR3 2.5 gives rise to preferred orientations of the flux-line lattice. The unusual lat- tice orientations we observe in the superconducting A-phase help identi- fy a particular symmetry for the unconventional superconductivity in this enigmatic material. In a superconductor the phases of individual electron wave functions become locked toge- ther. This rigidity underlies the frictionless passage of currents synonymous with a com- plete loss of electrical resistivity. Although a magnetic field induces angular momentum into the electron fluid, the phase rigidity can be conserved if the resulting motion is quan- 0.1 0.2 0.3 0.4 0.5 0.6 tized and the magnetic field is bunched to Temperature (Kelvin) thread the sample as an array of flux lines. In genera], repulsive forces between the flux Figure 1: The low temperature phase diagram of UPt3. Three different superconducting phases exist as lines favour the formation of a hexagonal a function of temperature and field denoted by A,B and C. The geometric figures show schematically the gap flux-line lattice (FLL) to maximise the sepa- structure in each phase, relative to the hexagonal crystal axis depicted by the frames. ration between the lines. However the scree- ning currents surrounding an isolated flux an angular dependence. In real materials it is The distinguishing feature is that the line need not be exactly cylindrical, and this also important to consider the complicating superconducting gap and not just the phase can lead to deviations from this geometry and effect of crystal defects. Flux lines trapped in of the order parameter is believed to have a Scientific 22 highlights maximum gap in the A-phase pointing along directions in between the symmetry axis of the crystal structure. The possible gap symmetries have previously been classified by symmetry analysis and only two possibilities appear compatible with the above observation, one of which can be eliminated by other measurements. The above experiment thus provides evidence in favour of a particular gap symmetry shown in Fig. 1. Finally if the lattice is formed at Tc by cooling in a constant field we observe a third FLL orientation that Figure 2: Symmetrized images of the small angle scattering from the flux-line lattice measured on D22. Panel (I) shows the image when the lattice was formed at lOOmK in the B phase. In (II) the lattice was grown at 500mK is aligned with extended crystal defects (Fig. in the A-phase. The second image is consistent with two domains of differently oriented nearly hexagonal 3). In this case the flux lattice shows lattices inclined at ±15° to the crystal symmetry axis shown by the solid and dashed lines (the a axis is vertical considerable longitudinal disorder, characte- and an a* axis is horizontal). rised by wide rocking curves compared to the resolution limited curves recorded in the other two cases. Figure 3: The image of the small angle scattering from a flux-line lattice grown at Tc (field cooled). The image, measured on D22 represents integrated diffracted intensities as in figure 2. The range of rocking angles over which the spots were illuminated is however much larger in this case. The FLL is aligned to extended metallur- gical defects which themselves give rise to small-angle scattering that was measured on Dll and is shown in panel II. lower rotational symmetry than the crystal lattices aligned at +15° and -15° to the structure (see Fig. 1) and further, this symmetry axes (see Fig. 2). These new symmetry changes with field and alignments were stable over a range of for- temperature, defining three distinct super- mation temperatures that span the A-phase. conducting phases denoted A,B and C. The natural explanation for the appearance We have used D22 to measure the of these new orientations is that they reflect orientation and geometry of the FLL with a the underlying symmetry of the superconduc- magnetic field along the hexagonal crystal ting gap. In the B-phase the gap appears axis for an applied field of 0.2 Tesla to isotropic about the hexagonal axis and the explore any evolution in the FLL between the diffraction spots are aligned with the direc- B and A superconducting phases [2], As the tions of maximum projected fermi-velocity. temperature at which the FLL was formed As the A-phase is approached however the REFERENCES was increased, we observed a change from gap becomes highly anisotropic about the [1] D. McK. PAUL, C. V. TOMY, C. M. AEGERTER, an hexagonal lattice aligned along crystal hexagonal axis and the diffraction spots align R. CUBITT, S. H. LLOYD, E. M. FORGAN, S. L.LEE AND M. YETHIRAJ, PHYS. REV. LETT. 80 (1998) 1517 symmetry directions, to two equally popula- with the directions of the maximum gap. The [2] A. HUXLEY, P. RODIERE, N. H. VAN DlJK, M. D. PAUL, ted domains of approximately hexagonal observed alignment therefore points to a R. CUBITT AND J. FLOUQUET, NATURE 406 (2000) 160 Scientific highlights 23 Dynamics of an itinerant magnet with orbital moments © A. HIESS (ILL) bution anticipated from the 5/ states we have extended our investigations Q N. BERNHOEFT (CEA-GRENOBLE) with the cold-source triple axis spectro- © S. COAD meter 1N14. (ILL AND INST. TRANSURANIUM ELEMENTS, KARLSRUHE) At low frequencies, within the spin wave gap, we have been able to characte- 0 G.H. LANDER (INST. TRANSURANIUM ELEMENTS, rise a response around Qo- Figure 3(aJ KARLSRUHE) gives the thermal evolution of the spec- © L. PAOLASIN1 (ESRF) tra and 3(b) the wave vector dependen- ce at 15 K, the solid lines have been cal- © D. KACZOROWSK1, A. CZOPNIK, R. TROC (W. TRZEBIATOWSKI INSTITUTE, WROCLAW) culated on the basis of a phenomenolo- 0.5 0.6 gical model developed previously [2]. (hhh) This consists of two modes, one slow UGa3, which has the simple cubic Figure 1: Typical constant-E scan with energy transfer of and overdamped, the other having a AuCu3 structure and orders at 14 meV. Data taken at T = 18 K with IN8. The two shoulders pole at finite frequency with the inter- TNeei = 67 K, is an itinerant 5/ antifer- represent a propagating mode symmetric about Qo. This mode becomes difficult to observe above -16 meV. The solid action between the two components romagnet with an orbital moment. We lines are Gaussian fits to the data. The Q-width (FWHM) of being taken into account in a mean-field have examined the dynamical magne- the (1 1 1) nuclear reflection is marked as black bars. approximation to yield the measured tic response with single crystals on cross section. As with the inelastic both IN8 and IN14. Two interesting scattering the quasielastic contribution features are present. First, there are whilst the slow, low frequency part has the dies rapidly as q increases from the zone two parts to this response, one qua- characteristics of an overdamped mode [1,2]. centre, this is evidenced by the decrease in sielastic and the other inelastic. UGa3 is a prototypical example of this class of intensity around AE = 0 in Second, all the scattering is localised materials for which the results of bulk Fig. 3(b). If one takes the low frequency around the magnetic zone centre. The thermodynamic [3] and microscopic de Haas spectral weight to be associated with the implications for our understanding of van Alphen [4] experiments yield unambi- propagation of quasiparticles at the Fermi the magnetic response of itinerant guous evidence for the itinerant nature of the / electrons are discussed. 5/ electron states. We have extended these investigations by neutron spectroscopic studies which give us detailed information on UGa3 - INS The challenge to understand the magnetic the dynamical magnetic response spectrum. 15 excitation spectra in actinide materials Initial experiments were performed with arises since the dominant 5/ states are single crystals on the thermal triple-axis considerably extended in real space and spectrometer 1T1 at LLB, Saclay, and were hybridise strongly with band levels involving then continued on 1N8 at ILL, Grenoble. These td both on site and neighbouring ligand wave- experiments, reported in Fig. 1 and 2, established an inelastic response at low functions. In addition to their hybrid nature 10 the 5/ states experience strong spin-orbit temperature in the antiferromagnetic phase coupling leading to anisotropic response around the magnetic zone centre, Qo =(j>7j), T = 18K functions and the possibility of a significant with a gap of about 5 meV. Interestingly this orbital moment. One manner to handle these spin wave like excitation propagates only over 0.05 0.1 0.15 complexities is to make a conceptual separa- a limited part (-25%) of the Brillouin zone in tion of the fast and slow degrees of freedom any of the high symmetry directions before i q I (r.l.u) of the 5/ levels whilst recognising that the the intensity dies away; previously, in ferro- magnetic metals similar effects have been Figure 2: Peak positions (deduced from Gaussian measured cross section is the total response fits, see Fig. 1) versus | Scientific 24 highlights consistent RPA [6] is able to provide a Q = (0.5,0.5, 0.5) 60K framework for understanding the antiferro- 2500 magnetic response of itinerant 3d com- 2000 pounds, as recently discussed both in the lanthanum cuprates and V2O3 [7], it is insuf- " 1500 ficient for the interpretation of the more complex 5/ spectra evidenced in this work. i One major feature of the work on UGa3 U 1000 reported here is to demonstrate unambi- guously the presence of a two component lineshape in the antiferromagnetic phase. The coupled mode phenomenology used in our analysis is a flexible generalisation of the theory of the Neel state and finds support in more formal theories based on, for example, the functional integral decoupling of fast and slow quasiparticle excitations [1]. We hope that these results will stimulate new activity on the magnetic response of metals in the presence of a large spin-orbit interaction. Figure 3: (a) Temperature dependence of the scat- tering at Oo versus energy transfer, (b) Q-depen- dence of the scattering along the [h h h] direction at T = 15 K. Note the displaced axes in both panels. The solid lines are fits to the interacting two com- ponent model as described in the text, and more fully in Ref. [2).(Data taken with IN14). surface, the topology of one particle calcula- tions [4] suggests a dominant contribution of states derived from the «rugby ball* part of the Fermi surface- the strong curvature implying a small amplitude for low-energy excitations having wave vectors substantial- ly different from Oo- Further detailed nume- rical analysis is required to confirm this conjecture. 2 4 6 E(meV) Figure 4: Colour contour map of the temperature and energy dependence at Oo as mea- Figure 4 gives a colour contour map of the sured on INI 4. Once the distinct inelastic feature disappears above 40 K the response is domi- temperature and energy dependence at Oo. nated by the quasielastic component {'Ma = 67 K.). This plot emphasises the merging, for T > 35 K, of the fast and slow contributions to the total response..The energy gap asso- ciated with the propagating part of the response, which in part may arise from the REFERENCES © [1] C. PEP1N AND M. LAVAGNA, PHYS. REV. B 59 (1999) spin-orbit interaction, decreases together 2591 0 [2] N. BERNHOEFT ET AL., PHYS. REV. LETTERS 81 with the staggered spin polarisation on (1998) 4244; PHYS1CA B 259-261 (1999) 614 approaching Twiei (67 K). In contrast, the 0 [3] D. KACZOROWSKI ET AL., PHYS. REV. B 48 (1993) 16425 © [4] A.L. CORNELIUS ET AL., PHYS. REV. B 59 strong quasielastic correlations in the dyna- (1999) 14473 O [5] Y. ISHIKAWA ET AL. PHYS. REV. B 25 mic magnetic response around Oo persist up (1982) 25 e [6] H. HASEGAWA AND T. MORIYA, J. PHYS. to at least 2TN&I- Soc. JAPAN 36 (1974) 1542 © [7] T.E. MASON ET AL., PHYS. REV. LETTERS 68 (1992) 1414; W. BAO ET In concluding we note that, whilst a self- AL., PHYS. REV. B 58 (1998) 12727 Scientific highlights 25 Spin excitations in a magnetic superlattice: the first inelastic neutron scattering measurements A. SCHREYER (RUHR-UNIV. BOCHUM AND ILL) 400 A. WILDES, W. SCHMIDT (ILL) C.F. MAJKRZAK, R.W. ERWIN, S. H. LEE (NIST) M. HONG, R. KWO (BELL LABORATORIES, MURRAY HILL) The observation of magnetic excita- tions in artificial systems with inelas- tic neutron scattering methods repre- sents a major challenge owing to the weak scattered signals. In this report we present measurements of spin waves in a Dy/Y superlattice using inelastic neutron scattering. The ingredients of our success have been Figure 1: Difference of inelastic constant 0 scans at T = 75 K and 10 K with fitted Gaussians. The projection onto the growth of a sample with a large the Q-E plane yields the dispersion curve. 0 is parallel to the Dy c-axis and the superlattice growth direction. number of layers and a large magne- tic moment and the use of modern high flux neutron instrumentation. Our periodicity would become accessible which a rather large 2.5 • 1.25 cm2 substrate. data indicates a folding of the can be dominated by the magnetic exchange Nevertheless, the Dy in the sample amounts Brillouin zone due to the superlattice coupling between the magnetic layers. Thus, to only 10 mg. Below its bulk Neel tempera- periodicity. INS would open up the opportunity to study ture of 179 K, Dy exhibits a helical phase the dynamic behaviour and the exchange cou- along the Dy c-axis which transforms into a pling in magnetic superlattices in more detail ferromagnetic structure below 89 K. In Dy/Y Up to now, information on magnetic excita- than was possible so far. superlattices a coherent helical phase occurs tions in magnetic thin films and superlattices Thin film samples made from Rare Earth which extends over many bilayers whereas has come from inelastic light scattering materials are the prime candidates for a the ferromagnetic phase is suppressed due (Brillouin scattering, BS) [1,2] and ferroma- study of magnetic excitations with inelastic to magnetoelastic effects [6]. The coherent gnetic resonance (FMR) experiments [3]. neutron scattering, since the Rare Earths helical phase is mediated by the diamagnetic FMR can probe spin waves at the centre of exhibit the largest magnetic moments of all Y interlayers via RKKY exchange coupling. the Brillouin zone (BZ), and at magnon wave elements, causing the largest possible cross- Elastic neutron scattering experiments have vectors q inversely proportional to the film section for magnetic neutron scattering. Rare confirmed that the present Dy/Y sample exhi- thickness. With Brillouin scattering, wave earth superlattices have been studied inten- bits the same long-range helical order obser- vectors on the order of the wave vector of sely over the last decade using elastic neu- ved previously. light are accessible, i.e. also close to the BZ tron scattering to determine their magnetic After initial work at the NIST Center for centre. Thus, it has not been possible to structures [4]. Neutron Research on the cold neutron spec- determine the dispersion of spin waves in The inelastic measurements were performed trometer NG-5 the measurements were magnetic thin film systems in the whole BZ. on a superlattice of the layer sequence continued at ILL on IN14 and IN12. The With inelastic neutron scattering (INS), on Y500A[Dy43A/Y28A]350/Y2340A/Nb2000A/Al2O3(S inelastic scans were performed at 75 K the other hand, the dispersion of spin waves ubstrate), which was grown by MBE methods where the Bose factor increases the spin- in much larger portions of the BZ is in prin- [5] along the c-axis of the hep structure of Dy wave signal to a sufficient level that it may be ciple accessible. Smaller wavelength (higher and Y. The amount of Dy in the sample was measured [7]. Equivalent scans at 4 K, 0) spin waves on the order of the superlattice maximised by growing 350 bilayers and using where the inelastic scattering is much wea- Scientific 26 highlights experiment, we have assumed to be bulk-like in Fig. 2. The resulting folded Brillouin zone (FBZ) is only 2re/A wide in q (instead of 2n/ao in bulk) and contains all the information about the dispersion curve. The obvious question at this point is if we can see a sign of this folding effect with neutrons. For example, instead of a single peak, we would expect a series of equidistant peaks in an energy scan at fixed q = n • 2jt/A. Such a scan at q = -2n/A performed with a higher instrumental resolution on IN 14 compared to the ones of Fig. 1 is shown in Fig. 3. Two peaks are now clearly visible. The energies -2-1012 roughly correspond to those which we expect q (2jt/A) assuming bulk-like dispersion curves (see Fig. 2). Figure 2: Schematic visualisation of the folding of the Brillouin zone in a superlattice on the spin wave disper- In conclusion these results demonstrate the sion curves. The resulting 1., 2. and 3. folded Brillouin zones (FBZ's) are marked. The dashed line indicates the feasibility of inelastic neutron scattering stu- scan of Fig. 3, the dots mark the expected peak positions. dies on spin waves in a rare earth superlat- tice for the first time. We find a bulk-like dis- persion curve and clear evidence of Brillouin zone folding effects. These results may open ker, serve as a quasi-background. The inten- through the superlattice stack due to the up a new field of research for inelastic neu- sity difference of these scans then is a , exchange coupling by the Y layers. The wave tron scattering providing access to much vector q of the spin wider regions of the Brillouin zone of magne- wave is then obtained tic superlattice structures than were acces- from the relation sible with other methods so far. Details and q = Q-Oheii. Comparison additional results will be published shortly [12]. with the data of bulk Dy [9] shows that the dis- persion curve observed here is essentially consistent with the bulk curve. In bulk the Brillouin zone is deter- REFERENCES 0,0 0,5 1,0 1,5 2,0 mined by the periodicity ::: [1] P. GRUNBERG, IN "LIGHT SCATTERING IN SOLIDS V", E (meV) EDITED BY M. CARDONA AND G. GUNTHERODT, SPRINGER of the lattice, i.e. by the VERLAG, BERLIN, HEIDELBERG, NEW YORK (1989) lattice constant ao. ::: [2] B. HILLEBRANDS AND G. GUNTHERODT, IN "ULTRATHIN However, an additional MAGNETIC STRUCTURES", EDITED BY B. HEINRICH AND Figure 3: High resolution scan along the dashed line of Fig. 2 yielding the two J. A. C. BLAND, SPRINGER VERLAG BERLIN, HEIDELBERG, expected peaks. period is present in a 1994, VOL. II, P. 258-277 0 [3] B. HEINRICH, IN REF. 2, superlattice along the P. 195-257 G [4] C.F. MAJKRZAK ET AL., ADV. IN PHYS. 40 (1991) 99; D.F. MCMORROW ET AL., PHYSICA B 192 (1993) measure of the spin-wave cross section. growth axis. This superlattice period L, which 150 0 [5] J. Kwo, IN "THIN FILM TECHNIQUES FOR LOW In Fig. 1 the difference between a few 75 K is the sum of the individual Dy and Y layer DIMENSIONAL STRUCTURES", EDITED BY R.F.C. FARROW, and 10 K data sets is plotted together with thicknesses, is expected to induce a folding of s.S.s. PARKIN, P.J. DOBSON, N.H. NEAVES AND A.S. ARROTT, NATO ASI SERIES B, PHYSICS V13, PLENUM PUBLISHER Gaussian fits and the projections of the peaks the Brillouin zone like in the case of phonons CORPORATION (1988) p. 337; M. HONG, R.M. FLEMING, onto the Q-I and Q-E planes [8]. in superlattices [10] or graphite intercalated J. KWO, L.F. SCHNEEMEYER, J.V. WASZCZAK, J.P. MANNAERTS, C.F. MAJKRZAK, D. GIBBS, AND J. BOHR, J. APPL. PHYS. 61 compounds [11]. This effect is visualised (1987) 4052 c: [6] M.B. SALOMON ET AL., PHYS. REV. LETT. We find a spin-wave excitation, which is well- schematically in Fig. 2. The superlattice per- 56 (1986) 259 O [7] SEE FOR EXAMPLE: G.L. SQUIRES, localised in energy. The resulting dispersion iod A causes satellite peaks in the elastic "INTRODUCTION TO THERMAL NEUTRON SCATTERING", CAMBRIDGE UNIVERSITY PRESS, CAMBRIDGE, NEW YORK, curve is shown in the Q-E plane. Its extrapo- scattering (at E = 0) at positions q = ±n • 1978 0 [8] A. SCHREYER ET AL., J. APPL. PHYS. 87 (2000) lation to higher 0 yields an elastic peak at 2JI/A (n = 1,2,3...) around the helimagnetic 5443 c [9] R.M. NICKLOW ET AL., PHYS. REV. LETT, 26 Oheii = 1-97 A"1 as its origin. This peak is (1971) 140 '.: [10] J. SAPRIEL AND B. DJAFARI ROUHAN1, peak at Qheii (i.e. q = 0). In principle, each of SURFACE SCIENCE REPORTS 10 (1989) 189 caused by the helical long-range magnetic these satellite peaks is the source of a dis- c [11] A. MAGERL AND H. ZABEL. PHYS. REV. LETT. 46 order in the superlattice which propagates persion curve, which, consistent with the (1981) 444; G. ORESSELHAUS ETAL., SOL. STATE COMM. 40 (1981) 229 '•'• [12] A. SCHREYER ETAL., IN PREPARATION Scientific highlights 27 Magnetic solitons in C11B2O4 M. BOHM (ILL AND LNS, ETHZ & PSI, VILL1GEN) B. OULADDIAF, J. KULDA (ILL) B. ROESSLI, J. SCHEFER (LNS, ETHZ & PSI, VILLIGEN) U. STAUB (Swiss LIGHT SOURCE, PSI, VILLIGEN) G. PETRAKOVSKII (INSTITUTE OF PHYSICS, KRASNOYARSK) Elastic and inelastic neutron scatte- ring measurements on single crystal have been performed to investigate the magnetic ground-state of copper metaborate CUB2O4. The compound exhibits a second order phase transition from a parama- gnetic to a commensurate antiferro- magnetic phase (CP) with k=0, followed by another second order transition to an incommensurate one (ICP) at J T*=10K. The association of the Dzyaloshinskii-Moriya(DM)-interac- tion, lattice symmetry and tetragonal Figure 1: Antiferromagnelic structure of CUB2O4 in the commensurate phase. anisotropy leads to the formation of a magnetic soliton lattice. romagnetically with a propagation vector k=0. (k=0) are those of the point group 42n. There The magnetic structure for this com are five irreducible representations. Four of mensuratc phase was determined with the them are one-dimensional (Fi, F2, F:s. P-i) help of group theory. The relevant irreducible and one labelled Ts is two-dimensional. The Magnetostatic measurements such as speci- representations of the magnetic structure reduction of the induction representation fic heat and susceptibility have revealed two phases transitions in GUB2O4 at TN=21I< and T*=10K. The magnetisation data show a very weak ferromagnetic component in the basal plane [1]. CUB2O4 crystallises in space group \42d (DM). The chemical contains 12 formula units. The Cu2+ ions occupy two non- equivalent sites namely Cu(A) at 4b site (local symmetry) and Gu(B) at 8d site (local symmetry 4) [2]. The neutron diffraction on single crystal measurements were carried out using the four-circle diffractometer D10 with X=2.36A. Below TN =21K, the intensity of some nuclear Bragg peaks and specially the forbidden reflections (110) and (0 0 2) increase with decreasing the temperature, which indicates that CUB2O4 orders antifer- Figure 2: Thermal dependence of the (0 0 2) reflection and the corresponding satellites. Scientific 28 highlights thermal dependence of the 0 0 2 the free energy, the spins acquire a phase reflection and the corresponding dependence cp(z) of the helix which is given satellites. The refinement of the neu- by a solution of the non-linear sine-Gordon tron data shows that the magnetic differential-equation [6]. The solution of this structure of CUB2O4 below T*=10K equation describes a situation where the consist of an helix with a constant phase of the spins along z will stay almost amplitude and phase shifts between constant over a given length L but changes different spins in the unit cell. At the abruptly over a short period § (Fig. 3). incommensurate-commensurate tran- Changing the temperature changes the ani- sition, the propagation vector k (T) sotropy term, which causes on the other L goes smoothly to zero. The transition hand an increase of L. Close to the phase is of second order. Therefore it is not transition T*, the magnetic structure can be Figure 3: Evolution of the spin arrangement in a soliton lattice. likely that competing interactions are represented as a periodic structure of at the origin of the helical structure. In domains which are separated by domain such case a first order phase transi- walls or equivalents solitons [5], In such a tion and a jump in the wave vector is case, higher order harmonics are expected to gives T4b = T3 + T4 +2 F5 and Tsd = T\ + produced [5]. be observed in neutron diffraction pattern at 2F2 +r5 +2r4+ 3Fs for Cu at site 4b and positions (n±1)k, (2n+l)k,..., where n gives 8d respectively. The magnetic modes of Ts On the basis of the Landau theory of phase the order of anisotropy [5]. Fig. 4 shows a and T4 of the 4b site correspond to a colli- transitions Dzyaloshinskii showed that scan along the crystallographic c*-axis near ferromagnetic and antiferromagnetic incommensurate structures can arise due to around the magnetic zone- ordering along the z-axis respectively. The centre (110) atT=9.5K. As modes associated with Ts describe a 90° 10000 the anisotropy in the basal configuration in the basal plane. Similar plane is of the order of 4 [1], magnetic modes for 8d site can be deduced. higher harmonics appear at A set of 25 pure magnetic reflections was 3 1000 - the positions 3k, 5k,.. The used in the refinement at T=l 2K. The magne- soliton lattice is further tic structure, which is obtained from these u confirmed by the appearan- calculations, can be described as a non-colli- I ce of considerable diffuse near arrangement of both the Cu(A) and scattering near the central Cu(B) spins along the diagonals of the tetra- peaks in agreement with the gonal plane (Fig. 1). The value of the magne- calculation of Izyumov [5]. tic moment is » ILIB for the Cu(A) with a The intensity of the diffuse small component along the c-axis H=0.25HB- scattering increases with The Cu(B) spins are confined within the ab- Figure 4: Neutron elastic scan along the [1 1 q] direction atT=9.4K sho- increasing temperature and plane and have a small magnetic moment wing the presence of higher order harmonics in addition to the principal magnetic satellites. exhibits a critical divergen- M^=0.25HB- The magnetic moments do not ce close to T*, in accordance with compensate and therefore a small sponta- the observation of susceptibility and specific neous ferromagnetic component arises in the the Dzyalloshinskii-Moriya (DM) interaction heat peaks at this temperature [1]. basal plane. in chemical structures belonging to the follo- wing eleven point groups: D2, D2d, C3h, D3, The magnetic structure belongs to two diffe- D311, S4, D4, D6, T, Td, 0 [6]. Symmetry ana- rent irreducible representations r& and T3. lysis of the chemical structure of CUB2O4 the spin Hamiltonian describing the system shows that the DM-interaction is allowed should be higher than order two [3]. On the between two Cu(A)- nearest-neighbour spins, other hand, the transition at TN being second with a DM-vector parallel to the tetragonal c- order, the transition temperature should be axes. The role of this interaction in forming considered as a bicritical point [4]. the magnetic ground state in copper metabo- rate is confirmed by the 90° configuration of REFERENCES o [i] 6. PETRAKOVSKII ET AL., J. MAG. MAG. MAT. 205 the magnetic moments in the commensurate Below T*. new magnetic satellites appear at (1999) 105-109 0 [2] M. MARTINEZ-RlPOLl ET AL., ACTA phase as the DM- interaction favours a non- CRYSTALLOGR. B 27 (1971) 677-681 O [3] E. F. BERTAUT, symmetrical points with respect to the collinear spin arrangement. According to ACTA. CRYST., A24 (1968) 217 O [4] B. BARBARA, commensurate reciprocal lattice points. The D. GLGNOUX AND C. VETTIER , LECTURE ON MODERN Dzyalloshinskii's theory the presence of an corresponding propagation vector is MAGNETISM, PUBLISHED BY SCIENCE PRESS BEIJING, additional crystal anisotropy distorts the SPRINGER-VERLAG, BERLIN HEIDELBERG 1988. P221 fc=(0,0,0.15) at T=1.8K. This indicates that regular helical arrangement of the spins © [5] YU.A. IZYUMOV, SOV. PHYS. Usp. 27( 11) (1984) 845 the magnetic structure becomes incommen- o [6] I.E. DZYALOSHINSKII, SOV. PHYS. JETP 20 (1965) along the helical axis. In order to minimise surate along the c-axis. Fig. 2 shows the 665-671 O [7] Yu.A. IZYUMOV, PHYSICA B 174(1991)9-17 Scientific highlights 29 Clusters or 'combed hair' in Fe-Zr metallic glasses? 0 A. R. WILDES (ILL) structure is free of crystal symmetry, invali- collinear components of the moments. One dating many approximations, and is subject to model proposes that the sample has within it 0 N. COWLAM (UNIVERSITY OF SHEFFIELD) magnetic frustration and short-range order. regions, or 'clusters'. All the moments in each As a result, many metallic glasses show cluster are ferromagnetically aligned, but at G S. AL-HENITI (ILL AND UNIVERSITY OF anomalous magnetic behaviour, frequently an angle to the collinear direction, as shown SHEFFIELD) interpreted as being due to 'non-collinear' fer- in Fig. l(c) [3,4]. In an alternative model, romagnetism. Such structures may take many shown in Fig l(d), the moments are ferromag- forms, and the exact nature of the ferromag- netically aligned, but the axis fluctuates netic structure of metallic glasses is still around the mean collinear direction. This Iron, with a small percentage of impu- being debated. Postulated examples are type of model is called a 'wandering axis' or rity, may be quench-cooled into an shown in Fig. 1. 'combed hair' model [1]. amorphous, or 'glassy', state. While these materials have been extensively researched - indeed, they are used for a wide variety of commercial purposes (b) today - their magnetic properties are still not fully understood. Many models have been proposed to explain the behaviour of these mate- rials, including magnetic cluster and wandering axis models. We have hhUl applied the techniques of polarised neutron scattering to measure the ferromagnetic structure of Fe-Zr, an c (d) example of a metallic glass, and are 4 ( ) _»*- able to determine which of the many proposed models is closest to the truth. V Ferromagnetic materials that readily align \ with an externally applied magnetic field - so- called 'soft' magnets - have a wide variety of applications, from magnetic recording heads to transformers for power grids. Iron-based metallic glasses are extremely soft and are currently commercially produced in large Figure 1: Previously proposed models for the ferromagnetic structure of metallic glasses. 1 (a) shows a collinear ferromagnet. 1 (b) shows a structure where the moments are randomly oriented on a cone. 1 (c) shows a 'canted quantities to satisfy the demand for these cluster' model. The moments in certain regions are ferromagnetically aligned, but the local axis is inclined with important materials. They are commonly respect to the mean ferromagnetic direction. l(d) shows a 'combed hair' model, where the moments are ferro- made by pouring a liquid melt of the consti- magnetically coupled, but the axis wanders about the mean direction. tuents onto a cooled, spinning wheel. The melt is quench-cooled and flies off the wheel Figure l(a) shows a conventional collinear All of these models have been used to describe in the form of a ribbon. The atoms in the rib- ferromagnet, the magnetic moments all colli- the metallic glass system Feioo-xZrx, bon have no long-range order and therefore near with a common direction. Of the propo- 7 s x s 12. It is not agreed which model best the atomic structure is amorphous. sed non-collinear models, the simplest has describes the magnetic structure. We have Besides being commercially important, these the moments randomly oriented on a cone, as therefore looked at FegnZrio and Fe92Zrs metallic glasses provide a test bed for the shown in Fig. l(b) [1, 2]. More complicated to attempt to resolve whether non-collinear understanding of magnetism. An amorphous models involve correlations between the non- ferromagnetism exists and, if so, which form Scientific 30 highlights *'••••••••• ' * ' "... Figure 2: The spin-flip cross-sections of FegoZrio and Feg2Zr8 at 135 K in 2T. This cross-section is directly pro- portional to the correlations between any non-collinear magnetism. The cross-section of FegoZrio is finite, but essentially featureless, suggesting it has a structure similar to 1 (b). Fe92Zr8 has peaks in the cross-section at the first two maxima of the atomic S(Q). Comparison of calculation with the data suggests that it has a 'combed hair' structure with some random disorder. does it take. We have used the technique of S(Q). Such features have never been seen neutron diffraction with polarisation analysis before and are representative of a correlated on the IN20 spectrometer. This is the only non-collinear ferromagnetic structure, such technique capable of unambiguously as those of Figs, l(c-d). detecting non-collinear magnetic structures. We have made detailed calculations of the The samples were subjected to a vertical model structures to see which will reproduce magnetic field of 20 kOe to saturate the the features in Fig 2(b). Two possibilities magnetic domains. In this configuration, any exist: the magnetic structure has either a spin-flip cross-section is directly proportional 'combed hair' component with some random to the structure factor of the non-collinear disorder; or it is a 'cluster canted' structure components of the moments. A collinear fer- with virtually every moment in a cluster and romagnet, therefore, will have no spin-flip very few collinear with the mean ferromagne- cross-section. Figure 2 shows the spin-flip tic direction. Looking carefully at the propo- cross-section for the two samples at 135 K, sed models and how they compare with our where they are ferromagnetic. conclusions, we believe that the former struc- The spin-flip cross-section of FegoZrio ture is the closest to the truth. Our results (Fig 2(a)) is finite and diffuse, decreasing can now be used to refine extremely detailed smoothly with increasing 0- Such a cross- theoretical calculations [6], providing vital section has been seen before [2,5] and is evidence to understand the complex magnetic representative of a non-collinear system with behaviour of metallic glasses. no correlations between the non-collinear components. This sample therefore has a REFERENCES magnetic structure similar to Fig. l(b). 0 [1] D. H. RYAN ET AL, PHYS. REV. B 35 (1987) 8630 0 [2] R A COWLEY ET AL., J. PHYS.: CONDENS. MATTER The spin-flip cross-section of Feg2Zr8 (Fig. 3 (1991) 9521 0 [3] S. N. KAUL, J. PHYS. F: MET. PHYS. 2(b)) has, in addition to a diffuse feature, dis- 18(1988) 2089 0 [4] L. F. KISS ET AL., J. MAG. MAG. MAT. tinct peaks at Q = 3.1 A"1 and <= 5.25 A'1, 135 (1994) 161 O [5] A R WILDES ET AL., J. PHYS.: matching the first two maxima in the atomic CONDENS. MATTER 10 (1998) 2617 o [6] R. LORENZ AND J. HAFNER, J. MAG. MAG. MAT. 139 (1995) 209 Scientific highlights 31 0_chemistry & structure Very short C-HO hydrogen bonds in phosphonium aryloxides • M.G. DAVIDSON (UNIVERSITY OF BATH) OH • C.K. BRODER, A.E. GOETA AND 1 + J. A. K HOWARD (UNIVERSITY OF DURHAM) ->- [Ph3PC(H)2R ] Ph • V. T. FORSYTH AND S.A. MASON (ILL) 1 2 ( \ _ ,H 1(R = R3= Me, R = Bu) C 1 2(R = R3= H, R2 = Ph) Pli 1 + 2[Ph3PC(H)2R ] 'Bu We have characterised a series of 3(R 1 = Me) three phosphonium aryloxides by single crystal neutron diffraction on D19. Definitive location of the hydro- Figure 1: The Synthesis of 1-3. gen atoms reveals uniquely short We have recently described the synthesis of a monomeric with interactions between cations C-HO hydrogen bonds. Using neu- range of organic phosphonium salts which and anion mediated via two C-H-0 interac- tron diffraction we have been able to contain extensive C-H X interactions tions, one from an alkyl C-H group and the address fundamental questions (where X = 0, N, P or n) in the solid state [4]. other from an aryl C-H group. The structural concerning the bonding in these inter- In order to gain insight into both the funda- parameters of the C-H-0 interactions in 1 actions. The three structures can be mental nature of C-H-0 hydrogen bonds them- have all been accurately determined and the rationalised in terms of a selves and their potential utility in crystal distances are at the lower limit of those found common supramolecular synthon engineering we have now undertaken single previously for C-HO hydrogen bonds. which may have significance for future By reducing the steric bulk of both the cation crystal engineering applications. and anion we were able to obtain the dimeric structure found in 2 [5] (Figs. 4 and 3(b)). Conventional 'strong' hydrogen bonds Here, each phosphonium cation partakes in (e.g., 0-H-O, N-H-0, etc.) are recognised to be the same alkyl/aryl chelation as was seen in 1 of fundamental importance in determining the but also hydrogen bonds to a second anion via supramolecular structure of organic solids an alkyl C-H donor. This generates the pucke- [1]. Among the many roles played by strong red eight-membered (-H-C-H-O)2 ring shown hydrogen bonds in structural chemistry and in Fig. 5. The C-H-0 hydrogen bonds within biology, one area of intensive research is their this ring are to our knowledge the shortest yet use to control supramolecular structure in determined by neutron diffraction, being up to crystal engineering [2]. In contrast, 'weak' Figure 2: Single crystal neutron structure of 1 5% shorter than any others found in the hydrogen bonds (e.g., C-H-0 and G-H-JI, etc.) (carbon, hydrogen, oxygen and phosphorus shown Cambridge Structural Database. have not been exploited in crystal engineering in grey, dark green, red and green, respectively). Accurate location and refinement of the hydro- to the same extent even though they have been crystal neutron diffraction studies of three gen atoms has allowed us to address funda- shown to be capable of exerting a profound phosphonium aryloxides (1-3, Fig. 1) using mental questions concerning the nature of the influence on supramolecular structure [2]. the D19 single-crystal diffractometer at the bonding in these very short interactions. Indeed, until relatively recently, even their ILL. Due to their sensitivity to air and moistu- Firstly, unlike strong hydrogen bonds which existence was still being seriously questioned re, all crystals were mounted under an inert can be partially covalent, even these very [3]. In this contribution we investigate the argon atmosphere which was maintained short C-H-0 interactions appear to be purely nature of very short C-H-0 hydrogen bonds using thin-walled quartz domes during data electrostatic in nature. There is no significant and consider whether they are useful as tools collection at 20 K. increase in the C-H bond lengths nor is there for crystal engineering. The structure of 1 [5] (Figs. 2 and 3(a)) is any distortion of the hydrogen atom Scientific 32 highlights hydrogen atoms we have shown that these a) b) c) uniquely short interactions are consistent with a 'normal' electrostatic bonding model for weak hydrogen bonds. All three supramolecu- •> lar structures possess the same supramolecu- lar synthon which may be applicable for use in crystal engineering although, consistent with the relative weakness of the interactions, pre- cise control of structure may be problematic. Figure 3: Cartoons of C-H-0 hydrogen bonded phosphonium salts (cations, anions and hydrogen bonds shown as red circles, blue squares and arrows, respectively). anisotropic displacement parameters along chelation of an 0 atom (Figs. 3(a) and (b)) we the C-H bond vectors (Fig. 5). Any covalent were interested in the possibility of utilising contribution to the bonding would be manifes- this motif as a so-called supramolecular ted in one or both of these effects. Secondly, synthon [2] in the design of more complex the neutron data provides unequivocal eviden- supramolecular structures. By analogy with ce that there is no disorder of the hydrogen compounds 1 and 2 we expected that the atom position between the two possible forms structure of a bisaryloxide salt would conform (C-H-O, ionic, or C-H-O, neutral) which might to one of two structures depicted in Figs. 3(c) be expected by analogy with very short O-H 0 and 3(d). In fact, the actual structure of 3 (Fig. 6) systems. is not so simple [6]. The synthon found in Because the structures of 1 and 2 contain the 1 and 2 is reproduced but only for one of the same structural feature of alkyl/aryl C-H cation-anion interactions. The second cation Figure 4: Single crystal neutron structure of 2 (car- Figure 6: Single crystal neutron structure of 3 bon, hydrogen, oxygen and phosphorus shown in (carbon, hydrogen, oxygen and phosphorus shown grey, dark green, red and green, respectively). in grey, dark green, red and green, respectively). REFERENCES • [I] G.A. JEFFREY, AN INTRODUCTION TO HYDROGEN interacts with two dianions in a previously BONDING, OXFORD UNIVERSITY PRESS INC, USA (1997) unobserved manner via two C-H-jt and one • [2] G.R. DESIRAJU, ANGEW. CHEM. INT. ED. ENGL., C-H-0 interactions thus generating the 34 (1995) 2311 • [3] G.R. DESIRAJU AND T. STEINER, THE WEAK HYDROGEN BOND: IN STRUCTURAL CHEMISTRY AND unpredlcted polymeric supramolecular struc- BIOLOGY, OXFORD UNIVERSITY PRESS, OXFORD (1999) ture shown in Figs. 6 and 3(e). • [4] M.G. DAVIDSON, K.B. DILLON, J.A.K. HOWARD, S. LAMB In conclusion, we have been able to fully cha- AND M.D. RODEN, J. ORGANOMET. CHEM., 550 (1998) 481; M.G. DAVIDSON AND S. LAMB, POLYHEDRON, 16 (1997) racterise a series of three C-H-0 and C-H-it 4393; M.G. DAVIDSON, CHEM. COMMUN. (1995) 919 hydrogen bonded phosphonium aryloxides by • [5] M.G. DAVIDSON, A.E. GOETA, J.A.K. HOWARD, S. LAMB, Figure 5: Detail of the (H-C-H 0)i ring in 2 (all single crystal neutron diffraction including the S.A. MASON, NEW J. CHEM 24, 477-479 (2000). • [6] C.K. anisotropic displacement parameters drawn at the BRODER, M.G. DAVIDSON, V.T. FORSYTH, J.A.K. HOWARD, 80% probability level; hydrogen bonds shown shortest interaction known to date. By accura- S. LAMB AND S.A. MASON, J. AM. CHEM. SOC. (SUBMITTED FOR in yellow). te location and anisotropic refinement of the PUBLICATION) Scientific highlights 33 Hydrogen fluoride: a model system for the understanding of the hydrogen bond • T. PFLEIDERER, I. WALDNER, H.BERTAGNOLLI (UNIVERSITY OF STUTTGART) • K. TODHEIDE (UNIVERSITY OF KARLSRUHE) • H, E. FISCHER (ILL & UNIVERS1TE PARIS-SUD, ORSAY) In view of its high importance as a model system for hydrogen bonds, we performed high pressure neutron diffraction experiments on the very corrosive and toxic hydrogen fluoride. Using neutrons from the hot source at D4B we were able to determine its microscopic structure and thereby the temperature dependence as well as density dependence of hydrogen bonds over a broad range of thermodynamic states [1]. During the past decades the importance of hydrogen bonds with respect to the struc- ture, intra- as well as intermolecular, has become more and more obvious. These bonds determine the structure of water, alcohols and also the conformation of macromole- cules, e.g. bio-organic molecules. Therefore, Figure 1: Snapshot of the ab initio molecular dynamic simulation of hydrogen fluoride at ambient conditions. investigations on hydrogen bonds are of great interest for many physical and chemical dis- [2] and neutron diffraction [3] studies. A synchronised pulse supersonic jet expan- ciplines. Hydrogen fluoride has become a great number of investigations clearly reveal sions by Suhm [5]. Indications of (HF)4 -7 popular model system for hydrogen bonds the presence of (HF)n chains in the liquid, too. clusters were found together with a great because of the strong influence of hydrogen This is also supported by the only elastic dif- amount of monomers and dimers. But the bonds on its properties - in fact hydrogen fraction study on liquid hydrogen fluoride. In reliability of the structural data for these fluoride forms the strongest hydrogen bonds 1985, Deraman et al. [4] determined the clusters, and for their temperature and den- known - and because of its low complexity in structure factor of liquid deuterated hydrogen sity dependencies, is debated. comparison to alternative systems, e. g. fluoride (deuterium fluoride) at ambient water with two hydrogen bonds per molecule. conditions using the diffractometer D4B at In addition, many theoretical investigations ILL. Undoubtedly, the liquid hydrogen fluoride and computer simulations have been carried Structural data of hydrogen fluoride are the consists of a network of winding chains. But a out to depict the properties of hydrogen fluo- ideal experimental reference for theoretical more detailed description was not accessible ride. In general, good agreement with ther- investigations and computer simulations, but by these experimental studies. modynamic results has been obtained by surprisingly little is known about the classical simulations using different potential structure of liquid and gaseous hydrogen The knowledge of the structure of gaseous models for the description of the hydrogen fluoride. In the solid state, hydrogen fluoride hydrogen fluoride is also restricted. Some bonds. But for all these models the structure forms (HF)n chains, as determined by x-ray progress was provided by IR spectroscopy in turns out to be rather different. The situation Scientific 34 highlights density /g cm Deuterium fluoride 1.0 hydrogen fluoride. Using the diffractometer \ 0.9 373 K D4B, we were able to determine the total atom pair correlation functions G(r) of deute- 403 K •*=*; 0.8 rated hydrogen fluoride (deuterium fluoride) 333 K_ 0.7 liquid^ at two liquid and four supercritical states in 0.6 the temperature range of 300 - 473 K and for pressures up to 320 bar [1]. The investigated 0.5 \ * (•) states are marked in Fig. 2. The sample \ 0.4 density is changing by a factor of 4 from critical point \ j liquid-like to gas-like densities. 0.3 / ' 0.2 In Fig. 3 we present the total atom pair cor- 0.1 gaseous ^^i^ ^ relation functions G(r) weighted with the •••----:•- ! • - • number density C and the factor 4KV-, i.e. 0 20 40 60 80 100 120 radial distribution functions. As the area pressure /bar under the first maximum is directly related to the average number of hydrogen bonds per deuterium fluoride (DF) molecule, these function give a quantitative impression of the Figure 2: Density as a function of pressure for deuterium fluoride: (I) performed, (T) planned experiments. changes in the hydrogen bonds. The left part of figure 3 illustrates the temperature de- was aggravated by the fact that the question are supported by the good agreement pendence in the range from 300 - 473 K and as to which model describes the structural between the total atom pair correlation func- the right part the density dependence in the properties of liquid hydrogen fluoride best, tion G(r) deduced from the simulation and supercritical region at 473 K. Both depen- relied on the comparison with the structural the one determined by Deraman et al. [4]. dencies are mainly described by a strong data of Deraman et al. [4] at only one ther- Further theoretical investigations and com- decrease of hydrogen bonds per molecule, modynamic state. A step forward was made puter simulations on the structure at other whereas only the variation of temperature by ab initio molecular dynamic simulations thermodynamic states are still missing affects the shape of the functions in the on liquid hydrogen fluoride at ambient condi- because of the lack of experimental structu- depicted distance range. tions [6], In Fig. 1 a snapshot of the liquid ral data. structure during a repetition of this simula- In order to extend the thermodynamic range tion is depicted. The winding chains are of available data, we have undertaken high Together with the planned experiments on clearly visible. The results of the simulation pressure neutron diffraction experiments on the saturated vapour at D4C, which are mar- ked in Fig. 2 as (T), these measurements represent the first comprehensive study of the structure of hydrogen fluoride from the temperature dependence density dependence liquid through the supercritical region to the vapour phase and thereby provide detailed information on how the structure changes from a chain-like arrangement in the liquid phase to small clusters in the vapour phase. It is hoped that these structural data will help to develop improved models for hydrogen bond potentials and to verify the results of theoretical investigations and computer simulations. REFERENCES • [I] T. PFLEIDERER, I. WALDNER, H. BERTAGNOLLI, K. TODHEIDE AND H. FISCHER, J. CHEM. PHYS. 113 (2000) 3690 • [2] M. ATOJ1 AND W. N. LlPSCOMB, ACTA CRYSTALLOGR. 7 (1954) 173 • [3] M. W. JOHNSON, E. SANDOR AND E. ARZI, ACTA CRYSTALLOGR., SECT. B: 160 200 120 160 200 STRUCT. CRYSTALLOGR. CRYST. CHEM. 3t (1975) 1998 distance r / pm • [4] M. DERAMAN, J. C. DORE, J. G. POWLES, J. H. HOLLOWAY, AND P. CH1EUX, MOL. PHYS. 55 (1985) 1351 • [5] M. A. SUHM, BER. BUNSENGES. PYS. CHEM. 99 (1995) 1159 • [6] U. R6THLINGSBERGER AND M. Figure 3: The weighted atom pair correlation functions of deuterium fluoride in the distances range PARRINELLO, J. CHEM. PHYS. 106 (1997) 4658 of the hydrogen bonds. Scientific highlights 35 structure Anomalously high temperature charge ordering in Bii/2Sri/2MnO3= the role of the Bi lone pair • J.L GARCIA-MUNOZ, C. FRONTERA, Tco. The bandwidth itself can be approxima- the CE magnetic order were clearly seen in A. LLOBET (ICMAB-CSIC, BELLATERRA) tely determined from the tilting of the MnO6 neutron diffraction data. In order to check for octahedra which is reflected in the average the presence of superlattice reflections we • M.A.G. ARANDA, (UNIVERSITY OF MALAGA) bonding angle Scientific 36 highlights cti©oiistrv 8L struct i Although Bi3* and La3* have similar radii in many oxides, replacement of La by Bi leads to 500 • an increase of the average Mn-O-Mn distor- Bi-Sr tion. The striking behavior of the Bii/2Sn/2 compound becomes clear comparing it to the 400 " Bii/2Cai/2 compound: As expected the average • Bi -Ca Mn-O-Mn distortion is more pronounced in the Ca-com- D 300 " pound, however, Tco (Bii^Srv2)» Tco (Bii/2Gai/2). • According to the classical bandwidth tuning [ Sm Pr 8 Q approach for the evolution of Tco as a func- Nd 200 " tion of the tilting of the octahedra in the D • • La Lni/2Mi/2MnO3 series, Tco for Bh/2Sn/2MnO3 is •x Pr Nd • expected at about 150 K, more than three i • . 100 i • i , i times lower than the measured value (Fig. 2). 150 155 160 165 170 175 It is clear that the extraordinarily high GO 9=(Mn-O-Mn) transition temperature of 475 K observed in Bi]/2Sri/2, which is unprecedented in Ln-Mn-0 Figure 2: Dependence of Tco for Lni/2Mi/2MnO3 (M = Ca, open manganites, cannot be explained by conside- squares, and Sr, solid squares) on the average Mn - 0 - Mn bending ring only the average buckling of the Mn-O-Mn angle. Crosses indicate the values of Tco for Bii/2Cai/2MnO3 and bonds. Obviously, an A-site mismatch effect Bii/2Sri/2MnO3 estimated from the bandwidth tuning mechanism. cannot explain such a high Tco as the A-size variance is larger for Bi]/2Sri/2 than 235 for Bii/2Cai/2 which should lead to even a lower Tco for Bii/2Sn/2. So why do the Sr and the Ga oxides behave so differently? Indeed, our data suggest that the lone pair of Bi3* plays a role in these apparently contra- dictory observations. In Fig. 3 the unit cell volumes of Lm/2Mi/2MnO3 (M = Ca, Sr) compounds at RT are plotted versus the ionic radii Scientific highlights 37 Crystal structure of new fast oxide-ion conductor La2Mo2(>9 leads to novel design concept • P. LACORRE, F. GOUTENO1RE (UNIVERSITY OF L.E MANS) • ILL-D1B A • 0. 1SNARD (CNRS GRENOBLE) • E. SUARD (ILL) •55 f ° n I l|aLa2IWo20s We have used neutron powder diffrac- tion to study the structural arrange- 30 34 ment of a recently discovered new 2 Theta (deg.) family of fast oxide-ion conductors. Figure 1: Neutron diffraction patterns of La2Mo2Ug collected below (533°C) and The parent compound of this family is above (617°C) the phase transition (DIB, X=1.28A). The stars point out the small lanthanum molybdate La2Mo2Og, superstructure peaks of a La2MO2O9. whose oxide-ion conduction above 600°C is comparable to or slightly higher than that of stabilised zirconia. ILL-D2B 670°C - pLa Mo 0 The structural arrangement of 2 2 9 La2Mo2Og is close to that of fSSnWO4, where divalent tin has an electronic lone pair. This structural similarity enabled us to explain the origin of anion conduction in La2Mo2O9, and to propose an original concept to design new families of fast oxide-ion conduc- Jt. tors, based on lone pair substitution. \Jb 11 III Niiiii minimi 11 HUM mini mini imiimmi nimi Fast oxide-ion conductors form a class of ••»• •-,'•'<• materials which are widely sought for and studied since they can be used in many diffe- 20 40 60 80 100 120 140 160 rent applications. They include Solid Oxide 21heta(deg.) Fuel Cells (SOFC) as low-polluting power generation systems, oxygen sensors for elec- Figure 2: D2B neutron diffraction pattern fit of the crystal structure of p La2M020g at 670°G. Note the strong decrease of diffracted intensities with the diffraction angle, tronic fuel injection systems in car motors, or due to large oxygen thermal factors (the modulated background is in this case due to oxygen pumping devices to separate nitrogen the diffusion from the glass container). from oxygen in air, for instance [1]. The most famous and widely used material of that kind efficiently used (usually above 700-800°G). tion toward the high T phase, the oxide-ion is probably stabilised zirconia, with an oxy- Recently, some of us have investigated a new conduction being around 6 102 S.cnr1 at gen-deficient fluorine-type structure. Other family of fast oxide-ion conductors based on 800°C, slightly higher than that, of usual oxide-ion conductors belong to different, scar- parent compound La2M0209 [2]. This com- stabilised zirconia. Electron and neutron ce, structural types, such as deficient- pound has been known for thirty years, but diffraction show superstructure reflections in perovskites, intergrowth perovskite/Bi2O2 neither its structure nor its conducting pro- the low temperature phase, most probably Aurivilius type phases, or pyrochlores. perties. We have shown that it undergoes a due to an oxygen/vacancy ordering (see Fig. 1), Research directions in the field have focussed reversible phase transition around 580°C as usually encountered in this type of mate- on improving the properties of already known from a slightly distorted low temperature rials. Neutron diffraction experiments were compounds or on finding new families with a phase to a cubic high temperature p phase. conducted on D2B and DIB at the ILL. The the main goal to lower the temperature range There is an abrupt jump in conductivity of neutron diffraction patterns show an impor- within which such materials can be almost two orders of magnitude at the transi- tant decrease of the diffracted intensities with Scientific 38 highlights necessary to fulfil the lanthanum valence. The lanthanum molybdate formula can thus be . ILL-D1B rewritten La2Mo208+i, with the extra oxygen atom allowed to migrate through the vacancy [2], Such an approach [7], called the LPS (for Lone Pair Substitution) concept can be applied to a variety of already known oxides of lone-pair elements (such as In+, Ge2t, Sb:i\ Se1*, P, among others). Substituting such elements by appropriate none-lone pair elements of similar size and identical or +1 extra charge is likely, if the structure is kept, to turn the insulating com- pounds into oxide-ion conductors. The large number of already known oxides of lone-pair ele- ments ensures a significant source of parent 0.9 1.4 1.9 2.4 2.9 3.4 3.9 4.4 4.9 5.4 compounds for future tests of the LPS concept. It is expected that such tests will be fruitful and will lead to new families of fast-oxide ion conductors, Figure 3: Detail of the neutron diffraction pattern of La2Mo2O9 at room temperature in order to improve the knowledge of such mate- versus Q=4JI sin 6/X, showing a large^diffusion peak around 2.31 A' (due to short range rials as well as their efficiency in applications order with pair distances around 2.5 A). Note that here, the container contribution to the background is negligible (vanadium container). such as solid oxide fuel cells. the diffraction angle (Fig. 2), an indication of the large thermal factors of oxygen atoms due to delocalisation. Another signature of oxygen diffusion can be found in the modulation of the neutron diffraction background, with a first maximum around 0 = 3.1 A1 (see Fig. 3). According to the Debye formula, which can be used to express the contribution of static structural disorder in diffuse elastic scatte- ring, the first marked maximum appears at Qmax=(2jtxl.23)/dm. where dm is a preferred atomic pair distance [3]. When applied to our data, such a formula gives a preferred pair distance around 2.5 A, which is characteris- 03 tic of minima] oxygen-oxygen distances. This is a good confirmation of some oxygen short pSnWO4 pLa2Mo2O9 range order in this compound. Figure 4: Cationic environments in (5SnWO4 and pLa2M020<). For comparison purpose, the environment of La is Neutron diffraction allowed us to determine limited to the nearest neighbours. Coloured circles indicate oxygen sites which are partially occupied. the crystal structure of pLa2M020g [4]. The cationic arrangement is close to that found in through its structural relationship with (3SnWO4. pSnW04 [5], as well as the location of two In this last compound, divalent tin is a cation over the three oxygen sites (see Fig. 4). All with an electronic lone pair. It is well known and the oxygen atoms have large anisotropic documented that electronic lone pairs, whatever REFERENCES thermal factors, and three forth of them are the electronic shell, have a spatial extension • [1] J.C. BOIVIN AND G. MAIRESSE, CHEM. MATER. 10 located on sites which are 50% occupied on close to that of an oxide ion, thus distorting the (1998) 2870 • [2] P. LACORRE, F. GOUTENOIRE, average. This leads to short distances bet- ligands environment of the cation [6]. Taking into 0. BOHNKE, R. RETOUX AND Y. LALIGANT, NATURE 404 (2000) 856 • [3] R. N. VANNIER, F. ABRAHAM, G. ween such sites, characteristic of short oxy- account the presence of a lone pair E in the for- NOW0GR0CK1 AND G. MAIRESSE, J. SOLID STATE CHEM. 142 gen-vacancy distances, and thus of the most mula of the tin tungstate leads to the formulation (1999) 294 • [4] F. GOUTENOIRE, 0. ISNARD, R. RETOUX, 2 probable conduction path in the structure, SnWCME, or Sn2W2OsE2. Substituting 2Sn * with AND P. LACORRE, CHEM. MATER. (2000), IN PRESS • [5] which appears to be tridimensional. The most lone pairs by 2hs^ without any lone pair creates W. JEITSCHKO AND A.W. SLEIGHT, ACTA CRYST. B 28 (1972) 3174 • [6] J. GALY, G. MEUNIER, S. ANDERSSON, AND interesting and suggestive way to understand two vacancies in place of the lone pairs, half of A. ASTROM, J. SOLID STATE CHEM. 13 (1975) 142 the origin of anion conduction in pLa2M020g is which being occupied by the extra oxygen atom • [7] P. LACORRE, SOLID STATE SCI. 2 (2000) 755 Scientific highlights 39 1 n f? rr FT FT Collective excitations in liquid metal alloys K-Cs: the role of the electron density O L.E. BOVE, C. PETRILLO, F. SACCHETT1 (ISTITUTO NAZIONALE PER LA FlSICA DELLA MATERIA - INFM) 0.2- 0 B. DORNER (ILL) We have investigated the ion dyna- mics of the liquid alloy K52CS48 by measuring the dynamic structure factor of the system. Well defined col- lective excitations were observed up to the maximum value of wave-vector transfer of the experiment, 0.7 A"1. We carried out a comparative analysis of the present data with those mea- sured in liquid Rubidium and Cesium and we found that a scaling relation holds for the dispersion curves mea- sured in all these systems. The derived scaling reveals the key role of the conduction electron density in the collective dynamics of alkali metals. Alkali metals can be regarded as prototype systems of simple liquid metals and they can be described as a two-component mixture of interacting electrons and ions. Such systems have focussed interest because they offer the opportunity to address the fundamental field of static and dynamic screening phenomena -4-2024-6 -2 2 in the interacting electron gas. Furthermore, the interplay between the electronic and the Ti Scientific 40 highlights dynamic properties of the alkali metals would density, and hence the dielectric function, of be quite attractive. The significance of such Cs is now different from that of the two other an approach can be established on experi- 6- AAAAA systems, the data were scaled according to A • A A A mental ground by studying the dynamics of A the following relationship liquid binary alloys of alkali metals with a V composition chosen to match the electron 4- 4 V Av densities appropriate to the pure metals. The *v 3 alloy would then resemble the pure metal in • V Using the Lindhard dielectric function, eva- terms of electronic density, although the mass 2- luated in the static limit, we obtained the sca- • K52CS48 differences, possibly present, could give rise • V Rb led dispersion relations that are plotted as a A to some dynamic features typical of an alloy. scaled Rb function of Q/Qo in Fig. 3 (Co is the position Following this idea, we investigated the ion 0- 1 1 • of the first maximum of the static structure dynamics of the binary liquid alloy K52CS48 0.0 0.4 0.8 1.2 factor). A remarkable agreement is observed which has a melting temperature of 236 K, an Q (A-1) for the three systems including Cs with a electron density matching that of pure Rb and quite different electron density. Note, that the Figure 2: Dispersion relation of the collective excita- an average atomic weight comparable with tions in K52CS48 in comparison with the results of the- result of a single scan on K32CS68, scaled as that of Rb within 1%. We carried out room neutron scattering experiment in pure Rb. The data of explained above, coincides with the other data. temperature measurements at the three-axis pure Rb after scaling by V( 1.45) are also shown. We believe that the present experiments give spectrometer IN8 operated at constant analy- a clear indication of the role of the electron ser energy (4.1 A"1 final neutron wave-vector). tial screened by the electron gas. In a pure gas in liquid alkali metals and an extended An evacuated flight path around the sample, metal, this model results in the well known investigation of the collective excitations in made it possible to collect high quality data long-wavelength limit for the energy of the these systems could help in deriving new down to 1° scattering angle. longitudinal collective excitations, as given by information on the interacting electron gas [7]. The experimental data were reduced follo- the Bohm-Staver formula [6], that is 2 2 wing the procedure described in [5], The COBS^O) = Q p/z(Q). Where Q P is the plasma resulting coherent dynamic structure factors frequency of the ionic gas and e(Q) is the S(C, a>) are shown in Fig. 1 as a function of (static) dielectric function of the homoge- 1 ,\J D K52CS48 the energy transfer and at the different wave- neous electron gas. We extended this approxi- A vectors. Side peaks originating from inelastic mation to the alloy system and found that a 0.8- Rb O 4 A (Brillouin) scattering can be observed up to relationship similar to the Bohm-Staver 0 Cs Qi the maximum momentum transfer, thus indi- result holds if the plasma frequency Qp is 0.6- oA^A cating the rather slow decay of these replaced by that of a gas of ions with the same [A B collective density fluctuations. Moreover, the average density as the alloy and an effective 0.4- presence of an associated dispersion is evi- mass given by 1/Meff = X1/M1 + X2/M2 (xi • * being the concentration of the i-th component dent from the low- Q spectra. We analysed the 0.2- D spectra of Fig. 1 by decomposing S(0, co) into in the alloy). This result can be understood on the sum of three lines, the central quasi-elas- intuitive grounds reminding that the frequen- 0.0- > 1 1 1 1 tic Rayleigh line and the two inelastic cy squared is proportional to the inverse of 0.0 0.2 0.4 Brillouin lines. The inelastic contribution was the mass. In the present sample the average Q/Qo represented by means of the Random Phase atomic mass is 84.1 a.u., that is nearly the same as Rb (85.5 a.u.), whereas Meff is 59.1 Approximation to the dynamic structure fac- Figure 3: Dispersion relation of the collective excita- tor of a simple liquid metal [6]. Further a.u., that is 1.45 times smaller than that of tions in K52CS48. K32CS68, pure Rb and pure Cs as sca- details can be found in [7]. The dispersion Rb. We found that multiplying the energy of Jed according to the relation described in the text and hw(Q) of the inelastic propagating modes, the collective excitations in pure Rb by versus QIQQ. 2 obtained from the fit to the experimental data, (1.45)" , the dispersion relation of Rb scales is shown in Fig. 2 in comparison with the data to that of the alloy. The scaled dispersion rela- measured in liquid Rb [1]. tion is also shown in Fig. 2 where the good agreement with the alloy curve is apparent. REFERENCES Even though K52CS48 and pure Rb have the O [1] J.R.D. COPLEY AND J.M. ROWE, PHYS. REV. LETT. 32 same electron density and a very similar ave- This result shows that the interaction gover- (1974) 49; J.R.D. COPLEY AND J.H. ROWE, PHYS. REV. A9 rage mass, the presence of two quite different ning the propagation of collective modes is (1974) 1656 O [2] T. BODENSTEINER, CHR. MORKEL, the (additive) Coulomb potential screened by W. GLASER AND B. DORNER, PHYS. REV. A45 (1992) 5709 ionic masses in the alloy can have an effect on o [3] P.H.K, DE JONG, P. VERKERK AND L.A. DE GRAAF, the dispersion relations. In order to under- the electron gas, with the repulsive core J. NON-CRYST. SOLIDS 156-158 o [4] H. SINN ET AL., stand the role of the masses, we referred to potential confined to a secondary role. The PHYS. REV. LETT. 78 (1997) 1715; T. SCOPIGNO ET AL., validity of this hypothesis can be further J. PHYS.: CONDENS. MATTER 12 (2000) 8009 the simplified model where the liquid metal or o [5] C. PETRILLO, F. SACCHETTI, B. DORNER AND the alloy is described by a gas of point-like checked by applying the scaling procedure to J.-B. SUCK, PHYS. REV. E62 (2000) 3611 ions interacting through the Coulomb poten- the energy of the collective excitations in O [6] N.H. MARCH, LIQUID METALS (CAMBRIDGE UNIVERSITY Rb[l], K52CS48 and Cs[2]. Since the electron PRESS, 1990) O [7] L.E. BOVE, F. SACCHETTI, C. PETRILW AND B. DORNER, PHYS. REV. LETT. 85 (2000) 5352 Scientific 41 highlights Diffusion in a simple liquid beyond Brownian motion O W.-C. PILGRIM (UNIVERSITY OF MARBURG) 380K 900K 1773K O C. MORKEL (TU MUNCHEN) 1.50 1.25 The single particle motion in liquid sodium was studied by quasi-elastic 1.00 neutron scattering over a wide density range along the liquid-vapour coexistence curve. The results are in 0.75 excellent agreement with mode cou- pling theory (MCT) and with computer simulations. They all reveal a distinct 0.50 universal deviation for the density dependence of the diffusion constant D from the expectations of simple 0.25 hard sphere theory. VA/. Since the earliest studies on diffusion by Figure 1: Reduced diffusion constant D/DE versus reduced volume V/Vo (DK. Enskog diffusion constant, R. Brown in 1827, single particle motion in Vo: volume at close packing). Circles: experimental results; solid line: computer simulations liquids has always been of particular interest (based on hard spheres). as the knowledge on the behaviour of single particles has always been viewed to provide a link between the macroscopic and microsco- system, mechanisms exist which suppress easily shown (see e.g. [6]) that in the limit pic properties of condensed matter. the forward motion of the particles at high q-» 0 the incoherent scattering law Ss (Q.co) Still today, most of the experimental results densities but support it at lower densities. is a Lorentzian with halfwidth, 001/2 = D • Q2. are interpreted in terms of rather During the last 20 years, progress has been indicating classical Brownian diffusion. phenomenological models. Among these, the made towards a more quantitative understan- However, with increasing q, the line shape Enskog theory [1] describes the dynamics of ding of the underlying processes [4,5] by changes and the classical description no a single particle in an assembly of hard introducing a coupling between diffusive longer holds. A useful quantity which spheres by means of two particle collisions. motion and collective density fluctuations. describes this departure is the reduced However, computer simulations [2], carried Our experimental results support this view halfwidth y (Q) = toTn I D.QCgiven by the ratio out already more than 30 years ago, showed and allow to draw a comprehensive picture of of the experimental halfwidth to the classical that the density dependence of the diffusion the single particle motion along the whole value. constant for a system of hard spheres density range of the liquid. Thus y (Q) * 1 indicates deviations from deviates considerably from the predictions of Since the behaviour of liquid alkali metals ordinary Brownian motion which must set in the simple Enskog picture. This is illustrated was often approximated by employing hard with increasing q where the quasielastically by the solid line in Fig. 1 [3] where the ratio sphere models the choice of liquid sodium as scattered intensity probes the particle motion D/DE of the actual diffusion constant to the the object of investigation appeared to be on a microscopic length scale. expected Enskog value is shown as a function near at hand. Additionally, the diffusion In our experiments, liquid sodium was studied of volume, normalized to closed packing constant can be determined in an inelastic over nearly the entire liquid range from mel- 5 (V/VO=T/2/D. a ). If the simple Enskog theo- scattering experiment from the incoherent ting [Tm=371 K, pm=0.98g/cnf) up to tempe- ry would apply, D/DE should be equal to one contribution to S(Q,co) and sodium has the ratures as high as 1773 K(p= 0.58 g/cnf). for all volumes (dashed line). However, Fig. 1 largest incoherent scattering cross section All experiments were performed at the best shows that even in a simple hard sphere among the alkalis (ai=1.62 barn). It can be cold neutron spectrometer for the study of Scientific 42 highlights quasielastic dynamics, the time-of-flight from one point to another. This mechanism instrument IN5. For each thermodynamic hinders the diffusion process compared to the state, the diffusion constant was determined simple Enskog dynamics. from the 5s (ftco) spectra at low momentum While single particle motion is hampered at transfer and an appropriate extrapolation to high density, it is supported at lower density Q = 0. The experimental results are given in (see Fig.1). In this case, the corresponding Fig. 1 as circles. They agree well with the MCT-ansatz combines the diffusive motion computer simulation. with a transverse density fluctuation [5]. In Fig. 2, the deviation from classical beha- Again, experimental and theoretical diffusion viour is shown in terms of y (Q) for melting constants are in excellent agreement point conditions (380 K, closed circles) and (Dut=2.98.10< cmVs; Dexp=2.95.10' ctrf/s). for an experiment carried out close to the The predicted Q-dependence of y(Q). a linear boiling point (900 K, open circles). deviation from 1, is also perfectly confirmed by experiment (see dashed line in Fig. 2). 1.1 The underlying physical picture can be clarified with the aid of computer simulation results [2], Coupling to a transverse mode generates a speci- fic velocity field around the propagating particle which enhances its for- ward motion, hence an increase in the diffusion 0.8 L- constant compared to 0.0 0.2 0.4 0.6 0.8 1.0 the Enskog value. q/A1 At even higher tempera- tures and corresponding lower density, the mode CX|) Figure 2: Reduced halfwidth y(Q) = col/2 / D.Q-'for sodium at 380 K (filled coupling effects die out circles) and at 900 K (open circles). Solid and dashed lines represent the corresponding mode-coupling results. and the particle dyna- mics becomes Enskog like. In Fig. 1 this is indicated by the asymp- The corresponding y (Q)s obtained from MCT totic decrease of D/DE to unity with increa- (solid and dashed line) are also shown. At sing volume. At very low density and high lower temperature, coupling between diffusi- temperature the particle dynamics is control- ve motion and longitudinal density fluctuation led by repulsive interactions resulting in a was considered [4] which leads to a quadra- simple two particle collision dynamics. tic deviation from unity for y (Q) (solid line in However, the experiments confirm qualitati- Fig. 2). This is in excellent agreement with vely the return to Enskog dynamics at large the experimental finding. In addition, the dif- volumes (see Fig. 1) although this process fusion constant obtained from the calcula- sets in later than predicted by computer tions is also in perfect conformity with expe- simulations. riment (DIIF=0.426. W crrf/s, Dexp=0.423.10' enf/s). The agreement between experiment and theo- ry allows to draw a clear physical picture The MCT results together with the experi- from the MCT ansatz. The Q -dependence of mental data presented here demonstrate that y(Q) arises from slowly decaying density cor- in a simple liquid the diffusion constant is relations on a next neighbouring length scale. determined by a coupling between single par- REFERENCES The particle under consideration oscillates in ticle motion and collective modes. The inter- o [1] S. CHAPMAN, T. COWLING, IN "THE MATHEMATICAL a stable cage built by its own coordination play of the underlying mechanisms which are THEORY OF NON-UNIFORM GASES", CAMBRIDGE UNIVERSITY sphere. This motion does not contribute to the PRESS, CAMBRIDGE 1952 ©[2] B. ALDER, T.E. WAINWRIGHT, governed by a density dependent competition PHYS. REV. A I (1970) 184© [3] J. ERPENBECK, W. WOOD, forward motion. Single particle diffusion is of mode coupling effects leads to a new PHYS. REV. A 43 (1991) 4254 O [4] G. WAHNSTROM, L only accomplished through a longitudinal understanding of single particle dynamics in SjOGREN, J. PHYS. C 15 (1982) 401 O [5] I.M. DE density mode which carries the particle col- SCHEPPER, M.H. ERNST, PHYSICA A 98 (1979) 189 0 [6] simple liquids on a purely microscopic basis. P. EGELSTAFF, "AN INTRODUCTION TO THE LIQUID STATE", lectively with its immediate neighbourhood CLARENDON PRESS, OXFORD 1994 Scientific highlights 43 Q materials science Origin of negative thermal expansion in cubic a high pressure neutron inelastic scattering study 9 R. MlTTAL, S. L. CHAPLOT, (BHABHA ATOMIC RESEARCH CENTER, INDIA) • H. SCHOBER (ILL) • T. A. MARY Cubic Calc. (INDIRA GANDHI CENTER FOR ATOMIC Orthorhombic Calc. RESEARCH, INDIA) o Cubic Expt. • Cubic Expt. Isotropic negative thermal expansion • Orthorhombic Expt. (NTE) has been reported in cubic ZrW208 over a wide range of tempe- rature T=0.3-1050 K. Our calculations of phonon spectra predicted a softe- ning at P=1.5 Kbar compared to that at ambient pressure, by about 100-200 jxeV for phonons of energy below 8 meV. This unusual phonon softening corresponding to negative Griineisen parameters is able to account for the anomalous thermal 1000 expansion. We report here our recent high pressure inelastic neutron scat- tering experiment performed at the Figure 1: The comparison between the experimental and calculated percentage relative thermal expansion in the IN6 time-of-flight spectrometer on two phases, (VT/V300-I) x 100%, VT and V300 being the cell volumes at temperature T and 300 K respectively. the cubic phase of ZrW2Os at Experimental data: cubic [1] (solid and open circles are neutron scattering and dilatometer data respectively), 160 K and pressures up to 1.7 Kbar. orlhorhombic [5] (solid squares are neutron scattering data). Excellent agreement is found with the phonon softening predicted by lattice around 2.1 Kbar [5]. An anomalous thermal In order to check our predictions of softening of dynamical calculations. expansion is also observed in the orthorhombic phonon mode at low energies we carried out phase [5]. The Gruneisen parameters of the inelastic neutron scattering experiment on a zone center Raman modes above 5 meV have polycrystalline sample of ZrW2Os in the ener- An isotropic negative thermal expansion been determined for the cubic and orthorhombic gy transfer range 0-10 meVat 160 K and 280 K, (NTE) has been reported in cubic ZrW20s phases of ZrW2O8 [6]. We have reported ear- and pressures at 0.3 Kbar, 1.0 Kbar and over a wide temperature range from 0.3 to lier lattice dynamical calculations for ZrW20s 1.7 Kbar. The used incident energy at IN6 was 1050 K [1]. The structure of ZrW20s consists [7] which reproduce the anomalous thermal 3.14 meV, therefore the measurements were of a framework of corner linked ZrO6 octahe- expansion in both the cubic and the orthorhom- performed in the energy gain mode. The dral and WO4 tetrahedral units [1]. The NTE bic phases (Fig. 1). The small sharp drop in sample was compressed using argon gas in a has been associated with the presence of volume at about 400 K is associated with a pressure cell available at ILL. The inelastic Zr-0-W transverse vibrational modes [1] and disordering phase transition. Our calculations neutron scattering signal from argon at each rigid unit modes [2]. The material finds appli- single out the unusually dominant contributions pressure and temperature, and the empty cell cations as a component in composites in from the transverse acoustic, and librational background were also measured for correcting order to reduce the overall thermal expan- and translational optic modes below 8 meV in the signal from the sample. sion near to zero. Specific heat [3] and neu- leading to a large NTE in the cubic and The compressed argon gas and the pressure tron inelastic scattering measurements orthorhombic phases. The calculations walls introduce background scattering. The cor- [4] have been reported for cubic ZrW20s. predicted that large softening of the phonon rected inelastic spectra for ZrW20s at ambient The neutron diffraction data show a cubic to spectrum at low energies would be responsible pressure and 1.7 Kbar are shown in Fig. 2. The orthorhombic phase transition at pressure for NTE in ZrW20s (Fig. 2). spectra at high pressures show an unusual large softening. In conformity with the predic- Scientific 44 highlights tions, the phonon modes of energy below about 5 meV soften by about 0.15 meV. For example, - Expt. P=0.0 at 3 meV, the softening of 0.15 meV corresponds 0.012- Expt. P=1.7Kbar to a Griineisen parameter of about -20. At ener- gies above 6 meV, the shift of the spectrum is much less than that at lower energies. o.ooa- This phonon softening on compression of the crystal is large and unusual since in normal materials the spectrum shifts only slightly to 0.004- higher energies. The experimental observa- tion of phonon softening is in good agreement 0.000' with our calculations (Fig. 2), which also 4 6 10 shows similar behavior. The experiment vali- E(meV) dates our lattice dynamical calculation which -Calc. P=0.0 showed that the NTE behaviour in ZrW2Os is 0.012- Calc. P=2.0 Kbar due to large negative Griineisen parameter of low energy phonon modes. 0.000 4 6 10 E(meV) Figure 2: The comparison between the calculated and experimental inelastic neutron scattering spectra for cubic ZrW2Oa. The experimental spectra at p = 0.3 and 1.0 Kbar fall in between those of p = 0 and 1.7 Kbar, and have not been shown here for clarity. REFERENCES • [1] T.A. MARY, J.S.O. EVANS, T. VOGT AND A.W. SLEIGHT SCIENCE, 272 (1996) 90 • [2] A.K.A. PYRDE, K.D. HAMMONDS, M.T. DOVE, V. HEINE, J.D. GALE AND MX. WARREN, J. PHYS. CONDENS. MATTER, 8 (1996) 10973 • [3] A.P. RAMIREZ AND C.R. KOWACH, PHYS. REV.LETT, 80 (1998) 4903 • [4] G. ERNST, C. BROHOLM, G.R. KOWACH AND A.P. RAMIREZ, NATURE 396 (1998) 147 • [5] J.D. JORSENSEN, Z. HU, S. TESLIC, D.N. ARGYRIOU, S. SHORT, J.S.O. EVANS, AND A.W. SLEIGHT, PHYS. REV B59, (1999) 21S • [6] T.R. RAVINDRAN, A.K. ARORA AND T.A. MARY, PHYS. REV. LETT.84 (2000) 3879 • [7] R. MITTAL AND S.L. CHAPLOT, PHYS. REV B 60 (1999) 7234 ; SOLID STATE COMMUN. 115 (2000) 319 Scientific highlights 45 The elementary jump in intermetallic alloys with CsCl-structure • J. COMBET (ILL & i.e. to observe direction, length and frequency UNIVERSITE STRASBOURG) of the diffusive jump in these alloys. Ni diffu- sion in NiGa and Co diffusion in CoGa are • B. FRICK (ILL) ideal for quasielastic neutron scattering stu- dies in terms of scattering cross sections. However, the rather slow diffusion in these alloys (10-':) to 10'2 m2/s) represents an The outstanding high temperature experimental challenge because it corresponds to properties of intermetallic alloys of a quasielastic broadening of at best 0.1 ueV. CsOI-structure are related to the self diffusion of atoms. A detailed understan- Nevertheless, experiments on NiGa and CoGa ding of the elementary jumps is single crystals could be successfully essential. Recent neutron backscatte- performed at the backscattering spectrome- ring experiments have shed light on ter 1N16. The problem of slow diffusion could the diffusion process through the Figure 1: Unit cell of the CsCl structure. The red be overcome quite successfully by crystal lattice and give support to a spheres represent A atoms (nickel or cobalt in the present case), the blue sphere a B atoms (gallium). i) carrying out the experiments close to the new model for diffusion. The two possible jumps for an A atom are jumps to melting point (-1450 K) and combining mea- NN sites (a) which belongs to the 'foreign' B sublat- tice or jumps to more distant but regular NNN sites surements of at least 20h per temperature Intermetallic alloys are increasingly used for (b). Please note that there has to be a vacancy on and sample orientation with an optimised high temperature applications, e. g. in the the target site in order to allow a jump. resolution function. A typical spectrum is aerospace industry, due to their corrosion shown in Fig. 2. stability and strength. Their extraordinary distance is considerably longer, the corres- ii) refining models simultaneously at all single properties at elevated temperatures are ponding jump being a jump to a next-nearest crystal orientations for one temperature and strongly connected to self diffusion, i.e. diffu- neighbour site (NNN). composition which reduces considerably the sion of the constituents themselves. The number of free parameters and compensates knowledge of the kinetics on the atomic scale It is obvious that the preference for one or the the weakness of the signal. in these systems permits the control of cru- other jump will be principally governed by the cial processes, e.g. different stages of preci- ordering energy of the alloy. While interme- In both alloys a diffusion process which pitation, and the optimisation of macroscopic tallic alloys with low ordering energies like involves two distinct time scales was detec- characteristics of these materials. However, FeAl [1] favour NN jumps, diffusion in com- ted: besides the principal quasielastic line, a even in intermetallics with structures as pounds with high ordering energies like NiAl second much broader line is present in the simple as the GsCl structure, the mechanism [2] is likely to happen via NNN jumps. signal (Fig. 2). These two time scales can be of self diffusion is not yet fully understood. However, an open question remains: What attributed to long residence times on the While it is generally believed that diffusion in happens in intermetallic alloys with interme- these alloys happens via jumps to vacant diate ordering energies like NIGa and CoGa? regular sublattice and short residence times sites in the lattice, the nature of the jumps is on the 'forbidden' antistructure sites [4]. Fig. 3 shows the measured line widths if the fast not yet understood. There exist two possibili- These alloys have been subject of extensive process is taken into account (red) or not ties which are illustrated in Fig. 1-. studies by tracer diffusion [3] which yield very precise values for the long range diffusi- (green). A comparison with calculations (line) a) An atom A may perform a short jump to a vittes at various compositions and tempera- shows clearly that diffusion happens via nearest neighbour site (NN). This jump is tures. However, the conclusions drawn from jumps to antistructure sites B and that NNN energetically expensive insofar as a nearest these data on the microscopic diffusion jumps (to A - sites directly) can be excluded. neighbour site in the CsCl structure is a so mechanism are rather indirect and some- called antistructure site, i.e. 'belonging' to the times contradictory. Further information on the residence time of other element B. the diffusing atoms on the antistructure sites In contrast, quasielastic neutron scattering is provided by the intensities and widths of b) The A-atom may prefer to stay on its own (QNS) allows to measure directly the the two components. These residence times sublattice (A-sites). In this case the jump elementary jump vector on single crystals, Scientific 46 highlights of jump frequencies from QNS and long range o diffusivities from tracer experiments, one can Ni51Ga49 1130 C conclude an unexpectedly weak correlation of CO subsequent jumps. This indicates that tradi- •t—» "c !l tional models based on cycles of strongly 1 correlated jumps cannot explain diffusion in 13 the investigated systems NiGa and CoGa. i III The neutron results on both alloys, NiGa [5] - l\\ and CoGa [6], point instead towards a diffu- In sion process which involves more than one o IV| defect and which leads to a practically unli- CO mited set of possible jump sequences. An ana- lytical treatment of this scenario is naturally ^^^^ not possible. Monte Carlo simulations, howe- ver, taking into account such high defect concentrations, can explain the observed data -4 -2 0 in alloys which are as different as FeAl or NiGa, solely by varying the interaction ener- E (peV) gies. Therefore we believe that the self diffu- sion at high temperatures in most intermetal- lic alloys with CsCl-structure can be explai- Figure 2: Quasielastic spectrum of NisiGa49 taken at 1400 K and 0 = 1.6 A' fitted with the sum (red) of a narrow (blue) and broad (black) Lorentzian, both folded with the resolution (dashed). The broadening is 0.3 ueV ned within a single and surprisingly simple for the narrow line and 3 ueV for the broad line (FWHM). model. > 0,3 0,0 1 Figure 3: The maximum of the line width at 0 = 1.8 A" near a reciprocal <100> lattice point which is obtained REFERENCES from a fit with a single Lorentzian (green circles) can not be explained by a model assuming jumps on the regu- • [1] R. WE1NKAMER, P. FRATZL, B. SEPIOL, G. VOGL, PHYS. lar sublattice. Jumps to antistructure sites, however, produce an additional broad line. If this line is subtracted REV. B 59 (1999) 8622 • [2] Y. MlSHIN AND D. FARKAS, 1 from the spectra the resulting narrow line (red) shows a minimum at 0 ~ 1-9 A' as expected for the NN jump SCR. MATER. 39 (1998) 625 • [3] FOR AN OVERVIEW SEE: model (solid line). Co64Ga36,1400 K. H. MEHRER, MATER. TRANS., JIM 37 (1996) 1259 • [4] O.G. RANDL, B. SEPIOL, G. VOGL, R. FELDWISCH, K. SCHROEDER, PHYS. REV. B 49 (1994) 8768 • [5] M. are very long, which indicates unusually high process by decreasing the degree of order in KA1SERMAYR, J. COMBET, H. IPSER, H. SCHICKETANZ, concentrations of thermal antistructure the system. With these neutron scattering B. SEPIOL, G. VOGL, PHYS. REV. B 61 (2000) 12038 atoms - up to several percent. These anti- • [6] M. KA1SERMAYR, J. COMBET, H. IPSER, data we are able to propose a new model for H. SCHICKETANZ, B. SEPIOL, G. VOGL, PHYS. REV. structure atoms in turn influence the diffusion self diffusion in these alloys. From a comparison B 63 (2001) 054303 Scientific highlights 47 The nature of ferroelectric transitions in Rochelle salt • J. KULDA (ILL) ferroelectric phase followed more than At the same time dielectric measurements, 30 years after and, even then, the limited far infrared and Raman spectroscopy [6,7] • J. HLINKA, S. KAMBA, J. PETZELT accuracy of the results did not permit more have proved the existence of a mode with a 1 (INSTITUTE OF PHYSICS CAS, PRAGUE) than to speculate on the possible structural frequency of about 22 cm (2.7 meV) at low changes. Only in the last decade high preci- temperatures, becoming soft and overdamped sion X-ray data [4,5] converged to a more or when approaching Ti. A relaxational mode in less consistent picture (Fig. 1) according to which the GHz range has been found to complement Inelastic neutron scattering study of a the soft mode - a feature typical for systems deuterated crystal of Rochelle salt • the structures of the high-temperature (HT) on the crossover between order-disorder and confirms the existence of a resonant and low-temperature (LT) paraelectric displacive nature of the phase transition. mode about 2.9 meV at 50 K which phases are different, mainly because of the softens with increasing temperature positions of three crystal water molecules and becomes overdamped on 08, O9 and O10 In an attempt to elucidate the relationship approaching the phase transition to between the structural and spectroscopic the ferroelectric phase. The analysis results we have used the IN3 and IN12 triple- of the inelastic neutron scattering • the ferroelectric phase is an ordered solid axis spectrometers at the ILL to perform neu- structure factors allows us to conclu- solution in which layers of the HT and LT tron inelastic scattering experiments on a de that this mode's eigenvector phases alternate along the b-axis. partially deuterated single crystal of RS [8]. involves mainly displacements of the same three molecules of crystal water that were found in recent structural c b studies to show the largest static dis- placements in the ferroelectric a distorsion. We claim that this reso- nant mode plays a central role in the ferroelectric phase transition. Na / Rochelle salt (RS), sel de Seignette(1675) or the sodium potasium tartrate (NaKC^-UOe- 4H2O), is the substance in which ferroelec- tricity was discovered for the first time [1]. Moreover, it remains the only system with a re-entrant paraelectric phase. The C22 #<2 09 (P2i1l, Z=4) ferroelectric phase is surroun- ded by the D23 (P2i2i2, Z=4) paraelectric phase, so that there are two Curie points, Tci = 255 K and Tc2 = 297 K. A phenomeno- logical explanation of this curiosity has been proposed in terms of two interpenetrating lat- tices [2,3]. However, an exact knowledge of the structure of the individual phases and a microscopic understanding of the transition mechanism remained an open problem up to recently. Figure 1: The crystal structure of Rochelle salt. The arrows indicate the main displacements of crystal water Although the structure of RS in its high molecules in the soft mode of 2.9 meV. On entering the ferroelectric phase at Tci the orange oxygen atoms start to occupy progressively the HT positions (displaced along their orange arrows), while the other half temperature paraelectric phase has been (the turquoise ones) rests in the LT positions. Their displacements (opposite to the turquoise arrows!) follow known since the 1940's, the structure of the only when all the "orange" positions are occupied. Scientific 48 highlights (a) Indeed a flat optical dispersion branch is rea- soft mode displacement pattern below 600 dily observed at 2.9 meV with an anomalous TCi and in the ferroelectric distortion. -•"• T = 50 K Q = (0 5 0) -o- - T=100K shift due to deuteration observed previously. IVIoreover the fact that Shiozaki's 500 -B- - T = 150 K -*- - T=175K As seen by comparing both parts of Fig. 2, its result [9] has been obtained with X- -•- - T=198K evolution with temperature closely follows rays practically excludes any contri- 400 the behaviour expected on the basis of pre- bution of hydrogen atoms and vious dielectric experiments [6]. The most confirms the displacive character of important consequence of this observation is their movement (no water molecule that the eigenvector of the phonon does not reorientation). change significantly with temperature. The information on the eigenvector of One of the essential advantages of coherent the resonant mode, which we have inelastic neutron scattering is that, in analo- obtained from the above analysis of gy to conventional crystallography, the inelas- the inelastic scattering intensities, tic structure factor provides information provides a link between the previous about the phonon displacement pattern spectroscopic and crystallographic (eigenvector). We have therefore collected results and leads to the following sce- intensity data for the soft mode in a number nario for the phase transition at Tci. of Brillouin zones in order to identify the The water molecules Oo and Oio, atoms that play a prominent role in the fer- involved together with Os in the roelectric phase transitions. This part of the 2.9 meV lattice mode, vibrate in a experiment was carried out on the 1N3 ther- double well potential. With increasing mal beam TAS, providing access to a more temperature this mode becomes pro- extended range of momentum transfers. gressively damped by their excursions into the shallow well. Tci corresponds Among the data from about 40 Brillouin zones to the temperature at which some of the strongest signal is observed in the [050] the new positions start to be occupied and [800] zones. The [050] Bragg reflection permanently. It appears that an alter- in the paralelectric phase is forbidden by the nating occupation of HT and LT posi- 2i screw axis. The 2.9 meV mode, breaking tions in layers perpendicular to the b this symmetry, softens and gets overdamped axis is energetically favourable. This at Tci, where the family of [0*0] (* odd) ordering breaks the screw axis sym- reflections, recognised as the fingerprint of metry along b and is responsible for the ferroelectric phase, appears. The Bragg the difference in the electric dipole Figure 2: The soft phonon of the ferroelectric transition: inelas- moments in the two halves of the unit tic scattering results from INI 2 (a) and their expected form (b) intensity of the [050], the strongest one of the calculated on the basis of the results of dielecti'ic measurements family, has been found by Shiozaki [9] to be cell arising due to different Os posi- exactly proportional to the square of the tions. As a result, the crystal exhibits electric polarisation P- the order parameter spontaneous electric polarisation in of the ferroelectric phase, an intriguing the temperature range up to Tc2, finding because the electric polarisation is where also the second half of the parallel to the a-axis. Our detailed analysis of water molecules (turquoise in Fig. 1) the contributions of the different atoms to the has passed into the high temperature crystallographic structure factor reveals that positions and the 2] screw axis symme- the [050] Bragg intensity is essentially due to try along b is re-established. the oxygens of the crystal water molecules 09 and O10- A similar analysis of the structu- re factor in the 800 Brillouin zone reveals an REFERENCES antiphase movement of the Os and Oio water • [I] J. VALASEK, PHYS. REV. 17 (1921) 475 • [2] T. MITSUI, PHYS. REV. Ill (1958) 1259 molecules (Fig.l). It is important to note that • [3] B. ZEKS, G.C. SHUKLA, R. BLINC, PHYS. REV. B3 all the Os molecules move in phase so that (1971) 2306 • [4] E. SUZUKI, Y. SHIOZAKI, PHYS. REV. their displacement can modify the electric B53 (1996) 5212 • [5] X. SOLANS, G. GONZALEZ-SlLGO, C. RUIZ-PEREZ, J. SOL. ST. CHEMISTRY 131 (1997) 350 polarisation of the corresponding layer. From • [6] A.A. VOLKOV, G.V. KOZLOV, E.B. KRYUKOVA, the fact that the soft mode eigenvector does J. PETZELT, SOV. PHYS. JETP. 63 (1986) 110 • [7] not vary with temperature, we may conclude S. KAMBA, G. SCHAACK, J. PETZELT, PHYS. REV. B51 (1 995) 14998 • [8] J. HLINKA, J. KULDA, S. KAMBA, J. PETZELT, that the three water molecules Os, O9 and PHYS. REV. B 63(2001) 052102 • [9] Y. SHIOZAKI, J. 010 play the most important role both in the PHYS. CHEM. SOL. 28 (1967) 2381 Scientific highlights 49 Thermostability of proteins: How thermal equilibrium fluctuations contribute to protein stability O J. FITTER (FZ JULICH) which have been related to increased thermo- stability (e.g., additional hydrogen bonds and O R. HERRMANN, N.A. DENCHER salt bridges, shorter loop regions, increased (TU DARMSTADT) internal hydrophobicity), dynamic features play also a crucial role for thermal adaptation. Therefore, we have studied thermal equili- brium fluctuations of a mesophilic and a ther- Thermozymes, which are stable and mostable a-amylase [2]. This enzyme cata- optimally active at elevated tempera- lyses the cleavage of glycosidic bonds in tures (60°C - 125°C), play a significant starch and related carbohydrates (see Fig. 1). role in many biotechnological applica- Since starch is best soluble at high tions like diagnostics, waste treat- temperatures, thermostable a-amylases play ment and paper industry as well as a key role in industrial hydrolysis. In a first starch-processing industry. A key to comparative analysis of conformational elucidate the molecular mechanisms fluctuations, we have applied neutron spectro- of this remarkable thermostability is scopy to highly concentrated protein solutions to understand the role of structural at room temperature. Incoherent neutron fluctuations for protein stability. The scattering from about 3,400 non-exchanged time-of-flight spectrometer IN6 offers hydrogen atoms (enzymes were dissolved in a powerful tool to monitor these local D2O), distributed quasi-homogenouses over conformational picosecond fluctua- the whole structure of each a-amylase mole- tions of protein structures in solution. cule, gives valuable information about structu- ral fluctuations with amplitudes in the range Biological polymers like proteins are characte- of 0.1 nm and on the picosecond time scale rised by complex and hierarchically organised (see Fig. 2). To our surprise the comparison of structures. One of the most remarkable pro- conformational fluctuations in both enzymes perties of these molecules is the fact that a revealed a higher conformational flexibility of Figurel: Crystal structure of the thermostable the thermostable enzyme as compared to the protein polypeptide chain (in aqueous solvent) a-amylase from Bac. licheniformis (PDB-entry:lBLI). generally folds only into a sole three dimen- mesophilic counterpart (see Fig. 3). Most of sional conformation. Only this unique fold Conformational flexibility can stabilise the previous studies analysing protein ther- ensures that the protein will operate. On the the folded state mostability indicate that thermoenzymes are other hand, this conformation exhibits a A promising approach to investigate the less flexible as compared to their mesophilic marginal stability, equivalent to the energy of underlying mechanisms of protein stability by analogues. It is assumed, that one of the main a few hydrogen bonds. The conformational concurrent functionality, is to compare pro- effects of these stabilisation features is to stability, defined as a free energy change teins which are adapted to very different tem- increase the rigidity of the protein structure. AGunf for the transition folded state *» unfol- peratures. Proteins adapted to extremely high The reduced conformational flexibility of loop ded state, is in the order of 30-60 kJ/mol at temperatures, known as thermozymes, are regions, of secondary structure elements, or of room temperature [1]. The biological signifi- produced by thermophilic or hyperthermophi- the core region is supposed to make the pro- cance of the relatively low AGunf is the requi- lic organisms (living at temperatures ranging tein structure less susceptible to thermal rement for a well balanced compromise bet- from 70 °C to far above 100 °G). They remain unfolding [3,4]. On the other hand, structural ween rigidity as a prerequisite of specificity folded and functional at elevated tempera- flexibility observed through equilibrium fluc- and flexibility ensuring a proper function of tures, and they are often less active at lower tuations contributes to the entropy change the protein. The details about the physical temperatures. In contrast to this, their meso- AS during unfolding and may have a profound nature of this balance and how proteins philic homologues (adapted to moderate tem- influence on the free energy change achieve this balance even under very different peratures) keep their specific conformation (AGunf = AH - TAS). Generally, the unfolded environmental conditions, like extreme and their functionality only up to approxima- state exhibits a higher conformational entropy temperatures, still are unclear. tely 60 °G. Besides some structural properties as compared to the folded state. Scientific 50 highlights ^*a»w' m 0- 0- -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 energy transfer [meV] energy transfer [meV] Figure 2: Steps in the data analysis of a-amylase in D2O solution, (a): Comparison of spectra as measured with enzyme solu- tion (triangles) and with pure buffer only (circles). At the given scattering angle the solvent scattering is about two third of the total scattering observed for enzyme solutions, (b): The difference spectrum (enzyme solution minus solvent as shown in (a)) at the given scattering angle 3.0- 1.0 I -«- thermostable (a) 2.5- 1 -»- mesophilic 0.8- jj — resolution 2.0- 1 0.6- 1.5- \ 0.4- 1.0- %— \ = 6 ps o thermostable T = 3 0.2- Tfe. ° c °- ps * mesophilic 0.5- 0.0 n n 0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.5 1.0 1.5 2.0 2.5 Q[A1] energy transfer [meV] Figure 3 (a): Incoherent neutron scattering spectra as measured for both a-amylases at 30 °C (solvent scattering subtracted). The figure represents an enlargement of the quasielastic scattering due to stochastic motions in a-amylase. In this window the quasielastic scattering is about 10% larger for the thermostable enzyme as compared to the mesophilic analogue, (b): The modelfit of elastic incoherent structure factors Ao as a function of momentum transfer 0 also indicates a higher conformatio- nal flexibility of the thermostable enzyme. Therefore, our results indicate that the higher alrealdy provided some hints to questions conformational flexibility of the folded state as about the role of protein dynamics in the observed for the thermostable enzyme is rela- functionality of these molecules. We have ted to the smaller entropy change AS, which shown that in addition neutron spectroscopy favours the folded state. Very recently, we offers a promising approach to answer another have obtained results measured which a-amy- important question: how do proteins achieve lase at higher temperatures and enzymes in their stability under very different or extreme the unfolded state which support the idea that environmental conditions? This question is of the mechanism of entropic stabilisation plays enormous importance for many biotechnological an important role for thermostability in the applications. case of a-amylase [5]. This work demonstrates how neutron spectro- REFERENCES scopy, in particular with enzymes in solution, G [1] P.L. PRIVALOV, ADV. PROTEIN CHEM. 33 (1979) 167 © [2] J. FITTER a J. HEBERLE, BIOPHYS. J. 79 (2000) 1629 can contribute significantly to solve major Q [3] P. ZAVODSZKr ET AL., PROC. NATL. ACAD. SCI. USA 95 problems in the field of molecular biophysics (1998) 7406 O [4] I. KORNDORFER ET AL., J. MOL. BIOL. and biochemistry. Neutron scattering has 246 (1995) 511 O [5] J. FITTER ET AL., PHYSICA B (2001), IN PRESS Scientific highlights 51 1—JL How ATP induces structural changes of an archaeal chaperonin I. GUTSCHE, M. ROSSLE, H. HEUMANN, or a misfolded protein ('substrate'), and a fol- for the thermosomes, they function without a W. BAUME1STER (MPI FUR BIOCHEM1E, ding-active state, encapsulating the substrate. GroES-like cofactor. How? The mechanism MART1NSRIED) The exact relationship between the nucleoti- remained enigmatic until two structures of the J. HOLZINGER (MPI FUR BlOCHEMIE, de-bound state and the conformation of the archaeal Thermoplasma acidophHum were MART1NSRIED & ILL) folding machine has only been established for published, one obtained by cryo-electron the bacterial protein GroEL [1]. However, it microscopy [3] and the other by X-ray crystal- R.P. MAY (ILL) has still to be elucidated for the eukaryotic lography [4]. The difference between the two chaperonins and their simpler archaeal ana- was amazing - the same protein appeared logues known as thermosomes [2]. cylindrical in vitrified ice, but spherical in the In the bacterium Escherichia coli, chapero- crystal (Fig. 1). A transformation of one struc- Chaperonins are double-ring protein nins consist of two major molecules, GroEL ture into the other requires a rotation of the folding machines fuelled by ATP bin- and GroES. GroEL can be described as a hol- outer regions of all subunits. Hence, it was ding and hydrolysis. Conformational low cylinder formed by two heptameric rings realised that the thermosome possessed a changes upon ATPase cycling of bac- ("double donut"). The co-factor GroES forms built-in lid instead of a detachable cofactor. terial chaperonins have been revealed a cap that binds on top of the ATP-loaded Therefore, by analogy with the GroEL/GroES mainly by means of cryo-electron GroEL to seal off the folding compartment. As system, the thermosome was proposed to be microscopy and X-ray crystallogra- phy. For archaeal chaperonins, how- ever, these methods have as yet failed to provide a correlation between the GroEL/S structural and the functional states. Small-angle neutron scattering expe- riments on D22 have given us an insight into the regulation of the + Thermoplasma acidophilum thermo- ATP some by adenine nucleotides. One of the central problems in structural bio- ATP logy is to understand how proteins, syntheti- hydrolysis sed as linear chains of amino acids, acquire their functional three-dimensional conforma- tion. Over the past decade, it was realised that Thermosome cellular protein folding critically depends on the assistance of a sophisticated machinery, for instance by so-called chaperonins. These essential multi-subunit protein assemblies exist in all three 'kingdoms' of life: bacteria, eucarya (higher organisms) and archaea (whose genome manifests both bacterial and eucaryotic features). Chaperonins are double-ring structures with a central cavity that provides a compartment for ATP-driven folding of proteins. Binding and hydrolysis of ATP are thought to switch the Figure 1: Conformational rearrangements of GroEL/ES and thermosome as proposed on the basis of cryo-elec- tron microscopy and X-ray crystallography. One ring of the chaperonins is shown in red, the second in yellow. chaperonins between an acceptor state, in The cofactor GroES is shown in green. Note, that both the left (cryo-electron microscopic) and the right (X-ray which the cavity is open to receive an unfolded crystal) structures of the thermosome were solved in the absence of nucleotides. Scientific 52 highlights open in the absence of nucleotides, whereas the crystallisation conditions were presumed to have trapped it in a folding-active state. In solution, this state was postulated to occur ADP ATP only upon ATP binding [5], Faced with conflic- ting data of cryo-electron microscopy and X- ray crystallography, we decided to verify these hypotheses with a third, independent tech- nique, and chose small-angle neutron scatte- Th-ADP Th-ATP ring. This method provides information about the overall structure and conformational changes of macromolecules in solution, where physiologically relevant conditions can be best I approached. The experimental scattering curves were interpreted in terms of the pair- distance distribution function, p(r), which gives the weighted frequency of occurrence of 200 distances between the elementary scatterers within the particle [6]. Th-ADP-R Th-ADP-P First of all, we recorded the scattering curve of the thermosome in the absence of any nucleotides, calculated the corresponding rate-limiting p(r), and compared it with the p(r) functions step _ I derived from the cryo-electron microscopic and the crystal data. In this way, we could Figure 2: Mode! for the ATPase cycling of the thermosome based on small-angle neutron scattering data. show that in solution, in the absence of Conformations of different nucleotide-bound species are schematised and colour-coded in accordance with the nucleotides, the conformation of the thermo- experimental p(r) functions, presented in the middle of the figure. some is well reflected by the open structure observed in vitrified ice. In order to mimic the transient hydrolytic • ATP, Adenosine triphosphate, a molecule Although the archaeon Thermoplasma acido- intermediates ADP-P and ADP-Pi, we incuba- serving as energy source in biological phttum thrives only at 55 to 59 °C in hot sul- ted the stable ADP-thermosome with AlFn, reactions. phur springs, our biochemical [7] and small- known as a phosphate analogue with higher • ADP, Adenosine diphosphate, degradation angle neutron scattering analyses demonstra- affinity and slower dissociation rate, and with product of ATP, can be reconverted into ATP. ted that the thermosome from this organism inorganic phosphate, respectively. We verified • ADP-P, transition state of hydrolysis of Adenosine could bind nucleotides (ADP, ATP or non- that the closure of the thermosome, observed triphosphate. hydrolysable ATP analogues) already at 10 °G. in ADP-Pi conditions, could not be triggered by • ADP-Pi, second transition state of hydrolysis of The amplitude of the resulting conformational phosphate ions alone, in the absence of ADP. Adenosine triphosphate, where the inorganic change was the same in the range of 10 to Remarkably however, we found out that such phosphate is not yet released from its binding site. 55°C, and only the rate of ATP hydrolysis a closure could be artificially induced in solu- decreased with temperature. tion by the buffer used for thermosome crys- Our results on the structural rearrangements tallisation, which contained high concentra- of the thermosome during the reaction cycle tions of sulphate ions. are summarised in Fig. 2. Surprisingly, ATP In conclusion, small-angle neutron scattering binding does not induce a closure of the cha- unravelled the reasons of the difference be- tween the thermosome structures in the crys- peronin as anticipated from the analogy to REFERENCES GroEIVGroES system. On the contrary, the tal and in vitrified ice, and gave us a first 9 [1] P.B. SIGLER, Z. XU, H.S. RYE, S.G. BURSTON, thermosome seems even to expand a bit fur- insight into conformational rearrangements of W.A. FENTON AND A.L. HORWICH, ANN. REV. BIOCHEM. 67 an archaeal chaperonin in solution upon ATP (1998) 581 O [2] [. GUTSCHE, L.-O. ESSEN AND W. ther. Then, it hydrolyses the nucleotide, pro- BAUMEISTER, J. MOL. BIOL. 293 (1999) 295 © [3] ceeding through a transition state of nearly binding and hydrolysis [8]. The next step M. N1TSCH, J. WALZ, D. TYPKE, M. KLUMPP, L.-O. ESSEN AND identical structure, and closes subsequently would now be to address the cycling of these W. BAUMEISTER, NATURE STRUCT. BIOL. 5 (1998) 855 folding machines in the presence of their cel- S [4] L. DITZEL, J. LOWE, D. STOCK, K. 0. STETTER, upon relaxation into an ADP-Pi species with a H. HUBER, R. HUBER AND S. STEINBACHER, CELL 93 (1998) low affinity for inorganic phosphate. This clo- lular substrates. 125 © [5] A. L. HORWICH AND H. R. SA1BIL, NATURE STRUCT. sure represents the rate-limiting step of the BIOL. 5 (1998) 333 © 16] 0. GLATTEH, J. APPL. cycle in the absence of denatured substrates. CRYSTALLOGR. 10 (1977) 415 6 [7] 1. GUTSCHE, Glossary: 0. MIHALACHE AND W. BAUMEISTER J. MOL. BIOL. 300 The release of phosphate triggers the re-ope- (2000) 187 • [8] I. GUTSCHE, J. HOLZINGER, M. ROSSLE, • Adenine nucleotides, a class of molecules ning of the chaperonin. H. HEUMANN, W. BAUMEISTER AND R. P. MAY CURR. BlOL. 10 containing an adenine base, like ATP or ADP. (2000) 405 Scientific highlights 53 Formation of DNA-cationic vesicle complexes investigated by time-resolved small-angle neutron scattering R. P. MAY (ILL) P. C. A. BARRELEIRO, T.D. LE, B. LlNDMAN (LUND UNIVERSITY) DNA and cationic vesicles form com- plexes that are promising candidates for gene therapy. The reaction be- tween DNA and cationic vesicles was followed in the time range of seconds using the high flux small-angle neu- tron diffractometer D22. Different contrast conditions were used to exa- mine the complexes as a function of time (s) charge ratio and membrane fluidity. Our experiments reveal that the for- mation of the complexes occurs in two steps, with characteristics times Figure 1: Time-resolved scattering intensity during the formation of DNA-cationic vesicle complexes in a of about 10 and 50 s. These findings DODAB:DOPE (1:1)/DNA mixture at a charge ratio of positive to negative charges of 2, 98% 2H20 and 25 °C. demonstrate the potential use of Interesting observations are that the neutron intensity decreases for low q, and the Bragg peak appearance at 0 - 0.09 A1 for times larger than 50 s. The initial counting time was 1 s (10 runs summed up), small-angle neutron scattering in the with an exponential increase of 1.053. elucidation of structure and associa- ted mechanism of fast reactions. drawback is their low transfection efficiency. different contrast conditions, we monitor the The key role of the cationic lipid is to provide time-dependent structural change as a func- an electrostatic attraction between the vesicle tion of the charge ratio and membrane fluidity. In the last few years gene therapy has attai- and the DNA molecule. The introduction of The present time-resolved experiments, which ned considerable interest. Gene therapy neutral or zwitterionic lipids in the cationic are lacking in the literature, examine the time- involves the introduction and expression of vesicle has been shown to increase the trans- dependent structural change that is useful for recombinant genes in cells. Its purpose is to fection efficiency of the complexes in different future developments and biological applica- cure the cause of a disease while the traditio- cell lines in vitro as well as in vivo. However, tions. Vesicles composed of a mixture of a nal therapies often are limited to treat the their role is not well understood. Despite the cationic (dimethyldioctadecylammonium bro- symptoms of a disease. In gene and oligonu- increasing number of publications describing mide) and a neutral lipid (1,2-dioleoyl-sn-gly- cleotide therapy, the weak link is not the gene, the structure and morphology little is known cero-3-phosphatidyethanolamine), are trans- but the vehicle used to deliver the gene, e. g., about the thermodynamics and kinetics of the formed into multilamellar structures upon the ability of the vehicles to transport the gene formation of DNA-cationic lipid vesicle com- DNA addition, as shown by small-angle X-ray inside the cells efficiently, safely and repea- plexes. The knowledge of the mechanism of [2] and neutron scattering experiments per- tedly. The most efficient gene transfer vectors the complex formation is an important step for formed in our group. currently in use are viruses that involve com- the improvement of gene therapy efficiency. The kinetic experiments were performed by plex technologies and suffer from severe limi- We have combined small-angle neutron scat- rapidly mixing equal volumes of solutions tations. Artificial self-assembling vectors, for- tering (SANS) with a "stopped-flow" appara- containing cationic vesicles (200 nm diame- med by the complexation of cationic lipids tus, in which two solutions are injected rapid- ter) and DNA (length 2000 base pairs) and and/or polymers with DNA, are currently ly into a measuring cell, to investigate the measuring the time evolution of the scattering being studied for transfection. These non-viral structural change occurring upon the interac- vector (O)-dependent scattered intensity, l(Q). carriers appear promising because of their tion of DNA with cationic vesicles. A key The process was observed for a total of about simplicity, apparent lack of immunogenicity advantage of the SANS technique is the ability 600 s using exponential exposure times, i.e., and inflammatory response, larger carrier to adjust the contrast in such a way that one the initial 2D spectrum was recorded for 1 s, capacity and high fusogenic potential. A major component can be monitored at a time. Under and subsequent spectra were recorded for a Scientific 54 highlights racteristic times (10 s and 50 s) suggest that 1 the reaction occurs in at least two steps. The first time constant is attributed to the cluste- ring of vesicles with DNA adsorbed on their surface; it has been independently confirmed 1 - by stopped-flow turbidity measurements per- formed in our laboratory [3]. The second time constant is made evident by the Bragg peak appearance. It is attributed to the reorganisa- tion of the complex, resulting in the formation of multilamellar structures (Fig. 3). The fact that we do not observe the slower rate constant in the turbidity measurements indi- cates that the rearrangement of the complex may occur without a significant size increase. A very fast time constant in the millisecond 0.1 time range, observed by stopped-flow, turbidi- 0.01 0.02 ty and fluorescence [3], was attributed to the J q(A" ) binding of DNA to cationic vesicles. The applicability of small-angle neutron scat- Figure 2: Scattering intensity from Fig. 1 for selected times at low 0- Times are, from top to bottom, (•) 0 s; (•) tering to probe the structural evolution during 2 s; ( •) 3.1 S; (•) 4.3 S; and (A) 6.8 s at 25 °C. the DNA-cationic vesicles interaction in dilute solutions is demonstrated in this report. The duration of 1,053 times the preceding one. For vesicle complex show the existence of clusters combination of small-angle neutron scatte- improved statistics, each reaction was repea- where the vesicles are deformed at the ring, turbidity and cryo-TEM measurements ted 10 times, and the respective runs of each contact regions but are not ruptured. The flat- has given us the relevant time-scales for the sample were summed up and analysed. An tening of the bilayers is caused by the addition different stages of the complex formation as well as the structures of the different interme- diates. Clusters of LUVcoated with DNA seconds Acknowledgements We are very grateful to Manfred Rossle for the use and help with the stopped-flow apparatus. This work was supported by grants from The Swedish Research Council for Engineering Sciences (TFR), Sweden and PRAXIS XXI, FCT, Portugal (scholarship BD/13788/97). Large Unilamellar Vesicles minutes Figure 3: Schematic representation of the formation of DNA-cationic vesicle complexes suggested from cryo- TEM, stopped-flow turbidity and small-angle neutron scattering experiments. example of scattering curves obtained from a of DNA. The "hump" at 0 = 0.025 A' is inter- time-resolved small-angle neutron scattering preted as an intermediate occurring while for- experiment is shown in Fig. 1. The SANS ming the multilamellar complexes. The Bragg intensity at low 0 decreases with time till peak at Q = 0.09 A1 corresponding to an inter- t ~ 10 s as is shown in Fig. 2 for times up to lamellar spacing of 68 A, arises from lipid REFERENCES 6.8 s. This decrease comes from the mixing of bilayer domains in the multilamellar com- 9 [1] K. ROEMER, T. FRIEDMANN, EUR. J. BlOCHEM. 208 the vesicles with DNA, since no change was plexes and only appears after -50 s. The (1992) 211 © [2] I. KOLTOVER, T. SALDITT, J.O. RADL.ER, C. R. SAFINYA, SCIENCE, 281 (1998) 78 © [3] P.C.A. observed when mixing vesicles with water. "hump" at 0 = 0.025 A' becomes less pro- BARRELEIRO, T. D. LE, B. LlNDMAN, BIOCHEMISTRY, Cryo-TEM measurements on the DNA-cationic nounced as the time progresses. The two cha SUBMITTED Scientific highlights 55 Slow relaxation process in DNA at different levels of hydration © A. P. SOKOLOV AND A. KlSLlUK (UNIVERSITY OF AKRON, USA) © H. GRIMM (IFF; FZ JULICH) ® A. J. DIANOUX (ILL) Sharp increase of atomic mean-squa- red displacements observed in hydra- ted bio-polymers at temperatures above T-200-230 K is usually ascribed to a dynamic transition. The nature of the dynamic transition is not yet clear but it is suggested that the dynamics transition is an important factor in enabling the protein's functions. Inelastic neutron scattering experi- ments have shown that the transition in DNA is related to the appearance of a slow relaxation process. Decreasing the hydration level suppresses the process and the dynamic transition. The decrease in water content is equi- valent in terms of dynamics to a decrease in temperature. These The structure of the most common form of the DNA molecule (B form). © CNRS. results support the idea that the dyna- mic transition is mediated by the -200 K for samples with high humidity level v<100 GHz. The latter exhibits a strong water of hydration, since bulk water and a gradual increase for the "dry" sample. dependence on both T and relative humidity. has a dynamic transition around the These results are similar to many previous Only the fast process is present at the lowest same temperature. observations obtained on bacteriorhodopsin1, temperature and the quasielastic contribu- myoglobin2 and other bio-polymers. They tion to the spectrum in dry sample is stron- reflect a general trend in the dependence of ger than in the samples with higher humidity Oriented Li-DNA-fibers, hydrated with D2O the dynamics of bio-polymers on (Fig. 1). This suggests that a decrease in water content to different levels (81%, 75% and 11%), temperature and humidity. increases flexibility of the DNA molecule at lower tem- have been studied on the time-of-flight spec- In order to emphasize details in the quasie- peratures. trometer INS at the ILL. Incoming neutrons, lastic spectra, imaginary part of the suscep- The situation changes at higher temperatures. Distinct of two different wavelengths (5 A and 8 A), tibility is shown in Fig. 1. Two processes can differences develop at T-250 K, le. just above the dyna- were used in order to cover a sufficiently be readily identified: the "fast" one that domi- mic transition: a decrease in the humidity level broad energy range. The quasielastic intensi- nates the spectra at v>100 GHz and the decreases the flexibility of DNA and is similar to a ty demonstrates a strong increase at T above "slow" one that dominates the response at decrease in temperature. Scientific 56 highlights 103- 1000 Figure 1: Susceptibility spectra of DNA samples at 11% r.h. (symbols), 75% r.h. (solid lines) and 81% r.h. (dashed line). The most important observation in Fig. 1 is related to the dynamic transition in bulk the absence of the slow process in the spec- water observed at the same temperature tra of the "dry" sample. Even at the highest range. The dynamic transition in water of temperatures, T-320 K, only the tail of the hydration makes the DNA molecule more fast process is evident at v<100 GHz. The flexible and enable slow conformational fast process gives aharmonic contribution to variations. That can be important for opening The observed dependence of the dynamics on REFERENCES O [I] M. FERRAND ET AL., PROC. NATL.AC.SCI. the level of hydration supports the idea of a USA 90 ( 1 993) 9668-9672 O [2] W. DOSTER, strong influence of the solvent in bio-polymer S. CUSACK, W. PETRY, NATURE 337 (1 989) 754- dynamics. In particular, it supports our 756 o [3] A.P. SOKOLOV, H. GRIMM, R. KAHN, 3 J.CHEM.PHYS. 110 (1999) 7053-7057 0 [4] previous speculations that the dynamic A.P. SOKOLOV ET AL., J.BIOL.PHYSICS 26 (2000) transition observed around 200-230 K can be S1-S5 Scientific highlights 57 Q soft matter Structure of clay-polymer-salt-water complexes: study by H/D isotope substitutions 9 J. SWENSON (CHALMERS UNIVERSITY OF TECHNOLOGY, GOTEBORG) 0 M.V. SMALLEY AND H.L.M. HATHARASINGHE (a) (UNIVERSITY COLLEGE LONDON) 0.1 M salt # G. FRAGNETO (ILL) H-PEO, H-salt In this report we show how the inter- D-PEO, H-salt I mediate-range structure of a complex four-component clay-polymer-salt- D-PEO, D-salt : water system, consisting of n-buty- lammonium vermiculite, poly-(ethylene oxide) (PEO), n-butylammonium chlo- •\ H-PEO, D-salt : ride and heavy water, can be determi- i '-,-•)'••-,....i ,.. ned by H/D isotope substitution of the 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 1 butylammonium chains and PEO. By Momentum Transfer Qz (A ) comparing the diffraction results of the four isotopically different samples we can show that a substantial part of the polymer chains is directly bound to the clay surfaces and that the remaining (at least 50%) polymer seg- ments have a Gaussian-like distribu- tion in the middle between the clay layers. A major part of the butylam- monium ions are located in a 4-5 A thick layer at a distance 12-16 A from the centre of the clay platelets. The clay surfaces are therefore covered by, first, one layer of adsorbed ethylene oxide segments and water molecules, second, another molecular layer of water, and, third, the layer of butylammonium ions (1). H-PEO, D-salt _,-•- Clay systems are not only a central problem of i" l"y--r--t---r' t , i geology and soil science, they are also 0.8 1.2 1.6 2 remarkably useful as model systems in colloid, 1 Momentum Transfer Qz (A' ) polymer and biological science. Vermiculite clays consist of millions of negatively charged parallel silicate platelets that are held Figure 1: Difference structure factors KS(Qz) (on different scales in (a) and (b)) obtained from neutron together by positively charged counterions diffraction experiments on butylammonium vermiculite gels in 0.1 M salt concentration and a volume fraction of located in the interlayer region. 4% PEO. The upper curves are shifted vertically for clarity. Scientific 58 highlights The usefulness of the clays as colloidal model materials lies in their ability to swell (up to 50 1 1 1 l 1 1 1 1 1 1 1 1 times) in the direction perpendicular to the silicate platelets by taking up water, salts, polymers and other materials in the interlayer region. The clay gel formed is an ideal one- dimensional colloidal system suitable for stu- dies of electrostatic interactions and inter- layer structure in colloidal suspensions and polyelectrolyte solutions. In the case of the present four-component system with added high molecular weight PEO the situation is rather complicated since one has to find the location of both the butyiam- (0 monium chains and the polymer segments. 0 10 20 30 40 50 60 However, due to the one-dimensional nature of rz(A) the complex colloidal system and the use of four isotopically different samples (H-PEO Figure 2: Scattering length density of butylammonium chains in the interlayer region as a function of r . The and H-salt, D-PEO and H-salt, D-PEO and z centre of the clay platelets are located at rz=0 and rz=63 A. The result was obtained from a modelling (see the D-salt, H-PEO and D-salt) of each composition text) of the diffraction data shown in Fig. 1. we have for the first time been able to deter- mine the intermediate-range structure of this crystallographic description is a method to technologically important system (e.g. for sta- produce a single particle distribution function, bilisation of drilling fluids and preparation of p(z), rather than a spatially averaged pair cor- relation function, G(rz). papers). Fig. 1 shows A.S(QZ) of these four isotope compositions of PEO and butyiammo- Using this method, by means of an inverse nium ions in an 0.1 M butyiammonium vermi- Monte Carlo based program which simulates the scattering density along the z-axis simul- culite gel, where S(QZ) of pure D2O has been subtracted from the measured S(Qz). The neu- taneously for several isotopically different tron diffraction experiments were performed samples, and the more traditional analysis of on the D16 diffractometer. The volume pair correlation functions, we have come to fraction of PEO was 4% and the polymer the following conclusions: molecular weight about 75000. In Fig. 1 (a) we 1. Polymer segments displace water mole- show the low Q-range of AS(QZ), containing cules immediately adjacent to the clay sur- four orders of Bragg peaks arising from inter- faces, bonding directly to them by physical platelet correlations, whereas Fig. 1 (b) shows adsorption. the diffuse scattering caused mainly by clay 2. The remaining polymer segments show a platelet-solution correlations in the higher Gaussian-like distribution with its centre in Q-range.The distribution of polymer segments the middle of the interlayer region. were determined from analysis of the Fourier 3. There are no indications that the presence transfoms of the AS(QZ) shown in Fig. 1, whe- of PEO affects the distribution of butyiammo- reas the location of the butyiammonium nium ions. This is evident in Fig. 2, which chains has been modelled by analysing the dif- shows that the majority of the butyiammonium fuse scattering and the intensity variation (as a chains are, as in the case of the corresponding function of ordering) of the Bragg peaks. The three-component system without added PEO, neutron structure factor, S(QZ), is related to the located in a 4-5 A thick layer situated just out- neutron scattering length density profile per- side the approximately 6 A thick layer of pendicular to the clay platelets (along the z-axis), adsorbed polymer segments and water molecules. p(z), via the crystallographic structure factor, F(QZ) S(Qz)=M(Oz)F(Qz)F*(Qz) F{Qz) = fp(z)[cos(Qz) + ism(Qz)]dz 0 where c is the clay layer spacing and M(Qz) is REFERENCES a Lorentzian that takes into account the • [1] J. SWENSON, M.V. SMALLEY, H.LM. HATHARASINGHE AND G. FRAGNETO, LANGMUIR, IN mosaic spread of the sample. This essentially PRESS Scientific highlights 59 Polymeric efficiency boosters in microemulsions: a SANS investigation of the polymer's role 9 H. ENDO, J. ALLGAIER, M. MONKENBUSCH, D. RlCHTER (IFF, FZ JiJLICH) 9 B. JAKOBS, T. SOTTMANN, R. STREY (UNIV. KOLN) ir S The boosting effect of amphiphilic block copolymers in the ternary microemulsion (water, oil and non- ionic surfactant) is investigated. Small amounts of PEP-PEO block copoly- mer lead to a dramatic expansion of 1 phase region where water and oil can .V be solubilised by the mediation of sur- factant molecules. Small-angle neu- tron scattering experiments have made use of a sophisticated 2-dimen- sional contrast variation technique. The results demonstrate that the Figure 1: Scheme of the partial scattering functions S;,/. The enough sets of SANS measurements with same com- polymer is distributed uniformly on ponents but different contrasts allow to decompose to each partial scattering function. the surfactant membrane, owing to the variation of the membrane curva- Here we report small angle neutron scattering i.e., deuterated and protonated contrasts have ture elasticity. (SANS) experiments with a high-precision to be mixed such that pw = po = pr * pp. contrast variation technique, which have been However, since the scattering from such poly- performed on samples in the bicontinuous mer is about 5 orders of magnitude lower than Microemulsions are thermoclynamically stable microemulsions consisting of equal amounts of the bulk contrast and 2 orders of magnitude and macroscopically homogeneous mixtures oil and water. With neutrons as probe, the lower than the film contrast scattering (only of oil and water, where the miscibility contrast variation techniques based on H/D- surfactant protonated) the required perfect is mediated by surfactant molecules. replacement can be used to modify the visibi- match of the scattering length densities of Microscopically the surfactants form an lity of the different components water, oil, water, oil, and surfactant is practically unfea- extended interfacial film separating water and surfactant film and polymer. sible. On the other hand, a careful analysis of oil on a local scale. Recently we have discove- In bulk contrast, with deuterated water and data obtained close to matching point yields red an enormous efficiency increase of the protonated oil, the scattering is dominated by (among others) the required polymer emulsification capacity of the non-ionic sur- the three-dimensional structure of the water scattering functions. The scattering intensity factant n-decyl-tetraoxyethylene (C10E4) by or oil network; contributions from any surfac- 1(0) is given by adding amphiphilic block copolymers of the tant film or polymer are marginal. In principle, the location and structure of the polymer in polyethylenepropylene/ polyethylenoxide 2 } (PEP-PEO) [1]. Theoretically, a universal mecha- the bicontinuous microemulsion can be exami- where S# is the partial scattering function of nism is proposed for the enhancement of the ned in a system where all components except components / and / which originates from the swelling behavior, which is due to the variation of the polymer have exactly the same scattering interference between /and /. In Eq. (1), water the membrane curvature elasticity by polymer densities pi [with i = w, 0, f, p for water, oil, is arbitrarily chosen as the component of the mushrooms anchored at the interface [2]. surfactant film and polymer, respectively]; system that is considered as background, and Scientific 60 highlights in this case the relevant partial scattering 0.15 Figure 2: Location of the samples used for the contrast matching experiment functions are S ,o. S[,r, S , , S ,i, Sr. . and S , . • o p p 0 p p 0 in the scattering length density plane Figure 1 presents the partial scattering func- 0.10 - (po-pw), (pr-pw). tions schematically. • The experiments have been performed at the g 0.05 high intensity SANS diffractometer D22. Contrast variation around the two-dimensional matching point was achieved by stepwise '•' 3 o.oo » increment, adding tiny amounts of protonated + decane and protonated surfactant to microe- § -0.05 - mulsions containing fully deutrated water, oil • and surfactant and protonated polymer. This -0.10 measure allowed to vary p0 and p/- in small • but defined steps and produced a set of 15 • • -0.15 1 1 1 1 samples [Fig. 2]. Their scattering data were -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 then used to extract the six partial scattering 10 2 (p -yp ) «10 l [cm ] functions by inserting the measured intensi- ^Fitm Water ' ties and the contrasts into Eq. (1) and solving Film the overdetermined set of equations by singular value decomposition separately for each 0 value. Figure 3: Experimental partial scatte- Figure 3a shows the result for the polymer, a '. ring functions, (a) Sp,p (o), in compari- Sp.p, and film, Sr,f, scattering functions. The son with Sr,f[o) and calculated polymer comparison of the scattering curves indicates coil scattering (solid line); see text, (b) that the polymer scattering Sp.p follows the Sr.p (D), which contains the interference 2 between surfactant film and extended S//.io- : form of Sfj for small Q (< 0.02 A'). This s polymer layer, and fit to the calculated demonstrates clearly that the polymer "deco- : ii^ intensity (solid line) for ideal chains rates" the surfactant film. Because of the dilu- anchored to a planar membrane; see text. Note the linear scale of the oixlina- tion of scattering density, however, the corres- teofFig. 3(b). ponding intensity is about 2 orders of magni- I tude lower than the pure film-contrast scatte- I ring. For Q > 0.03 A', on the other hand, Sp,p 10"' D D ..XN reflects the scattering from the polymer coils. o \ D . The solid line in Fig. 2a shows a fit to the Zimm D • b formula [3], which contains the Debye func- a', tion and includes the effect of second virial coefficient to account the interactions in the \ interface. Any polymer-polymer aggregation 0 - (micellisation) would immediately lead to i order of magnitude higher values for Sp,p and 0.01 0.1 can be safely excluded. The film- polymer QtA1] scattering Sr.p, shown in Fig. 3b exhibits a minimum at Q => 0.03 A', which mainly cor- responds to maxima of the polymer density on the film surface normal. The solid line is a fit of the Fourier transform of the monomer den- sity including a Q2 factor to account for the random orientation of the membrane. The fit- oil phase and by PEO chain in water are not ted average end-to-end radius amounts to noticeably influenced by the presence of 155 A, which compares quite well with the neighboring membranes. end-to-end distance RPEO = 140 A as determi- ned from homopolymer solutions; it is much REFERENCES smaller than the average bulk-domain size of • [l] B. JAKOBS ET AL., LANGMUIR 15, (1999) 550 A. The experimental results prove that the Acknowledgements: 6707 • [2] H. ENDO ET AL., PHYS. REV. LETT. 85, polymer is tethered to the interface, and that (2000) 102 • [3] J. S. HlGGINS AND H. C. We thank Isabelle Grillo (ILL) for her assistance BENOIT, POLYMER AND NEUTRON SCATTERING "mushrooms" formed by the PEP chain in the in performing the measurements at D22. (CLARENDON PRESS, OXFORD, 1994) Scientific highlights 61 latter Is the dynamics of counter-ions a probe of the microscopic mechanisms of the electronic transport in conducting polyanilines ? • D. DJURADO, M. BEE (UN1VERSITE JOSEPH FOURIER, GRENOBLE) • J. COMBET (ILL & UN1VERSITE STRASBOURG) • B. DUFOUR, P. RANNOU, A. PRON, J.P. TRAVERS (CEA-GRENOBLE) (a) Polyaniline, protonated by an organic sulfonic acid, is a new generation of electronic conducting polymers. Stable, conducting, flexible films (conductivity = 150 to 500 S/cm at room temperature) can be cast from a solution. In general, these films exhi- bit a broad transition from a metal-like regime to an insulating one when the temperature is lowered. The origin of this transition is still debated [1]. Lattice disorder is generally thought to be responsible for this behaviour but the respective roles played by polymer chains and counter-ions in this transition remain unclear. Incoherent neutron scattering tech- niques have been used in order to study the lattice dynamics without being obscured by the presence of mobile electronic charges. The results suggest that the dynamical disorder of the counter-ions and the metal-like conduction regime are related. In the solid state, the system under study consists of polymeric chains separated by acid counter-ion layers (see Fig. la). In such sys- tems the possible sources of dynamical mole- cular disorder are numerous and characteristic times of lattice dynamics may span a wide range of values. By using both the time of flight spectrometer IN6 and the high resolution Figure 1: Chemical formula of components of studied compounds, (a) Emeraldine salt conducting form of polyanili- backscattering spectrometer IN 16 it was pos- ne. (b) Camphor sulfonic acid, (c) Di n-pentyl ester of phthalosulfonic acid, (d) Di ethylhexyl ester of phthasulfonic sible to cover the range of time scales from acid. Scientific 62 highlights The temperature of the electrical transition is significantly lowered (Figs. 2b and 2c) and the dynamical transition is observed at a corres- pondingly lower temperature (Figs. 2b and 2c). However, in both of these cases, the dyna- mical transition is measured on IN6, the ther- mally activated motion probably being that of 0 50 100 ISO 200 250 300 0 50 100 150 200 250 300 Temperature (K) protons bound to the aliphatic tails of the Temperature (K) counter-ions. The systematic correspondence (a) of the temperature range of these two types of transition strongly suggests that the phenome- na are related. In conclusion, these results shed new light on the key role played by the dynamical disorder of counter-ions in influencing the transport ' 0 50 100 150 200 250 300 properties in conducting polyaniline. Of course, Temperature (K) many questions remain, for example: i) Are the electronic excitations always confi- (c) ned on vibrating chains whose dynamics are modulated via the interaction with the coun- Figure 2: Comparison between thermal variations of electrical conductivity and elastic intensity of (a) polyani- ter-ions? line/camphorsulfonic acid, (b) polyani!ine/di n-pentyl ester of phthaiosuifonic acid, (c) polyaniline/di ethylhexyl ester of phthaiosuifonic acid compounds. ii) In a given range of temperature, is any adiabatic transfer of electronic excitations to 109 to 1013s. In particular, systematic measu- tions have been found on IN6 and IN 16. In the the counter-ions possible, as stated in an exis- rements of the elastic intensity have been car- dynamic range of IN6, mainly the motion of ting theory [4]? ried out as a function of temperature. With methyl groups was observed. Above 300K the this method any emergence of diffusive motion rotors could be described as one set of equi- These experiments have opened a new field of in the measuring window is easily detected valent scatterers but, when the temperature is investigation of metal-insulator transitions in since it results in a marked lowering of the decreased, the values of correlation times disordered, organic systems containing intensity of the elastic peak. This was readily become more and more widely distributed due counter-ions. They should stimulate new done on IN 16 using the fixed window mode to the progressive freezing of the molecular theoretical studies, even if a detailed (Doppler drive stopped) while on 1N6, scatte- environment, which results in energetically description of the microscopic interplay red intensity in the two channels correspon- inequivalent rotors. Results were successfully between the chains and the counter-ions is ding to the elastic peak maximum was traced modelled with the so-called Rotation Rate currently lacking. as a function of temperature. Finally, the dis- Distribution Model developed for methyl crimination between contributions from poly- groups in amorphous polymers [2]. In addi- meric chains and from counter-ions was made tion, the temperature dependence of the by comparing the results obtained from fully elastic intensity measured on IN 16 exhibits hydrogenated samples and samples in which two inflexion points (Fig. 2a). One is centred the benzene rings of polyaniline chains were at 50 K and is also a signature of methyl group selectively deuterated. The different systems we rotation. The second inflexion point is centred have studied are shown in the Figs. 1 b, c, d. at 250 K and reveals the appearance of diffe- rent dynamics, assigned to a rigid-body motion An initial and somewhat surprising result is of the counter-ions. It is significant that the that in the whole time range probed in these onset of counter-ion motion coincides with the experiments, the polyaniline chains behave as temperature at which the sign of the slope of stiff, non-diffusive vibrating objects. This the conductivity curve is also inverted (Fig. 2a). REFERENCES • [1] SEE T.A. SKOTHE1M, R.L. ELSENBAUMER, result has been confirmed by comparing the J.R. REYNOLDS (EDS), HANDBOOK OF CONDUC- additional elastic intensity measured with The correspondence between electrical and TING POLYMERS, MARCEL DEKKER [NC. (1998) fully hydrogenated samples to that obtained dynamical transitions was further confirmed P. 27 AND P. 85 • [2] R. MUKHOPADHYAY, A. ALEGRIA, J. COLMENERO, B. FRICK, with partially deuterated ones. Accordingly, by the results obtained with the two other MACROMOLECULES 31 (1998) 3985 • [3] T.E. the whole quasielastic signal was attributed to compounds shown in figures la,c and la,d. OLINGA, J. FRAYSSE, J.P. TRAVERS, A. DUFRESNE, the dynamics of the counter-ions. With these counter-ions, while keeping similar A. PRON, MACROMOLECULES 33 (2000) 2107 • [4] L. ZUPPIROL1, M.N. BUSSAC, S. PASCHEN, Concerning the camphor sulfonic acid com- electrical properties, the films exhibit an 0. CHAUVET, L. FORRO, PHYS. REV.B SO (1994) pound (Figs. 1 a & b) two distinct contribu- improved mechanical flexibility [3], 5196 Scientific highlights 63 undamental & nuclear physics Quantum states of the neutron in the gravitational field • V.V. NESVIZHEVSKY, A.K. PETUKHOV, To our knowledge, the Planck constant and the in textbooks on neutron physics (for instance H.G. BORNER (ILL) neutron acceleration in the gravitational field [12,13]). As in such a situation no forces act have been considered simultaneously only in in the horizontal plane, quantum states can • H. ABELE, S. BAESSLER, N. HAVERKAMP, F.J. RUESS, T. STÖFERLE, A. WESTPHAL few papers (see for instance [1,2,3]). As poin- only be observed in the vertical direction. (UNIVERSITY HEIDELBERG) ted out in Ref. [4], it is possible to observe quantum states of neutrons in the Earth's gra- Figure 1 illustrates how quantum states of • A.M. GAGARSKY (PNPI, GATCH1NA) vitational field. Earlier, a complete analytical neutrons in the Earth's gravitational field can solution of the mathematical problem related be observed. The neutron wave is reflected • A.V. STRELKOV (JINR, DUBNA) to the quantum states of particles in a gravi- from the bottom mirror and interferes with tational field was published in textbooks on itself. Such self-interference provides a stan- quantum mechanics (see, for instance [5,6]). ding wave of neutron density. In a vertical 100 years ago, Max Planck introduced Ultracold neutrons provide a convenient slice, the neutron beam above the bottom mir- the concept of minimal energy method for such a study [7,8]. The experimen- ror consists of plane neutron "jets" with a neu- quantum, which can be transmitted tal technique using ultracold neutrons was tron density which varies with the vertical dis- through physical interactions. Soon it developed during the last 30 years. Recently a tance from the mirror. became evident that quantum one-component neutron gravitational spectro- (discrete) properties of matter are meter of high resolution was built at the ILL One lets neutrons "flow" with a certain hori- dominant in a variety of phenomena: [9]. The first measurement at the ultracold zontal velocity distribution through a slit. An quantum states of electrons in an beam of the ILL [10] with this device showed absorber/scatterer placed above the incident electromagnetic field are responsible the existence of the for the structure of atoms, quantum lowest neutron quantum states of nucléons in a strong nuclear state but hinted also to the potential for further field are responsible for the structure 0) ai a. O. of atomic nuclei. An example for investigations [11]. When 8. a, quantum effects in the weak interac- neutrons with extremely n low energy fall down in i i E=4 . tion is the possible neutrino i, fini oscillations. We report here on the the Earth's gravitational II i i i c e a first observation of quantum states of field - above a reflecting 50 a neutrons in the gravitational field. mirror - then they do not move continuously along 40 the vertical direction: The Pauli principle couples the Planck they must fall in quantum constant to the minimum values of the bound states because distance Az and the velocity AKat which the they experienced a deep quantum phenomena become dominant for the and wide potential well. particle with the mass m-.AzAV-him Obviously, the Earth's (AF « c). The smaller are m and AF, the lar- gravitational field alone ger is Az. When the particle is placed in a does not create such a potential well during a time period potential well as it does longer than AT ~ ñ I A£,quantum states with not allow particles only to an energy difference A£can be resolved. In move upwards too high; order to allow for an experimental observa- the other wall of the well tion, all other interactions must be so small is obtained by introducing a horizontal reflecting Figure 1 : Quantum states of neutrons are formed in the potential well be- that their interference with the gravitational tween the Earth's gravitational field on top and the horizontal mirror on quantum phenomena can be neglected. The mirror. A description of bottom. The probability of finding neutrons at the height z, corresponding to the ii1" quantum state, is proportional to the square of the neutron wave choice of a neutron, a neutral long-lived the interaction of neutrons with surface can be found function ipl{z). The vertical axis z gives an idea about the spatial scale of particle, satisfies the above mentioned conditions. this phenomenon. Scientific 64 highlights ii ui & The observed effects are consistent with the non-transparency of the slit formed by the bot- tom mirror and the absorber/scatterer for neutrons as long as the slit height is smaller 0.1•*. than the spatial width of the lowest quantum state. Careful analysis of the experiment allo- wed to rule out any systematic errors. In par- ticular, the shape of the transmission curve 0,01 (Figs. 2,3) does not depend on the value of the horizontal velocity component but it depends only on the vertical velocity component, as 1E-3 expected. A 15 pun-wide slit is not yet trans- parent for neutrons but it is sufficiently large to observe its transparency for visible light. This is a good check of the setup as the light 1E-4 wavelength of ~0.4[xm is much larger than the too 120 140 160 neutron wavelength of -0.01 pun • Evidently, the difference results from the fact that under Figure 2: The count rate versus the slit height measured with a low-background gaseous 3He neutron detector. normal laboratory conditions the Earth's gra- Blue circles show the data measured in the broad discrimination window of the detector signal. Solid blue curve is the classic iV-Ah'Tit for these data. Red circles correspond to the data measured in the narrow "peak" dis- vitational field does not act noticeably on light crimination window. Dotted red curve is the corresponding classic fit. Horizontal straight lines show the detec- beams but forms bound quantum states for tor background values and their uncertainties measured with the reactor "off. neutrons. Figure 3 shows in a larger scale and with addi- neutrons and the reflecting mirror below act absorber height should not increase the neu- tional statistics the initial part of the depen- as a selector for the vertical velocity compo- tron transmission as long as the height is dence analogous to Fig. 2. One can even noti- nent. From Fig.l we expect a stepwise depen- smaller than the spatial width of the second ce here indications of the stepwise character dence of the neutron flux as function of the quantum state. Then again the transmission of such a dependence not only for the lowest height of the absorber: if its height is smaller should increase stepwise. At sufficiently high quantum level. than the spatial width of the lowest quantum absorber position one should approach the state, then the neutron transmission should be classical dependence and the stepwise increa- This first experimental observation of quan- zero. When the height is equal to the spatial se should be washed out. Measurements at tum state in the gravitational field showed once again the universality of the quantum properties of matter. As the parameters of quantum states are defined in such a system mainly by the interaction of the neutron with the gravitational field, the presented pheno- menon can be considered as a tool for further investigations of fundamental properties of matter. 0.01 - REFERENCES • [1] R.A. COLELLA ET AL., PHYS. REV. LETT. 34 1E-3- (1975) 1472 • [2] V.G. BARYSHEVSKII ET AL., PHYS. LETT. A 153 (1991) 229 • [3] A.I. FRANK, SOV. PHYS. USP. 34 (11) (1991) 980 • [4] V.I. LUSCHIKOV AND A.I. FRANK, JETP LETT 28(9) (1978) 559-561 • [5] L.D. LANDAU AND 20 30 40 E.M. LIFSHITZ, IN QUANTUM MECHANICS, PAR. 28 (1963) • [6] S. FLUGGE, IN PROBLEMS IN QUANTUM MECHANICS, PROBL. 40 (1971) • [7] Figure 3: The dependence of the count rate versus the slit height. The data are summed up in intervals of 2jxm. V.I. LUSCHIKOV ET AL., JETP LETT. 9 (1968) 40- Solid and open circles and lines as well as the horizontal straight lines are analogous to those in Fig. 2. Vertical 45 • [8] A. STEYERL, PHYS. LETT. 29B (1969) lines correspond to the height values, at which approximately a next step in the count rate is predicted by cal- 33 • [9] V.V. NESVIZHEVSKY ET AL., NIM 440(3) culation. (2000) 754-759 • [10] A. STEYERL ET AL., PHYS. LETT. A (1986) 347 • [11] width of the lowest quantum state then the small absorber height showed the existence of V.V. NESVIZHEVSKY ET AL., SUBMITTED TO transmission is expected to increase sharply. the lowest quantum state; at sufficiently large PHYSICS LETTERS, ALSO IN ISINN-8, DUBNA (2000) • [12] V.K. IGNATOVICH, "PHYSICS OF The height of such a step is equal to the first absorber height the purely classical behavior ULTRACOLD NEUTRONS", 1986 • [13] R. GOLUB state population. Further increase in the is observed (Fig. 2) [11]. ET AL., "ULTRACOLD NEUTRONS", 1991 Scientific highlights 65 leotal & nuclear phys Phase coexistence and phase transitions in finite nuclei • R.F. CASTEN, V. ZAMFIR, M. CAPRIO (YALE UNIVERSITY) • H.G. BORNER, M. JENTSCHEL, H. L.EHMANN (ILL) Data from GRID measurements at the ILL, Grenoble, complemented by other measurements at Yale and Koln, are leading to a new understanding of structural evolution in nuclear sys- tems. This work has raised the pos- sibility that finite nuclei evolve in ways very close to those of systems showing real phase transitional behaviour, with coexisting phases, critical points, and order parameters. Figure 1: Energy surface in the Interacting Boson Model as a function of a phase transition parameter §, showing the discrete jump in p [5]. The positions of different Sm isotopes in the parameter space are also illustrated. The nature of collectivity and coherence in nuclei and their evolution with proton and GAMS 4 Spectrometer in '52Sm and in the energy ~1 MeV, must be detected with a reso- neutron number is one of the most fundamen- spherical neighboring nuclei '48l5OSm. In the lution of AE/E ~ 10'6, three orders of magnitu- tal issues in nuclear structure. Although GRID technique [3] the lifetime of a nuclear de better than obtainable with the usual Ge nuclei can change properties rapidly with state is obtained by an analysis of the Doppler semi-conductor detectors. Fortunately this is changes in the number of their constituents, broadening of a deexcitation y-ray transition: feasible for the GAMS4 and GAMS5 crystal phase coexistence and phase transitions, as a highly excited nuclei are produced by thermal spectrometers. The lifetimes x of the excited function of nucleon number, in the condensed neutron capture. Those deexcite to lower states are directly related to absolute transi- matter sense of these concepts, have general- energy states via emission of y-quanta which tion rates: The probability for the decay of an ly been discounted in finite nuclei. However, induce a recoil to the nuclei. The excited excited state is proportional to the square of recent studies [1,2] mark a significant change atoms move In the bulk of the target material the matrix element of the transition operator in this situation: they have revealed that and are slowed down by collisions with the between the initial and final state wave func- 152Sm is a rare example of phase coexistence surrounding atoms. Meanwhile the still exci- tions. with a deformed ground state and a more ted atomic nuclei deexcite by secondary spherical set of vibrator-like excited states. Y emission. The probability of these emissions An extensive set of about 50 new transition This phenomenon differs from that of intruder depends on the ratios (i/t) where t is counted rates were measured for the first time and states, as seen in Cd, Hg, and Pb nuclei in that from the emission of y's inducing the recoil and they provide a critical basis for evaluating the both sets of shape coexisting states arise from x is the lifetime of the intermediate state. If structure in this phase/shape transition a single set of basis states, that is, a single the nucleus is in motion when it deexcites region. In Fig. 2 is shown a typical example for Hilbert space. (fc-t) the y-ray energy will be Doppler-shifted. a GRID profile. These results alter in a signifi- If it is stopped, the normal transition energy cant way our understanding of how collectivi- will be measured. The Doppler effects are on To further map out the phase transition, we ty and deformation develop in certain regions the order of eV. Thus the y-ray, typically with have carried out GRID studies of lifetimes with of nuclei. Instead of a gradual softening of the Scientific 66 highlights yoaanienrcai & nuciear physics of two intrinsic excitations. The mechanism 800 r seems to resemble a Jahn-Teller symmetry breaking in favor of the lower symmetry defor- 150 Sm med state. 600 Interestingly, this feature appears in collecti- ve models such as the Interacting Boson Model or Geometric Collective Model, although it corresponds to a miniscule portion of their respective parameter spaces and was £ 400 unrecognized before these structures motiva- ted its discovery in the model framework. 8 These results have directly inspired the theo- retical development of a new class of dynami- 200 - cal symmetry for nuclei, the first dynamical symmetry applicable at a critical point itself, where structure is changing most rapidly [6]. There are two possibilities for this new class -30 -20 -10 0 10 20 30 of symmetry, one describing a transition from Doppler shift (eV) a vibrator to a deformed y - unstable struc- ture and the other a vibrator to axially sym- 15 Figure 2: Example for a GRID profile: The i 193 keV 2*3 -» g.s. transition in °Sm. The dashed line is the ins- metric transition region. These critical point trumental response, the full line is the fitted GRID line shape. The lifetime deduced from the Tit is - 0.5 ps. symmetries are the first entirely new para- digms for the structure of atomic nuclei since A New Class of Symmetry the 1950s. Their empirical discovery has relied in a significant way on GRID data from Critical Point Phase Transitional Nuclei the ILL. The first symmetry [6], called E(5), has been found [7] to exist empirically in 134Ba. The second symmetry [8], X(5) is manifested [9] in 152Sm as shown in Fig. 3. New GRID data will further test these ideas in this region. This new type of symmetry should have applica- tions in other mesoscopic systems. E(4)/E(2) 2.91 2.80 3.01 2.69 E(02)/E(2)=5.67 E(02)/E(2)=5.62 152 X(5) Sm REFERENCES • [ 1 ] R.F. CASTEN ET AL., PHYS. REV. C57 ( 1 998) 152 Figure 3: Comparison of X(5) and the data for Sm (based on ref. [9]). R 1 553 • [2] N.V. ZAMFIR ET AL., PHYS. REV. C60 (1999) 0543)2 • [3] H.G. BORNER AND J. JOL1E, J. PHYS. G 19 (1993)217 • [4] nucleus to deformation as valence nucleons which describes the magnitude of ellipsoidal F. 1ACHELLO, N.V. ZAMFIR, AND R.F. CASTEN, are added, the nucleus remains essentially deformation, is 'bl-valued': either ~ 0 or fair- PHYS. REV. LETT. 81 (1998) 1191 • [5] spherical in going away from closed shells, ly large, without a transitional region in bet- R.F. CASTEN, D. KUSNEZOV, AND N.V. ZAMFIR, PHYS. REV. LETT. 82 (1999)5000 • [6] albeit developing greater anharmonicities, ween (Fig. 1). The physical picture is that of F. 1ACHELLO, PHYS. REV. LETT. - 85 (2000) 3580 until enough correlations build up that the a crossing of two potential energy surfaces, • [7] R.F. CASTEN AND N.V. ZAMFIR, PHYS. REV. nucleus 'snaps' to a deformed structure [4,5]. with spherical and deformed minima. The LETT. - 85 (2000) 3584 • [8] F. lACHELLO, TO BE PUBLISHED • [9] R.F. CASTEN AND N.V. Geometrically, the deformation variable p, phase transition occurs at the crossing point ZAMFIR, TO BE PUBLISHED Scientific 67 highlights 0 modelling & theory Preface The following three articles reflect the growing use of numerical methods, based on modelling atom-atom interactions, in the analysis of experimental data. Once the model of interacting atoms has been shown to reproduce the existing experimental data, it can then be used to predict physical properties or effects, which are difficult to observe directly in measurements. Neutron scattering data lends itself par- ticularly well to such numerical modelling since the neutrons are scattered principally by nuclei. It is sufficient, therefore, to determine the trajectories of nuclei, which can be done without recourse to the electronic structure, as in force-field based methods. However, in cer- tain cases the electrons must be treated explicitly, using quantum chemistry techniques, in order to obtain more accurate results. A col- lection of twenty research articles in the field of neutron scattering and numerical modelling has recently been published in a special issue of Chemical Physics (vol. 261 nos. 1-2 (2000) Eds. M.R. Johnson, G.J. Kearley and J. Eckert). Nearly forty shorter articles can also be found in the proceedings of an ILL workshop (AIP conference proceedings 479 (1999) Eds. M.R. Johnson, G.J. Kearley and H.G. Biittner). Probing host-guest interactions in zeolites: a diffraction and molecular modelling study • C. BAEHTZ, H. FUESS (TU-DARMSTADT) tals. We have focussed on the nature of the rated, only the relative intensities of reflec- binding of the guests in the host lattice, which tions at low scattering angle (< 60°) in the • M. JOHNSON, A.W. HEW AT (ILL) is a basic question for the understanding of the powder diffraction pattern change (Fig. 1). host-guest interactions of these systems. From these intensity changes, the non-zeolite TCNQ and TTF are model substances for the framework scattering densities are extracted Powder diffraction, Fourier analysis and Rietveld refinement were perfor- med to localise guest molecules in mesoporous zeolites [1]. We are investigating 7,7,8,8-tetracyano-p- quinodimethane (TCNQ) and tetra- thiafulvalene (TTF) separately and as a charge transfer complex in faujasite NaY (high sodium content) and HY (low sodium content). Diffraction pat- terns were recorded at 5 K at the neu- tron powder diffractometers D1A and D2B. Experimental results were com- pared with force-field-based adsorp- tion simulations, modelling directly the host-guest interactions, for the 20 30 40 2-Theta [•] NaY host. Figure 1: D1A diffraction data of TTF in the NaY zeolite (black crosses). The red curve is a fit to the bare zeoli- te pattern, the scattering densities for TTF are determined from the blue difference curve. Interest in host-guest-systems has increased in the last few years, due to their potential applications in the field of sensors, gas sepa- investigation of binding sites in zeolites. by difference Fourier analysis and reveal the ration and heterogeneous catalysis [2]. Also The crystal structure of the host remains positions of the adsorbed molecules. The posi- dyes are incorporated into mesoporous crys- unchanged when guest molecules are incorpo- tion, orientation and occupancy (7 parame- Scientific 68 highlights ters) of rigid guest molecules are then refined found in the positions as described above. So by the Rietveld method, introducing a mini- the intermolecular interactions between the mum number of additional parameters (6). guests are weaker than the host-guest-inter- The NaY and HY zeolites show identical crys- actions. The complex could not be incorpora- tal structures but with different sodium ted in the HY zeolite, because of its lower content. The sodium content of HY at position sodium concentration, only TTF penetrates II in the crystal structure, which is accessible into the pore system. for the guest, is 42% of the corresponding The overall agreement between experiment NaY. and adsorption simulation validates the force field used, in the rigid molecule approxima- Figure 2 displays the arrangement of TCNQ in tion, and gives an energy-based confirmation the framework (yellow) of NaY and HY. The of the diffraction results. The simulations also Figure 2: TCNQ in the 12-ring window of the NaY molecule is in the centre of a 12-ring window, and HY framework; Na=pink, C=grey, H=white, offer more information in the more difficultly which connects two supercages of the zeolite N=blue; (200)-pIane=blue. determined binding sites of TTF. In principle, pore system. The cyano-groups point into additional information concerning the defor- these two cages and are coordinated to the mation of the guest molecule and the host sodium cations (pink) in front of the small matrix could also be sought by unconstrained 6-ring windows, the distance between sodium geometry optimisation in the simulation. and nitrogen is 2.7 A. The dimension and the However, UV spectroscopy reveals, through high symmetry of TCNQ are such that the shifts in vibrational frequencies, significant molecule fits precisely into this position. Such modification of the molecule and host matrix a 4-fold coordination of a guest to a zeolite fra- electron densities on binding, which would not mework has not been reported in literature be well-modelled by force-field simulations. before. Changes in molecular geometry on binding are Adsorption simulations using the 'sorption' therefore not expected to be reproduced accu- module in the Gerius2 molecular modelling rately using force-fields. Similarly, since the charge distribution of the guest molecules package and the Burchardt-Universal-Force- Figure 3: TTF in front of a sodium cation of the NaY Field confirmed this binding site to be the and HY framework; Na=pink, C=grey, S=yellow, changes when forming the charge transfer complex and the lifetime and degree of elec- most stable. For a rigid molecule adsorbate H=white. tron transfer is unknown, no adsorption simu- and a rigid zeolite framework, binding sites lations of the complex were performed. Such are probed randomly in the free volume of the electronic effects can only be investigated cage using a Monte Carlo algorithm. Binding using quantum chemistry (QC) methods, energies are determined in this way from Van which are typically employed following a der Waals and Coulomb interactions. force-field investigation. The size of the zeolite unit cell (15000 A3) presents a problem for Although TTF and TCNQ have similar molecu- solid state QC calculations and a more limited lar dimensions, the binding of the two mole- model of the binding sites would have to be cules in NaY is somewhat different. By simul- constructed. Finally, the knowledge of the taneous crystal structure refinement of the adsorption site of the guest molecules is the zeolite with one TTF molecule per super-cage, Figure 4: Second arrangement of TTF in the NaY framework; Na=pink, C=grey, S=yellow, H=whitc, basis for the understanding of spectroscopic using neutron and synchrotron radiation dif- found by simulation. results [3]. Henceforth, this force field can be fraction patterns, a distribution of TTF sites, employed to model the dynamics of guest giving rise to good R-factors, is proposed. also observed (Fig. 4). This binding geometry molecules in zeolite cages [4], which have an These sites have TTF in the super-cage with was not obtained in the original refned set of obvious importance, for example, in gas sepa- two sulphur atoms pointing to the cation in binding sites, but retrospectively the diffrac- ration processes. front of the 6-ring window (Fig. 3). The dis- tion data does not rule-out its existence. tance between the centre of mass of TTF and Simulations also confirm that the different this cation varies between 3.5 and 4.4 A, the nature of the binding of TTF and TCNQ arises rotation about the long molecular axis is also from separation of the electronegative atoms not well defined. which co-ordinate to the Na cations; 3.2 A for TTF and 8.3 A for TCNQ on the long molecular REFERENCES Again simulations give similar results, simula- axis. • [I] H. KLEIN, C. KIRSCHHOCK, H. FUESS, J. PHYS. CHEM. 98 (1994) 12345-12360 ted adsorption giving a range of almost ener- • [2] D. WOEHRLE AND G. SCHULZ-EKLOFF, ADV. getically degenerate sites. However, a coordi- When the charge-transfer-complex of TCNQ MATER. 6 (1994) 875-880 • [3] H. FORSTER ET At. PHYS. CHEM. CHEM. PHYS. 1 (1999) nation of all 4 sulphur atoms of TTF to the and TTF is adsorbed in NaY, the two-molecule 593-603 • [4] H. JOB1C AND M. BEE, ILL ANNUAL sodium with slightly lower sorption energy is complex is distorted so that the guests are REPORT 1997, p. 16 Scientific highlights 69 niodelSIno & theory Dynamics of plastic crystals: a combined computer simulation and quasielastic neutron scattering analysis • J. COMBET, A. MARTIN package in order to simulate molecular dyna- (ILL & UNIVERSITE STRASBOURG) mics. One week of calculation was needed to simulate around 200 ps on a biprocessor Silicon Graphics workstation (Origin 200). Simulations were first realised in the NPT ensemble (constant number of molecules, Molecular dynamics simulation is a pressure and temperature) to determine the powerful technique for determining evolution of the lattice parameters at different the structural and dynamical proper- temperatures. The initial unit cell configura- ties of molecular compounds. tion was generated from the well-ordered, low Molecular modelling and neutron temperature, monoclinic structure determin- scattering are complementary ed by X-ray powder diffraction [1]. The simu- methods since they probe similar Figure 1: The molecular geometry of norbornane lation box contained 27 unit cells and periodic length and time scales (A and ps-ns). (G7H12), or bicyclo [2,2,1] heptane. The full arrow boundary conditions were used in order to represents the axis of the dipole moment. Detailed analysis of the calculated mimic an infinite crystalline environment. trajectories enables to predict neu- The temperature was increased from 50 K to scattering. Motions have been described as tron scattering intensities. Thus, a 320 K using 20 K steps. About 100 ps were rapid isotropic reorientations. Long-range dif- comparison with experimental spec- simulated for each temperature in order to get fusion was also evidenced on a slower time tra provides a means for testing the a correct equilibration of the system. Lattice scale. We have started molecular dynamics accuracy of theoretical description. parameters extracted from these simulations simulation in order to get a more accurate This report presents a molecular exhibit high modifications around 155 K. The description of the dynamical properties of this dynamics simulation performed on monoclinic ordered phase rapidly transforms compound, including greater insight into the two plastic phases of a molecular toward a disordered hexagonal phase. This phase transitions. compound: the norbornane. corresponds to the first order-disorder phase transition experimentally observed at 131 K. Dynamical Simulations This very encouraging result indicates that the The norbornane plastic crystal intermolecular interactions could be well We have used the COMPASS empirical represented by the COMPASS force field. force field implemented in the CER1US2 Globular molecules often form plastic crystal We also started NVT ensemble simulations below the melting point. These phases exhibit (constant number of molecules, volume and temperature) in the two plastic phases in a high degree of disorder: while the centre of • 1 • • • • 0 experimental 1 • order to generate the molecular trajectories. mass is rather well localised in space, the simulation 1 • 0.8 orientation varies from one site to another and isotiopic rotation!—" The comparison with experimental quasielas- changes with time. When the temperature is tic neutron scattering experiment was perfor- decreased, these substances exhibit one or 0.6 med with the nMOLDYN package [2]. several solid-solid transitions into a fully well \ ordered crystalline phase. Simulation analysis and neutron Norbornane (Fig. 1) is a simple bridged mole- scattering cule and has been considered as a key com- 0.2 \ \ pound for strained materials. This system is The simulation results were compared with the orientationally disordered at ambient tempe- **T7 1 1 0.5 1 1.5 measurements performed at ILL on 1N6 [3]. The rature, transforming from cubic to hexagonal Q (A"1) trajectories analysis enable the EISF (Elastic at 306 K and to a ordered monoclinic phase at Incoherent Structure Factor) to be extracted. 131 K. Figure 2: Experimental EISF extracted from IN6 This quantity is determined by the probability The dynamics of norbornane in its two disor- experiment (similar in both plastic phases), simu- distribution of the hydrogen atoms, and thus lated EISF extracted from the trajectories analysis dered phases has been extensively studied by (identical for both plastic phases), and theoretical provides a detailed description of the geometry NMR and incoherent quasielastic neutron evolution in case of isotropic rotation. of the motions. EISF are represented on Fig. 2. Scientific 70 highlights plex than simple isotropic reorientation and the orientational distributions are different in each phase. Spectroscopic and diffraction measurements have been unable to detect these differences. Molecular dynamics simulations is a powerful method for the description of structure and dynamics in molecular compounds. Combined studies with quasielastic neutron scattering experiments enable the same quantities to be probed, and thus offer a unique way to valida- te the simulations and, thereafter, exploit the wealth of microscopic information available from the simulation model. -1 0 Energy transfer (meV) Figure 3: Comparison between experimental spectra and dynamical structure factor extracted from the molecu- lar dynamics simulation in the cubic disordered phase (0 = 0.7 A1) The simulations are clearly able to explain the phase. The excellent agreement definitively experimental evolution of the EISF. The small demonstrates the validity of the simulations differencies observed between the isotropic and the accuracy of the empirical force field. rotation description and the simulated dyna- The detailed analysis of the computed trajec- mics could be explained by the existence of tories allowed the orientational distribution to preferential orientations and needs a careful be probed. Figure 4 represents the results for investigation of the simulated trajectories. the dipole moment axis represented in Fig. 1 Figure 4: 2D representation of the orientational distribution of the molecular symmetry axis (representations realised with LAMP, the Large Array Manipulation Programme developed at the ILL). Left side: cubic phase, right side: hexagonal phase. Light blue spots indicate the occurrence of pre- ferential orientations. The EISF only gives geometrical information for both plastic phases. Although the simula- on the molecular motions. A more stringent tions are unable to reproduce directly the high REFERENCES test of the simulation involves comparing the temperature phase transition, stable simula- • [1] A. N. FITCH AND H. JOBIC, J. CHEM. SOC, experimental spectra with the dynamical tions in the hexagonal and cubic phases show CHEM. COMMUN. 1993, 15)6-1517 • [2] structure factor extracted from the trajecto- clearly broad maxima, indicating the existen- G. R. KNELLER, V. KEINER, M. KNELLER, M. SCHILLER, J. COMPUT. PHYS. COMMUN. 23 ries. Figure 3 represents the experimental ce of preferred orientations. The dynamics of (1977) 327 • [3] M. BEE, H. JOBIC AND and simulated spectra in the cubic disordered norbornane molecules is therefore more com- C. CAUCHETEUX, J. CHIM. PHYS. 83 (1986) 623 Scientific highlights 71 Proton vibrational dynamics in nucleic acid building blocks • N. LEULLIOT, M.P. GA1GEOT, M. GH0M1 bulges, etc). The chemical repeat of an RNA is forces is necessary. To achieve this task for (UNIVERSITY P. & M. CURIE, PARIS) a ribonucleotide, whereas that of a DNA is a RNA and DNA building blocks, vibrational data 2'-deoxyribonucleotide. Each of these mono- can be used in junction with quantum mecha- • H. JOBIC nical calculations. Raman scattering and 1R (INSTITUT DE RECHERCHE SUR LA CATALYSE, mers is formed by a phosphate group, a sugar VILLEURBANNE) (ribose in RNA and 2'-deoxyribose in DNA) and absorption cannot give sufficient data in this a base (uracil): U (RNA), thymine: T (DNA), framework, because their intensities are basi- cytosine: C, guanine: G, adenine: A). cally monitored by the displacements of heavy Inelastic neutron scattering (INS) spectra of two major RNA (RiboNucleic Acid) building blocks, i.e. adenosine (rA) and guanosine (rG), as W3 X well as those of two major DNA / W2 (DeoxyriboNucleic Acid) building blocks, i.e. 2'-deoxycytidine and 2'- deoxythymidine, have been recorded in solid phase at T = 15 K using the IN1-BeF spectrometer. These spectra gave us the opportunity to perform theoretical investigations at the den- RTOONirOLF.OTIDK sity functional theory (DFT) level in order to access both geometrical and vibrational features of these molecu- PHOSPHATE GROUP lar compounds. The joint use of INS spectra and ab initio calculations allo- wed us to particularly probe the hydrogen vibrational dynamics of the nucleic acid building blocks, and to analyse the effect of solid phase inter- molecular H-bonding. This set of information is not accessible by tradi- Figure 1: (a) Chemical structure of RNA. DNA chemical structure is similar, the 0-H group in the 2'-position of tional optical spectroscopy, i.e. sugar should be replaced by a hydrogen and uracil by a thymine (5-methyluracil). (b) Graphical representation of uracil bound to three water molecules through N-H, C=0 and C5-H groups. Geometry optimisation has been Raman scattering and IR absorption. made at the DFT/B3LYP/6-31++G* level [6]. A ribonucleoside or a 2'-deoxyribonucleoside, atoms (e.g. G, N, 0 and P). Hydrogen vibratio- The knowledge of the structure of biological which is formed by a base and a sugar, can nal dynamics can only be accessed by molecules is an important task in order to also be considered as an important RNA or inelastic neutron scattering (INS) spectra in understand their functions. Nucleic acids DNA building block. Hence, there are four which the vibrational modes arising from (DNA and RNA) are linear polymers (Fig. la) major ribonucleosides, i.e. uridine (rtJ), cytidi- hydrogen atoms are particularly exalted. which take part in various biological pro- ne (rC), guanosine (rG) and adenosine (rA), To give an example of our investigations on cesses such as replication (DNA to DNA), and four major 2'-deoxyribonucleosides, i.e. ribonucleosides and 2'-deoxyribonucleosides, transcription (DNA to RNA), translation (DNA 2'-deoxythymidine (dT), 2'-deoxycytidine (dC), we present in Fig. 2 the INS spectra of rG and to protein) and catalysis (RNA splicing or self- 2'-deoxyguanosine (dG) and 2'-deoxyadenosine rA recorded at T=15 K on INl-BeF, in the splicing). Nucleic acids have a great confor- (dA). To study the conformational flexibility of spectral region below 2000 cnr1 (250 meV). To mational flexibility and can adopt a variety of nucleic acids by different theoretical methods, analyse these spectra, we resorted to DFT conformations (double-helices, hairpins. the knowledge of the intra and inter-molecular calculations with B3LYP exchange-correlation Scientific 72 highlights functional and 6-31G* (or 6-31++G*) basis 110 meV) region. However, some discrepan- phase, at a reasonable computational cost. sets [1-5]. First-order INS spectra calculated cies appear in the spectral region below 900 Similar calculations are now in progress on with the harmonic force field of isolated rA and cm"' (Fig. 2). We have shown [4] that the poor other bases, as well as on ribonucleosides and rG, are reported in Fig. 2 for comparison [4]. agreement below 900 cm-' (110 meV) between 2'-deoxyribonucleosides. We can emphasize here the new information the calculated and observed INS spectra, is obtained from the INS analysis of these mole- mainly due to intermolecular H-bonding that cular compounds. First of all, in all ribonu- affect (/) the base vibrational N-H wagging cleosides and 2'-deoxyribonucIeosides, one motions in the 900-500 cm"' (110-60 meV) can observe intense INS bands in the 1500- region, and {//) the ring (base or sugar) torsio- 1200 cm-' (185 - 150 meV) spectral region nal motions in the region below 500 cm-' (60 meV). which mainly arise from the CGH, OGH, COH We have attempted to account for the inter- and NGH angular bendings of sugar [3]. We molecular H-bonding through simple theoreti- have verified the above-mentioned assign- cal models in which the main acceptor and ments with the use of the INS spectra obtained donor sites of RNA building blocks interact from the labile hydrogen deuterated species with water molecules. The example which is also recorded on the INl-BeF. Moreover, the completely treated up to now, is that of uracil C-H and N-H waggings of the bases as well as bound to three water molecules (Fig. lb) the torsion modes of the bases and sugars, mimicking the solid phase H-bond network. which induce large hydrogen displacements, The calculated results based on these models can also be observed in the 900-700 cm' have shown, that the main H-bonding effects rG (calc.) yv uMI w \.A..P W,Av-- V 1000 1500 Wavenumber (cm-1) REFERENCES Figure 2: Comparison between the observed (powder samples atT=15 K) and calculated (first-order) INS spec- • [I] N. LEULL1OT, H. JOBIC, M. GHOMI, IN tra of rG and rA in the spectral region below 2000 cm'(250 meV), as obtained from the quantum mechanical NEUTRONS AND NUMERICAL METHODES-N2M, ED. calculations at the DFT/B3LYP/6-31G* level. M.R. JOHNSON, G.J. KEARLEY AND H.G. BUTTNER, A1P, NEW YORK (1999) 179 • [2] N. LEULUOT, M. GHOMI, G. SCALMANI, G. BERTH1ER, J. PHYS.CHEM. A 103 (1999) 9716 • [3] (110 - 85 meV) spectral region. All this vibra- appear in the N-H wagging modes which are N. LEULLIOT, M. GHOMI, H. JOBIC, O. BOULOUSSA, tional information is not accessible in Raman upshifted by -250 cm' (-30 meV) in going V. BAUMRUK, C. COULOMBEAU J. PHYS.CHEM. B and IR spectra. from the isolated to H-bonded uracil, thus in 103 (1999) 10934 • [4] M.P. GAIGEOT, N. LEULLIOT, M. GHOMI, H. JOBIC, O. BOULOUSSA, On the basis of the careful analysis of the good agreement with the observed INS C. COULOMBEAU, CHEM. PHYS. 261 (2000) 217 whole set of results, we can conclude that the spectrum in solid phase [5-6]. • [5] M. GHOMI, A. AAMOUCHE, B. CAD1OLI, calculated INS spectra of isolated nucleic acid In conclusion, such modelling (Fig. lb) repre- G. BERTH I ER, L. GRAJCAR, M. H. BARON, J. MOL. sents acceptable compromise to account for STRUCT. 411 (1997) 323 • [6] M.P. GAIGEOT, building blocks reproduce satisfactorily the N. LEULLIOT, H. JOBIC, M. GHOMI, CHEM. PHYS. observed ones in the 2000-900 cm-' (250 - complicated intermolecular H-bonding in solid 261 (2000) 217 Scientific 73 highlights Network of hydrogen bonds as a medium for DNA interaction in solvents • V.L. GOLO (UNIVERSITY OF MOSCOW) knots represent the oxygen atoms and the nematic phases, which are observed in the threads correspond to the hydrogen bonds. DNA dispersions, have a number of features • E.I. KATS The conformation of the net and the distribu- which cannot be understood within the (ILL & ON LEAVE FROM LANDAU INSTITUTE, MOSCOW) tion of the protons are determined by Bernal- conventional framework of electrostatic and Fowler's rules [2], i.e. there are two protons Van der Waals forces [8]. In the presence of 9 YU.M. YEVDOKIMOV in the neighbourhood of an atom of the oxygen, DNA molecules, the network of the hydrogen (INSTITUTE OF MOLECULAR BIOLOGY, and one proton is assigned to each hydrogen bonds has a conformation which is different MOSCOW) bond. Defects also should be allowed for: close to the DNA molecules and in the bulk, a We suggest that the DNA molecules could form a cholesteric phase owing to an interaction mediated by the net- work of the hydrogen bonds (H-net- work) in the solvent. Using the experi- mental data for the cholesteric phase of the DNA dispersion, we obtain a rough estimate for the energy given by our model, and show that it should be taken into account as well as the energy due to the steric repulsion, Van der Waals, and electrostatic forces, generally used for studying the DNA molecules. The elastic constant of the H-network generating the interaction between the DNA molecules is deter- mined by the energy due to the pro- 230 280 330 420 470 520 570 ton's vibration in the hydrogen bonds. A, nm It is generally accepted that the formation of the cholesteric phases of the DNA dispersions Figure 1: The CD (circular dichroism) spectra of the liquid-crystalline dispersion treated by daunomycin (DAU) (curvel - CDAU = 0, curve 2 - CDAU = 6.8 -lO6 M, curve 3 - CDAU = 27.2 -10* M, where CDAU is the is intimately related to the interplay among concentration of daunomycin. the basic properties of the DNA and the sol- vent, but, the nature of the interactions bet- some of the threads (H-bonds) can be consi- ween the molecules of DNA in the dispersions dered as broken due to the thermal fluctua- few layers of the water molecules away from has remained obscure. It is worth noting that tions. The conformation of the network of the the DNA. One can imagine that close to the the hydrogen bonds, although weak hydrogen bonds in the DNA dispersions has DNA molecules, the net is rolled round a pole; (5 kcal/mol), are still larger than the Van der certain properties which make it quite diffe- it is spreading more leisurely away from the Waals ones, and are strong enough to hold the rent from the network of the hydrogen bonds DNA molecules. Interaction between DNA water molecules in a rigid structure in solid in the bulk solvent, so that the interaction bet- molecules and solvent leads to substantial water (ice) [1]. The structures of the water ween the DNA molecules which are separated modification of the structure of H-bonds in the and the ice are similar, and either of them is by less than 50 A, may induce new phenomena. nearest aqueous environment. If we believe determined by the network of the hydrogen Experimental data and numerical simulations that very close to DNA molecules the network bonds [2], even though in liquid water some of [3] - [10] suggest that the inter-molecular of the H-bonds is more or less stable (on cha- the hydrogen bonds are broken, their number medium of the DNA dispersions plays an racteristic time-scale relevant for intermole- depending on temperature. The whole system important role in the phase stability. Indeed, cular interactions), there is a certain elastici- may be visualised as a fisherman's net: the the transitions from the cholesteric to the ty of the net (it means that for any deformation Scientific 74 highlights of its structure one has to pay a certain ener- topological constraints are also essential for can only expect a small modification of the gy) and therefore effective elastic interaction the proton dynamics of the H-bonds. In fact, direct interparticle interaction (electrostatic between DNA molecules mediated by the the deformation of the net by DNA molecules or Van der Waals) rather than the fundamental H-net. Therefore, we shall assume that the can be described in certain cases as the defor- rearrangement of the entire structure network of the H-bonds in solvent has a cer- mation field created by the dislocation in the observed experimentally [10]. tain amount of elasticity (see also [11]), net, i.e. by topological constraints, and besides and in fact enough to account for the interac- the interaction changes the probability of pro- tion of DNA molecules. The total energy, which ton tunnelling (near DNA the potential for tun- contains the elastic and the entropy term nelling is not the same as is is in bulk water). under the constraints imposed by the bounda- In this respect it is worth noting the phenome- ry effects, should result in an effective inter- non reported in [4] that the protons could action between the DNA molecules, through perform cooperative or concerted motions the medium of the network of the hydrogen (flip-flop) by jumping from one state to ano- bonds. The picture given above implies that in ther. Saenger et al. [4] claim that one could the neighbourhood of the DNA molecule in the find chains of the water hydroxyl groups solvent, the conformation of water molecules 0 - H ••• 0 - H - 0 - H in which the protons is strongly influenced by the geometry of the would oscillate clockwise and anti-clockwise 1400 230 280 330 420 470 520 570 nm REFERENCES Figure 2: The CD spectra of the liquid-crystalline dispersions formed by DNA - DAU complexes • [1] L. PAULING, THE NATURE OF CHEMICAL 6 6 (curve 1 - CDAU = 13.6 • 10- M ; curve 2 - CDAU = 37.6 • 10 M). BOND, CORNELL UNIVERSITY PRESS (1960) • [2] J.D. BERNAL AND R. FOWLER, J. CHEM.PHYS. 1 (1933) 515 • [3] S. NEIDLE, DNA helix. In this neighbourhood, which in a flip- flop manner. Obviously, the choice of H.M. BERMAN, AND H.S.SHIEH, NATURE 288 spreads out at a distance of approximately 10 the circles allowing for the flip- flop motion is (1980) 129 • [4] W. SAENGER, CH. BETZEL, A, the conformation of the H-bonds follows the determined by the topology of the network of B. HlNGERTY, AND G.M. BROWN, NATURE 296 (1982) 581 • [5] p. GALLO, SINGLE PARTICLE grooves of the DNA and has the shape resem- the H-bonds. SLOW DYNAMICS OF CONFINED WATER, COND- bling that of the helix. In this respect, it differs Our hypothesis on the nature of the interac- MAT/0003027 (2000) • [6] D.C. RAU AND considerably from that in the bulk (compare tion between DNA molecules that leads to the V.A. PARSEGIAN, BlOPHYS.J. 61 (1992) 260 • [7] R. PODGORNIK, D.C. RAU, AND V.A. [4], [5]). formation of the cholesteric phase, is in agree- PARSEGIAN, BlOPHYS. J. 66 (1994) 962 • [8] It is important that stringent topological ment with experimental data [10] (see Fig.l A.A. KORNYSHEV AND S. LEIKIN, PHYS.REV.LETT. conditions are imposed on the network owing and Fig.2), which show the change in the 84 (2000) 2537 • [9] YU.M. YEVDOKIMOV, V.I. SALYANOV, A.T. DEMBO, AND F. SPENER, SENSORY sense of the cholesteric twist due to the inter- to the requirements due to the structure of ice SYSTEMS13, (1999), 158 • [10] YU.M.YEVDOKIMOV, [ 1 ], i.e. among 16 possible combinations of the calation of DNA molecules by the daunomy- S.G. SKURIDIN, S.V. SEMENOV, v.l. SALYANOV, AND G.B. protons round an atom of the oxygen only 6 cin. At this intercalation, from the viewpoint of LORTKIPANIDZE, BIOPHYSICS 43 (1998) 240 molecular properties, modification of the DNA • [11] D. FARAGO AND Y. KANTOR, THE ELASTIC are allowed, so that the residual entropy of ice BEHAVIOUR OF ENTROPIC "FISHERMAN'S NET", turns out to be equal to So = Nks ln(3/2). The molecules is quite negligible and therefore one COND-MAT/0004276 (2000) Scientific highlights 75 Off-diagonal geometric phases • F. PISTOLESI (ILL) tries of the vibronic levels [2]. Among others, components along the other orthogonal initial a field where Berry's phase has provided a states |a|)k(si)>. Recently we have thus inves- • N. MANINI (IST1TUTO NAZIONALE particularly useful interpretation scheme is tigated the phase factor q/ of When a quantum system undergoes a cyclic evolution, the initial and final wave functions differ by at most a phase factor. This phase factor has an intrinsically geometric compo- nent, as it was first recognised by Berry [1], In this context, "geometric" means that the phase depends only on the sequence of quan- tum states visited by the system. A simple classical analogous is the rotation by a finite Figure 1: Two vectors tangential to a curved surface are driven along a path. Despite being parallel-transported angle of a parallel-transported reference along the path (remaining as parallel as possible to the direction they were pointing at before each infinitesimal displacement), when they return to the original point, the two vectors find themselves rotated with respect to frame along a closed loop on a curved surface, the initial directions. These rotation angles measure the curvature of the surface enclosed in the loop, and are as drawn in Fig. 1. analogous to the Berry phase acquired by the quantum wavefunctions. Berry's phase is almost ubiquitous in present- day physics, and it has been a fruitful tool for |ijjk(si)>. Pancharatnam considered the phase orthogonal states. We found [5] that such arbi- interpreting disparate physical phenomena. A associated to the projection Scientific 76 highlights e=o 9=Jt in such a way that the energy levels would cross. Whenever no special symmetry selec- tion rule prevents level repulsion, we will get a sequence of avoided crossings with the cha- racteristic approximated swapping of the pair of involved states. Basically, after each avoi- ded crossing, we have a permutation of the original states. In the region between two avoided crossings, the only well-defined geo- metric phases are the off-diagonal ones asso- ciated to that specific permutation. The off-diagonal geometric phases can be a useful tool to characterise the evolution of any multi-dimensional quantum system. Specifically, they could be observed for example with neutron interferometry experi- ments similar to those used successfully to measure diagonal phases [8]. Very recently, Figure 2: The observed initial (6=0, left), intermediate (8=n, central) and final (9=2jt, right) eigcnstates of the the neutron-spin off-diagonal phase has been microwave cavity deformed following adiabaticaiiy the path of Ref. (6] around a 3-fold degenerate point. Red/yel- low indicate opposite signs of the wavefunctions. succesfully measured at the ILL [9]. only geometric phase information is represen- parameterises the displacement of the upper ted by the diagonal Berry phases ojf=yf. For right corner of the resonator away from the a generic non-cyclic path, both diagonal yf rectangular-shape position. At this position, and off-diagonal 7// phase factors carry the three energy levels are degenerate. In this same kind of information, and can be conside- experiment, the three nearly degenerate red on the same footing. When the n final wavefunctions are followed adiabaticaiiy as states are a nontrivial permutation of the ini- the distortion is driven in small steps through tial ones, the diagonal scalar products a loop 6=0 to 2re around the degenerate point. Scientific highlights 77 Isabelle Griilo fills the D22 stop-flow apparatus. The PERK.EO installation at the new beam of polarised/unpolarised cold neutrons for Particle Physics PF1B. View of the collimation system and the spectrometer in the iron shielding and the people involved in the project (from left): M. Kreuz (University of Heidelberg), V. Nesvizhevsky (ILL), C.Vogel and U. Mayer (University of Heidelberg). I'T Rebuilding the D20 multidetector. Giovanna Fragneto making model membranes on the ILL Langmuir trough Millennium programme 78 and new developments Millennium programme and new developments A sustained effort has been devoted this year into establishing a solid foundation for the regeneration of the Institute's instruments and infrastructure through the launching of the ILL'S Millennium Programme. Perhaps it is more than symbolic that ILL is an integral part of the word millennium. Such an ambitious programme requires a concurrence of three essential ingredients - motivated and well-qualified manpower, adequate sources of finance and inspired ideas. It also requires good luck and a willingness to take risks. As the first year of the Millennium Programme draws to a close, there are hopeful signs that all these elements are in place. Building upon the newly rebuilt reactor, the momentum generated by instrument projects already in place, the increasingly youthful staff and the guidance of our users, our funding bodies have responded to our appeal for the investment necessary to fulfil the vision which is illustrated in the ILL'S long-term strategy document - the Road Map. Millennium programme ana new developments 79 The ILL millennium programme C.J. CARLILE The third Millennium rolls on - only 999 ! #? years left until we celebrate the next one. But in the meantime there is much activity in Part of the Laue diffraction pattern of 4% Ga-doped FeGc2 at 77 K, observed in a trial with the LADI image-plate the Millennium Programme at the ILL with detector on a thermal beam. Shown in inset is the evolution of one set of magnetic satellites towards the com- mensurate 1/10 phase below -170 K. five projects currently on the menu. VIVALDI, the Very Intense, Vertical Axis, Laue Elsewhere, as part of our continuing detector the length of the detector by an appropriate Diffractometer (previously known as Thermal development programme, we have commis- mix of absorber/quench gases and anode vol- LADI) with its image plate detector is well sioned and received five prototype linear tage has been exceeded in recent tests. An into its manufacturing phase. We expect position sensitive detectors 7 mm active dia- array of 140 such detectors would then deli- delivery of the complete instrument - a colla- meter and 1 m active length. The aim to ver the appropriate spatial resolution boration with the EMBL - during spring 2001 achieve a position resolution of 7 mm along (7 mm x 7 mm) for the D22 and later the Dl 1 and installation on its guide position before small-angle scattering machines summer. Tests with the prototype version at an overall count rate of 2 gave remarkably good quality data with none MHz. Commissioning tests indi- of the background problems which had been cate that a rate of 5 MHz can be feared. Weak superlattice reflections in achieved with dead time losses magnetic materials were clearly seen. E.F. of 10%. Currently these instru- Schumacher's catch phrase "Small is ments are limited to around 50 Beautiful" appears to apply here and we kHz and must use beam attenua- shall be exploring other possible applications tors in order to avoid data cor- of image plates. Certainly speed of manufac- ruption, even with the detectors ture, relative cheapness and small sample at large distances from the size, are all qualities in favour of such ins- sample. A hundred-fold increase truments. in data rate will now allow expe- riments only dreamed of to be The rebuild of the single crystal diffractometer tackled. D3 as a spherical polarimeter is also moving ahead quickly, but it is conceptually and techno- The Strain Imager is a joint pro- logically much more complex and will come on ject financed partly by the ILL line gradually over a three to four year period of and partly via a grant to Prof. development and commissioning. Decpol, a spin Philip Withers of the University sensitive detector for the new instrument, com- of Manchester. Beam line design prising polarised 3He plus a 3He detector within The first experiment carried out on D3 with Decpol, a compact is now complete, integrating single detector with spin analysis, aimed at measuring the antifer- its magnetic shield, has already been received romagnetic form factor of Co in LiCoPCM using poiarimetric scat- D1A, DIB, the Strain Imager and commissioned on the instrument. tering. Upper left: Hubert Humblot delivering a 3He spin filter. and VIVALDI along the same Millennium programme 80 and new developments been an iterative process as the guide feeding D1A, DIB, the Strain with initial in-house seis- Imager and VIVALDI. mic calculations acting as We also recognise that ILL cannot, from input to the engineering within even the increased budget voted by design prior to the final the Associates at the November 2000 quality-assured simula- Steering Committee, finance all the projects tions by accredited consul- necessary to fully rejuvenate the instrument tants. The first pillars suite, the neutron guides and the ancillary have already appeared infractructure which are necessary to deliver during the present winter the best science for our users. We therefore shutdown and the have entered into a number of collaborative 2-dimensional detector has ventures with teams of users to bid for addi- already been ordered. tional funds to accelerate instrument renewal. During 2001 we will start These include the Strain Imager, PF1 bis, the upgrade of the D7 pola- D2b, D19 and IN13 as well as bids for a risation analysis spectro- Facility for Materials Engineering FaME meter with its huge pro- (in collaboration with the ESRF) and for a gramme of supermirror Bacteriological Deuteration Facility spin analysers, to begin (in collaboration with the EMBL). the rebuild of the D2b high We still have a large number of projects resolution powder diffrac- queued up to follow on from the nine identi- tometer from the mono- fied above, but we are very keen to ensure chromator to the detector, that bright new ideas continue to be added to and we intend also to the list of contenders. We therefore strongly implement the Miniball encourage anyone with such ideas to contact detector options for Prototypes of the D22 tubular position sensitive detectors one of the Directors so that we can see whe- (length of the tube: lm) Lohengrin aimed at inves- ther it is possible. Equally well, we encoura- tigating the properties of ge consortia of users interested to bid for renewed neutron guide. The site for the in- short-lived isotopes near the neutron drip funds from external sources for their favourite strument has already been excavated down line to furnish data for cosmological models. instrument or project also to contact us. to a 2 metre depth ready for the foundations At the same time we will start the neutron which will support the 1 tonne capacity, guide renewal programme with the rebuild of 10 \im precision hexapod orientation device the Dl 1 guide and collimation system as well which will sit on the granite sample table. A considerably increased polarised neutron flux of 6x107 n per cm~2s"' on the sample will be obtained by remodelling the IN20 primary spectrometer. A larger beam tube using a source diameter of 170 mm has been instal- led in January 2001 which will be implemen- ted together with a new large (230 mm x 140 mm) double focussing Heusler monochroma- tor in spring 2001. BRISP, the Brillouin scattering spectrometer being built as a CRG by an Italian 1NFM team comprising Professors Fabrizio Barocchi, Caterina Petrillo and Francesco Sacchetti, together with Professor Jens-Boie Suck from Chemnitz, is now out of the starting blocks following an intense year of engineering opti- misation. In particular the seismic stability of the large instrument, to be installed on an Schematic drawing of the new IN20 monochromator inclined beam hole in the reactor hall, has assembly, the biggest one ever built (230x150 mm2, had to be meticulously simulated. This has composed of 75 crystals). Millennium programme and new developments i81 Review of developments in instrumentation C.J. CARLILE The instrument rebuild programme is now firmly established as a central and continuous theme in ILL life and earlier investments are bearing fruit whilst the new projects in the Millennium Programme have been launched. In addition, the need for infrastructure renewal has been reco- gnised and we shall begin replacing two neutron guides in 2001. Progress with Ongoing Upgrades Whilst the Millennium Programme is now in full swing, the earlier investment programme, colloquially known as UFI - the Upgrade of Five Instruments - is taking centre stage. In some senses the two programmes now form Simon Wood on D16. a single continuous programme of instru- ment build which we intend to maintain as a gent pulse recognition algorithm the maxi- ment in detector stability - much improved constant thread in ILL's activities. mum count-rate of the delay line detector on even what was previously the most stable has been lifted from its original specification liquids instrument in the world. The D16 long wavelength diffractometer has of 175 kHz to 400 kHz. Undoubtedly this is a been operating in its new: configuration with really welcome addition to the IUL's instru- The IN4 time-of-flight spectrometer, an scheduled experiments for over a year now. ment park. ILL/INFM Italy collaboration, is nearing the Special sample environment equipment - end of a painful rebirth. The two new hori- shear cells and controlled humidity cells - The rebuild of the D4 liquids and amorphous zontal axis choppers are both installed on the has been commissioned. The project for an diffractometer was completed in June 2000 instrument and operational. Final authorisa- improved detector is underway and a proto- and the nine sets of microstrip detectors in tion has been given to remove the aluminium type two-dimensional wire device, designed their new shielding and collimator housing beam blocker, signalling the start of first and assembled in-house, has been tested were installed on the beam-line. The instru- measurements which we eagerly await. with great success. ment immediately operated successfully and Improvements to the instrument have been within days first users were quickly brought made in parallel - detector electronics have After major commissioning work, the D17 in to assay the quality of the new instrument. been replaced, the radial collimator has been reflectometer is open to users in both mono- The original aim to improve the precision of refurbished and the low-angle detector has chromator and time-of-flight modes. the instrument by an order of magnitude been put in place for neutron tests. Spurious signals and sources of background through improved counting statistics and have been hunted down and reflectivity pat- reduced systematic errors appears to have The D20 microstrip detector in its mark-Il terns over eight decades of intensity are now been realised. A five times increase in data version is now delivering high quality data to feasible. By the implementation of an intelli- rate is combined with a five times improve- the user programme. The meticulous manu- Millennium programme 82 and new developments all detector channels is complete and since October the user programme has taken off to great acclaim. At the same time as the mark-II detector was installed, the ability to take off at 122° from the monochromator was imple- mented by a long overdue modification to the shielding of the neighbouring instrument. We shall now set in place a programmed sequen- ce of detector maintenance. The new triple-axis spectrometer IN8, an ILL/GSIG Spain collaboration, is showing visible progress on-site. The monochromator drum was received in late September and all monochromator crystals have been spark- eroded at ILL for the 3-faced monochromator itself. First neutrons are expected during summer 2001. The rebuild of the IN5 primary spectrometer is underway. Work commenced immediately Gabriel CuJL, h_ii./ Fisclici and 1'iuit l'alleau during the first tests of D4. following the end of reactor operations in mid-December 2000 with the removal of the present guide and chopper systems and the facturing and assembly process, combined off. First diffraction patterns from standard new focussing guide and chopper pedestals with online diagnostic software to monitor samples including vanadium were taken in are currently being installed. The manufactu- the detector conditions, appears to have paid September and the delicate task of balancing re of the precision discs for the choppers has passed a significant milestone with the suc- cessful machining of four of the aluminium chopper blades. A number of initial manufac- turing techniques had failed to achieve the necessary precision at the circumference of the disc until a cold-rolled (and hence prefe- rentially oriented) aluminium ingot was used. Rather counter-intuitively this has resulted in a disc well within specification and rota- tional tests are to begin shortly. The IN5 upgrade, when completed at the end of 2001, promises an order of magnitude increase in data rate. We are presently loo- king at design options for the secondary spectrometer. A new high-intensity cold neutron guide - HI 13, built in collaboration with the University of Heidelberg - is now delivering neutrons to the rebuilt PF1 machine in the ILL's first guide hall. The guide, with a super- mirror coating and a cigar-shaped profile along its length, is a so-called ballistic guide and delivers an astonishing intensity 80 metres from the cold source of almost 1012n/sec. The new wheel choppers of IN4 (maximum speed: 5500 rpm). Millennium programme and new developments 83 A strain imager at the ILL THILO PIRLING AND STEVEN ROWE (ILL) ried out simulations of the monochromator determine the size of the gauge volume which and the neutron guide design. The specifica- defines the lateral resolution. The stress dis- tions of the monochromator and the guide tribution in the specimen is mapped by scan- have now been defined. During the current ning across the sample. The precise position reactor shut-down, the excavation work for of the scattered peak contains information the experimental area - down to a depth of about the variation of the lattice spacing 2 metres - has begun. (strain e) in the crystalline material, which is As part of the ILL Millennium related to stress by Hooke's law. In principle Programme, ILL is constructing a General design and technical our strain imager is a 2-axis high resolution neutron strain imager, dedicated to aspects of the strain imager powder diffractometer, applied with special the determination of (residual) A well-defined monochromatic neutron beam optics for the definition of the gauge volume [1]. stresses, in collaboration with the with a cross section of typically 1 mm2 pene- University of Manchester and partly trates the sample. Part of the beam is reflec- The strain imager will be located in the first funded by EPSRC. Its construction ted by crystalline planes of the specimen and guide-hall, on the H22 guide, between and commissioning is foreseen to last is imaged on the detector by secondary D1A/DIB and the new VIVALDI instrument. for 4 years, after which time the in- optics. The primary and secondary optics Space is an essential requirement for the strument will become a fully schedu- led ILL instrument. A strain imager is an instrument for the non- destructive determination of residual stresses inside matter. Thanks to the high penetration power of neutrons, both small and large components can be investigated. The technique covers a large range of appli- cations in basic, applied and industrial research and materials testing. Examples include welds, interfaces, coatings, harde- ning, composite materials and fatigue pro- cesses. Both high penetration and high reso- lution are required at the same time to allow high position accuracy for the smallest and the largest sample. The construction of a reliable instrument dealing with such a broad range of applica- tions and specimens is the challenge of the strain imager project, Not only are the technical specifications our concern, but also a user-friendly operation. The project was started at ILL in January 2000 and the design and feasibility studies of \ the overall instrument have already been completed. During the summer we have car- Design study of the new strain imager. Millennium programme and new developments study of large samples. Therefore, the expe- focusing monochromator for the definition of rimental area will be quite extensive sitting the gauge volume. They take advantage of the about 1.6 m below the beam axis. Fig.l divergent neutron beam and focus it on to the shows the overall view of the future instru- sample, shaping a precisely defined gauge ment. A 50° range in the take-off angles for volume. This concept was developed and tes- the monochromator will enable a continuous ted on D1A [1]. To be efficient, the mono- choice of wavelengths between 1.3 and 4.5 A. chromator itself requires a minimum diver- The instrument will slide on air-pads over a gence from the neutron guide. The calcula- marble floor. tions have shown that a super-mirror guide with a relative angle of total reflection of Compared to most of the instruments at ILL, m=2 provides the optimum divergence. a strain imager deals with "real" engineering Fortunately H22 will be replaced by such a components. They can have any shape, but guide starting in 2001. Compared to the pre- they must be very precisely aligned, often in sent situation on D1A, this will provide a gain orientation not easy to handle. in neutron flux of between 3 and 5, depending on wavelength. Our sample table (a Steward platform, or hexapod known from flight simulators for The neutron strain imager is unique in seve- pilots) is ideal for this task. In fact, it is ral respects: not only will it be the first ILL capable of positioning a sample over a large instrument to use a hexapod sample positio- range of tilting angles and translations. ning system, but it will also incorporate a Rotations around any freely defined pivot variable double focusing monochromator - point and scans along a freely defined trajec- the first of its kind at the ILL. tory are possible. It will be possible to load samples up to 1000 kg and position them to The year 2000 has seen a lot of progress an accuracy of 10 [xm on short range. towards the realisation of the instrument. We are looking forward to the collaboration with Lasers and a closed circuit TV camera Prof. Withers and his co-workers (University of system will help us to align any sample in a Manchester), starting in 2001 which will inspire quick and reliable way. and push forward the project tremendously. In order to be able to mount a specimen with its centre of gravity above the centre of the sample table, the hexapod can be displaced on the omega table, using an air cushion sys- tem. A displacement of 60 cm is easily possible, but we are envisaging up to 1 m. This would give us the possibility to mount samples of 2 m length. A feasibility study is in progress. Monte Carlo calculations, performed by Jan Saroun (Nuclear Physics Institute of Rez, Prague), have shown that the optimum design of the monochromator is a double focusing assembly, consisting of bent Ge-crystals. The curvature must be variable in both directions: horizontal and vertical. It has to be optimised for each diffracting angle 26 and it shows best resolution over a rela- tively small 20 range. But this matches well to the needs of strain measurements, since there is only one reflection under investiga- tion at a time. REFERENCES « [1] SEE ARTICLE "NEW BEAM OPTICS FOR THE DIA Radial collimators are best suited to a double STRAIN-IMAGER" IN THIS REPORT (P. 86 ) Millennium programme and new deveropmSnnts 85 Optics for strain imaging: radial collimators or slits? T. PIRLING (ILL) by the collimators. During the year 2000 we d -, have installed a primary radial collimator • = -A©coten dn and tested it successfully. It has led to better A number of neutron strain imaging performance, higher resolution and to more Strain is mostly reported in (is (10r> E) which applications require high resolution reliable results at surfaces and interfaces. is 1 ppm lattice distortion. Hooke's law [1]. These are mainly measurements This will extend the range of applications of connects strain e and stress a. Here the at interfaces or surfaces, and along the neutron strain imaging technique. First general formula for a cubic material: steep stress gradients. Unfortunately, experiments have already been performed at ILL, using the new optics. E vE the available flux of a neutron source -£„- + - sets practical limits to the smallest useable gauge size, and thus to the The principle of strain imaging lateral resolution. Furthermore, the The idea of strain imaging is to determine the Whereas v is the Poissons-ratio and E choice of the beam optics is crucial strain in the crystalline lattice of the mate- Youngs modulus. To determine the stress due to the so-called surface error, rial, from which the stress is then calculated. tensor, measurements in 6 different orienta- which occurs during interface, or sur- The strain s is defined by the variation of the tions have to be performed. In most cases, face scans, when the sample does not lattice spacing d relative to a stress-free the direction of the principal axes can be entirely fill the gauge volume. reference value do. A diffraction method is assumed lo coincide with the principal axes very well suited for its determination. By dif- of the sample. In this case three measure- ferentiating the Bragg equation we get a rela- ments in these directions are sufficient to tion that allows us to determine strain just by determine the complete tensor. Most strain imagers (or scanners) use slit- measuring the variation A6 of the diffracted To understand the difficulties of a measure- apertures for the definition of the gauge volu- peak position referred to its position 8o in a ment and the different error sources, the me. But just where the highest lateral reso- stress free sample: principle of the experiment will be explained. lution is demanded - at interfaces - they Fig. 1 and 2 show a sketch introduce the largest errors. To overcome the of different experimental experimental inaccuracies, the strain imager Neutron guide configurations. The pri- Monochromator mary optics defines a option on D1A has been equipped success- neutron beam with a fully with a secondary radial collimator in specific cross-section, 1997 [2]. Many inaccuracies and inconve- which penetrates the niences could be improved [3]. But the sur- sample and is scattered face error still remained. It lies in the range by the material. The of several 100% to several 1000%! Its cor- secondary optics selects rection is very difficult and uncertain since primary sample a fraction of the scatte- the measuring conditions are not very well slit red beam and images it defined by the primary slit and, of course, big onto the detector. The numbers have to be corrected to obtain rela- secondary intersection of the two tively small values. slit beams is the gauge volu- The work on D1A has shown [4] that the sur- me. The precision of the face-error is mainly caused by the wavelength definition of the gauge distribution in the primary beam, which is volume determines the caused by the monochromator/slit combina- lateral resolution. In the tion. A collimator in place of a primary slit PSD simplest case the optical can equalise the wavelength-distribution and components are just slit therefore reduce the surface error. Only a Figure 1: Set-up of a strain imager, using slits for the definition of apertures as in Fig. 1. At small geometrical error remains which can the gauge volume. Due to the divergence of the neutron beams the slits should be positioned as close as possible towards the sample. ILL we are using radial be analytically corrected, thanks to the per- The gauge volume is diffuse and diverging, even in a 90' scattering collimators, as shown in fect definition of the experimental conditions geometry. Millennium programme 86 and new developments exceed 20 mm. This com- centre of gravity of the effective scattering plicates the alignment gauge volume is moving while the sample is Neutron guide and increases the risk of passing out of the gauge volume. Monochromator collisions during the This leads to a movement of the peak position \ scan. Apart from surface on the PSD, which can be misinterpreted as scans the slits are fur- an angular peak shift. Furthermore the ther away and measure- reflected beam profile is cut by the slits. This ments can only be perfor- leads to an asymmetric peak with a reduced primary med with less resolution peak width. Since the original peak profile - 1111111 f than expected from the can not be reconstructed, an erroneous radial slit dimensions. This is a determination of the peak position and width collimators big disadvantage for the is the result, which creates errors in the secondary investigation of internal stress values. interfaces. Furthermore, The third source of error, the wavelength dis- \ \ohime the slit optics provides tribution in the primary beam, contributes \ wrong values when scan- the biggest fraction [4]. It is caused by the ning along steep stress primary slit: for any point in the gauge IPSD gradients, getting worse volume the slit aperture cuts out a different with increasing distance fraction of the divergent neutrons from the of the slits. monochromator. Since the wavelength is Figure 2: A strain imager equipped with radial collimators for the Especially in the vertical related to divergence, the wavelength-distri- gauge definition. The collimators focus the whole diverging neutrons bution across the beam is not homogeneous. onto the sample, but define a perfectly shaped gauge volume with direction, this error is parallel borders, which is rhombic for any scattering geometry. Due extremely high, particu- In surface scans when the sample passes out to the fixed distance of the collimators, no realignment needs to be larly when a focusing of the gauge volume, the varying wavelength- done after sample changes. The focal length of the collimators - at distribution leads to huge peak shifts. D1A 150 mm - leaves a lot of space for sample movement and sample monochromator is instal- environment. led. But for the efficient Whilst the errors, caused by the first effects use of neutrons this is described are in the range of several 100 ^e, Fig. 2. To be efficient, the detector is a PSD indispensable. On D1A a 200 mm high beam the latter adds several 1000 ^e! A correction (Position Sensitive Detector) since it detects is focussed down to about 15 mm on the is not reliable and neither is it practical, due the whole of a reflected peak at one time. To sample table over a distance of 1500 mm. to changing measuring geometries. perform a complete mapping, the sample is The resulting beam divergence is about 7.5°. scanned. Under these conditions the gauge volume Focusing collimators From this we can deduce the requirements defined by a slit aperture is several times Replacing the slits by collimators has howe- on the experimental set-up: a well-defined bigger than the aperture and quite diffuse, ver many practical advantages. Since their and sharp-edged gauge volume for high late- even at relatively small distances to the focus is fixed, they stay at the same position ral resolution; perfect imaging of the peak gauge. A high-resolution vertical scan is not and no changes have to be made when chan- shape; and high angular resolution for preci- feasible. This is unsatisfactory, because the ging the scan direction or the sample. No se determination of strain and thus stress. appeal of vertical scans is that they are not realignment is necessary and neither does High neutron flux enables either small gauge affected by one of the biggest problems of the experimenter need to worry about the volumes for better lateral resolution to be neutron strain-scanning: the surface error! best location of the optical elements - unlike probed or enables measurements to be made slits. Furthermore, the lateral resolution is deeper inside large samples. Surface error only determined by the collimators. Scans at The surface error is an instrumental effect, interfaces deep inside the material can be Slit optics which occurs during scans in the scattering performed with the same precision as at sur- Using slits for the definition of the gauge plane when the diffracting material passes faces. Since the focal length is quite large, in volume is relatively simple and allows a out of the gauge volume. This is the case at the case of the collimators on D1A it is flexible design because it is easy to adapt the surfaces or at interfaces between two diffe- 150 mm, there is the necessary space for gauge size to the sample. But it has many rent materials. The peak fit provides shifts, sample movements and for large samples. So weak points. which easily can be between 4 and 10 times this technique is very well matched to one of To achieve good lateral resolution, the slits higher than the real variation in the sample. the biggest advantages of neutrons, which is should be positioned as close as possible to Three different effects cause these shifts: their penetration power. Furthermore, the gauge volume. The reason is that on the geometrical position of the centre of gravity, sample environment - like i.e. a furnace, primary side the beam divergence and secon- peak clipping and the wavelength distribution stress-rig, or cryostat - does not allow slits dary the peak width increases the gauge size. across the primary beam. to be placed close to the sample. Collimators leave enough room for the equipment. For good results the distance should not The geometrical shift occurs because the Millennium programme 87 and new developments The gauge size is defined only by the collima- collimator, reflection, 2mrn, 95" tors and does not depend on the beam diver- slit, transmission, 95° gence or peak width [3]. The collimator takes advantage of the beam divergence without collimator,trans., 95° reducing the resolution. So the primary colli- collimator trans. 109° mator can also be used to define the vertical surface gauge height by turning it 90''. Vertical scans can then be performed with the same high resolution as horizontal scans. This is a big advantage, since these scans, as we have said earlier, are not affected by the surface error! But that is not all: the negative effects, descri- bed in the last section, for scans in the scatte- ring plane, are avoided or reduced. Peak clip- ping is totally inhibited by the secondary colli- mator. Peak shape analysis, used for the eva- luation of second order stresses, can therefore be performed close to the surface or interface. Since the primary collimator focalises the whole divergence of the neutron beam coming position / mm from the monochromator onto the sample, the wavelength-distribution inside the gauge volume is equalised. This means, that the mean wave- Figure 3: Through-surface scans of an iron powder sample in different scattering geometries. The Sary slit. All other curves were measured using the two collimators. The diffracting angle was length is constant across the gauge, which, as 95'(Fe(110)) and 109' 20 (Fe(211)). In one case the secondary collimator was approximated to described in the previous section, eliminates the sample so that the gauge size was 2 mm (red curve). The error of the collimator configuration does not exceed the geometrical error. the biggest part of the surface-error. Only the geometrical error remains, since the defined gauge volume, and ensure a constant high combination PSD / secondary collimator is not lateral resolution at any point inside the sample. a totally angle sensitive system. But due to the They leave room for large samples, sample envi- perfect defined gauge volume, and the well-defi- ronment and sample movement. And last but not ned measuring geometry, this error can be cor- least, they simplify the alignment. rected analytically. Radial collimators fit very well to a - horizontal Fig. 3 shows a comparative measurement of a pri- and vertical - focusing monochromator since they mary slit and a collimator set-up. The sample was focalise the whole divergent neutrons onto the an iron powder to make sure that there is no strain sample while at the same time achieving high gradient. Such an experiment shows directly the resolution. We are now optimising the mechanical surface error. In case of a 2 mm wide secondary support for the two-collimator configuration. gauge width and taking into account the detector Unfortunately the space on the D1A diffractome- distance of 800 mm, the pure geometrical error ter is limited. So only small samples up to approxi- should be maximally 604 ue and 360 ixs for a mately 100*100*30 mm3 can be measured in the gauge size of 1.2 mm. This is perfectly verified by new arrangement. They will be mounted on an the measurements. The maximum measured eulerian cradle, equipped with a small xy-table. error of the slit set-up is 1600 UE using a 1 mm pri- For larger samples the primary slit system in mary slit. And this is the optimum slit configuration combination with the secondary collimator will be of DlA Often the error reaches more than 2000 LIE. REFERENCES used, as before. [1] SEE AS WELL "THE ILL STRAIN-IMAGER" IN THIS The gained experience will be applied to the ANNUAL REPORT, P. 84 [2] T. PlRLING, R.C. WlMPORY, Conclusion design of the new strain-imager currently under STRESS MEASUREMENTS ON D1 A: A NEW HIGH PRECISION STRAIN SCANNER, ILL ANNUAL REPORT 1997, 87-89 The introduction of a primary radial collimator on construction. [3] T. PlRLING, A NEW HIGH PRECISION STRAIN SCANNER the strain imaging option at D1A has solved the AT THE ILL, MATERIALS SCIENCE FORUM VOLS 312-324 outstanding problem of the surface error. This (2000) 206-21 1 [4] T. PIRLING, NEUTRON STRAIN SCAN- NING AT INTERFACES: OPTIMISED BEAM OPTICS TO REDUCE makes high resolution experiments possible. THE SURFACE EFFECT; MATERIALS SCIENCE FORUM VOLS 347- Furthermore, radial collimators provide a precise 349 (2000), 207-112 Millennium programme and new developments New 1.8K cryogen free cryostat available to the users • M. DE PALMA, F. THOMAS AND S. PUJOL (ILL) The ILL Sample Environment Service has developed a Joule-Thomson low temperature low cooling power stage to expand the temperature range of commercial cryocoolers down to 1.8 K. Two prototypes have been made. The first one is incorporated into an ILL top-loading cryocooler and therefore, routinely available to the ILL users. The second one is mounted on the four-circle bottom-loading cryocooler Figure 1: Picture of the Joule-Thomson stage of the of the XMAS line, the UK CRG at XMAS cryocooler. ESRF. For the top-loading cryocooler, the set-up is 1 .OLU J. ! ILL_1_ 1,1 l.._ ILL _Jp ljJL...o _IJL_LJ.jr (left) and of the XMAS bottom-loading cryocooler similar except that the liquid-nitrogen trap (right). In foreground, the sample-holder of the top- is not needed. loading machine (length 910 mm). A descriptive of the Joule-Thomson stage is The base temperature is 1.8 K, and the shown in Fig. 1. Special attention has been maximum tilt angle to keep the base tempe- paid to achieving a compact design and a rature below 2 K is 50°. The typical cooling positive, we plan to put more such cryostats base temperature below 2 K. This stage can power is 2 mW at 2 K in a vertical position. into operation since they use less material be added to any commercial cryocooler Further improvements of the devices await resources than an Orange Cryostat, do not which can reach base temperature below users' comments following field tests cur- need frequent cryogenics fluids filling, and 10 K. The overall dimensions of the stage rently underway. Should these tests prove promise greater reliability. are typically 34 mm diameter x 85 mm high. Fig. 2 shows the ILL top-loading cryocooler (left) and the XMAS bottom-loading cryocooler (right). In the foreground, one can see the sample holder of the top-loading machine. This sample holder is very similar to those used in an Orange Gryostat, with the same outer diameter, and an adjustable length with a sample at low temperature. In the photograph, the actual length is 910 mm. Fig. 3 describes schematically the bottom- loading cryocooler set-up. To operate it, one needs only a standard pressurised helium- gas bottle, a liquid-nitrogen trap and a 5 m3 / h primary pump. The working pressure on the condensing line is around 10 bar and the Figure 3: Schematic drawing of the XMAS cryocooler set-up. The Joule-Thomson (JT) stage is added to an APD 3 helium-gas consumption is 4 m /week STP. cryocooler to expand the temperature range down to 1.8 K. Millennium programme and new developments 8 H113 neutron guide w. KAISER (ILL) 4,5E+10 -j e HI 13 (measured) 4,0E+10 •^ miscalculated) If) 3,5E+10 A H53 (measured) The project for a new guide on the 3,0E+10 vertical could source was first propo- sed by P. Ageron, J.M. Astruc and A. X 2,5E+10 '- Robert in 1991. It has been finally rea- lised with supplementary external oQ> 2,0E+10 : funding of the Universities of •** Q. 1,5E+1O : Darmstadt, Munich and Heidelberg CO and delivers by far the highest intensi- o 1,OE+IO r ty cold beam from a guide, exceeding c 2 5,0E+O9 - 10 n/sec. The new H113 guide is desi- 0 gned on a so called ballistic guide. O,OE+OO — The present guide has a divergent 0 10 20 30 40 50 60 70 80 section at its entrance and a conver- Distance (in metre) from Vertical Cold Source ging section at its exit which provides Figure 1: Neutron capture flux measured (filled circles) and calculated (empty circles) along the ballistic super- a very rough approximation to this mirror cold-neutron guide HI 13. For comparison, the capture flux at the present beam station PFl (H53) is also profile. given. The 6 m out-of-pile part length in the swim- and the guide elements were able to be Design, Manufacture, Mounting and ming pool, installed during the July/August mounted without cadmium foils. Flux Measurements 1999 reactor shutdown, is made in 6 mm thick float glass, again with m = 2 supermir- The final part in the neutron guide hall, and In-pile part ror coating, assembled with Aluminium pro- 46 m in length, has been installed during The HI 13 guide starts 2.2 m away from the files. The B2O3 content of the glass to avoid the winter shutdown 1999/2000 with the vertical cold source of the reactor. Over a radiation damage in this case is less than design identical to the 20 m upstream sec- length of 3.7 m the guide is straight and 1% and the Na20 content is about 12 to tion. The guide comes to the end in the prima- slightly convergent with a cross section of 15%. The guide elements are activated by ry casemate of the new PF1B beam facility. 200 x 68 mm2, exit section 200 x 62 mm2 (n,y) reactions in Na and Al. A 0.5 mm thick Flux measurement with gold foils confirm and is coated with Ni58. The in-pile part of cadmium foil around the guide elements pro- the computer simulation [1], the HI 13 guide, mounted during the reactor tects the housings which were made in a shutdown in 1994, is made of borated (19% stainless steel with low cobalt content The installation of this intense neutron B2O3) glass with low soda content (0.4% (~ 0.1%). This guide section penetrates half guide has been a story of determination, Na2O) from Corning, designed and manufac- a metre into the swimming pool door in the which in the end has paid off handsomely. tured by Cilas in Orleans. H1-H2 casemate in the reactor hall up to the The beam flux can be further increased by safety valve. the addition of a multi-bladed mirror assem- Out-of-pile part bly. Amongst other aims, this beam position In the out-of-pile part the guide is designed The succeeding part, 20 m in length, passes will be used to push down further the upper as a "ballistic guide", straight and divergent through the reactor hall and the reactor limit of the electric dipole moment of the over 10 m, expanding to an exit section of shell. The guide elements were installed neutron. 200 x 90 mm2 and curved with R = 4 km during the May 1999 reactor shutdown. over 52 m, then straight and convergent These guide elements were made in 6.5 mm over 10 m to 200 x 60 mm2. It is coated with thick borated float glass from Schott in supermirror m = 2. This section was desi- Mainz, using the same supermirror coating gned and manufactured in collaboration and assembly technology. The B2O3 content is about 12%, and the Na2O content around with the University of Heidelberg and the REFERENCES 8%. The housings downstream from the company Sputter-Diinnschichttechnik, also # [1] H. HANSEN ET AL., NUCL. INSTR. METH. A, 2001, in Heidelberg. safety valve were made from normal steel SUBMITTED Millennium programme 90 and new developments The D17 reflectometer R. CUBITT (ILL) The main requirements for a reflectome- ter are high flux, flexible resolution, accu- rate collimation and precise spatial detection. D17 has all of these proper- ties. This article shows some of the first results taken with D17 using the time-of- flight fTOF) option, including data from the first official experiment. The principal reason for having a TOF option on this instrument was to enable experiments on the kinetics of planar materials to be performed. This is possible because a range of q is measured simultaneously using a white beam as opposed to the monochromatic mode where the sample angle has to be rotated for each point in q. The TOF is less efficient than the monochromatic Figure 1: TOF alignment data taken in 10 seconds. The sample was a S1/D2O interface. The right-hand side shows the transmitted refracted beam and on the left is the specular reflection. mode in terms of the flux available at each q point as the flux at the extremes of usable wave- horizon showing that neutrons, unlike light, have modelling cell membranes. As the water thick- lengths are much lower than the peak flux used a refractive index less than one (for a material ness is largely defined by the first minimum in in the monochromatic mode. Having a chopper with positive scattering length density). The cur- the reflectivity, a limited q-range (0.005-0.05 A' system where the resolution can be continuous- vature of the diffracted beam with wavelength is measurable in 7 minutes) is sufficient to follow ly varied, however, compensates for this as flux due to the fact that the formula for neutron the water thickness through the transition. In the can be gained when high resolution is not requi- refractive index has a wavelength squared term. red. A second advantage is that when setting up experiments it is easy to check the alignment of For the first officially scheduled experiment (July the sample after only a few seconds of data 2000) neutron reflectivity curves were measured acquisition. For example, Fig. 1 shows raw data from adsorbed lipid bi-layers at a silicon/water from counting for only 10 seconds. The vertical interface [1,2]. The lipid layers undergo a phase resolution of the detector has been summed transition from a gel to a fluid phase and it is resulting in a picture with horizontal pixels cor- known that around the transition a substantial responding to 26, and vertical pixels represen- increase of the thickness at the interface is due to ting wavelength. It can be seen that above the critical wavelength where total reflection occurs an increase in the quantity of water between the there is no intensity in the direction of the inco- two bi-layers. The q-range explored on D17 (see ming beam. This means that no neutrons fly past Fig. 2) was larger than that measured in the past the sample surface and this surface is under illu- on this kind of sample as the lowest reflectivity is minated and correctly aligned with respect to the usually limited by background from bulk water incoming beam. The picture demonstrates the and sample cell. With the high flux and wide-angle Figure 2: TOF reflectivity from a lipid bi-layer at 57°C in multi-detector [3], reflectivities of ~W could be the fluid phase in D2O on a Si base. The bottom left insert fact that neutrons obey simple optical laws. It shows the scattering length density profile fitted to the can be seen that below the critical wavelength measured in the reasonable time of one hour. data with the water layer thickness Dw. The top right Analysis is still in progress but the increased insert is the fitted water thickness as a function of tempe- the transmitted beam is refracted towards the rature showing the sharp transitions. Measurement time q-range should give additional information for was 90 minutes. Millennium programme and new developments 91 top-right insert of Fig. 2 the fitted thickness of the water layer between the two bi-layers is given at the different temperatures shown. In addition to the ability to subtract incoherent backgrounds more efficiently there is another advantage in having a large area multi-detector. About the specular reflection there can be off- specular scattering arising from surface rough- ness or, as in the case described below7, low- angle diffraction. The sample wras a diffraction grating consisting of a glass substrate with thou- sands of strips of nickel, 900 A deep separated by 10 um (Ott and Menelle, Saclay). If this gra- ting were placed perpendicular to the incoming beam then 10 A neutrons would be diffracted only a few thousandths of a degree (qx = 6x10' A'). beyond the range of any existing small angle ins- trument. However, if the incoming beam strikes the grating surface at a glancing angle, then many diffraction orders can be seen in both the reflected and transmitted beams at measurable angles of deflection (Fig. 3). In addition to infor- mation on the stripe separation, the depth profi- le (consistent with 900 A) is revealed in the ripples of intensity found running along the spe- cular line and the various reflected diffraction orders. The diagonal line of intensity coming from where Figure 3 : TOF diffraction and subsequent refraction, reflection and interferen- ce from a Ni grating on a glass substrate. Measurement time was 30 minutes. the specular line just totally reflects is a Yoneda wing and is a consequence of the roughness along the surface of the nickel strips. Dl 7 is ideally suited to investigate not just struc- tures as a function of depth but also within the plane such as magnetic grain boundaries or poly- mer droplets. Recently, both the monochromatic and the polarised neutron modes of the instru- ment have been successfully tested. The polari- sation analyser will be installed in the winter shutdown 2000. Useful wavelength range 2-20 A Oz range (normal to a surface) 2x10~3 - 3 A"1 Ox range (within a surface): 1 xl0~6- 2x10"2 A'1 white beam flux 10:CI n/s/cm2 Detector size 250 x 500 mm Detector resolution 1.5x3 mm REFERENCES [1] T. CHARITAT ET AL., EUR. PHYS. J. B (1999) 583 [2] G. FRAGNETO ET AL., EUR. PHYS. J.B., IN PRESS [3] GABRIEL ET AL. J. MACROMOL. SCI. PHYS. B37 (4) (1998)463 Millennium programme 92 and new developments The D4C diffractometer for liquids and glasses H.E. FISCHER (LURE, ORSAY) Detection Plane GJ. CUELLO, P. PALLEAU, D. FELTIN (ILL) Shielding Immediately following its successful Monochromator commissioning tests in May/June 2000, the D4C instrument became operational and welcomed its first Primary users. Although the absence of the Shutter hot source allowed only 0.7 A neu- trons to be used, the first D4C experi- ments confirmed the anticipated per- formance of the new instrument. The scientific motivation of the D4C pro- ject was to increase the overall preci- sion of the former D4B instrument by nearly an order of magnitude, Shielding implying not only a large increase in counting rate to reduce random error, but also a corresponding reduction in Figure 1: Top-view schematic of D4C showing the arrangement of the 9 microstrip detectors and their collimation tubes. systematic error through improve- ments in detector stability and back- placed its two detectors at radii of about 1.5 stability of 2-10"4 over nearly 3 days was first ground counts. Higher instrumental m and 0.75 m (giving 26 widths of 0.1° and measured for the D4C prototype detector as precision was necessary to extend 0.2° per cell, respectively). The 9 D4G detec- compared to only 10-3 for a D4B detector the capacity of D4 to a greater range tors at a radius of 1.146 m (0.125° per cell) (Fig. 3), and then confirmed for the 9 D4C of samples and in particular to new provide a factor of 4 increase in total solid detectors during the May/June 2000 com- elements for which isotopic substitu- angle of detection as compared to D4B's missioning tests. The factor of 5 increase in tion experiments can be performed, usual configuration, amounting to a factor of total counting rate for D4C is thus matched such as silicon and carbon, the latter 5 increase in total counting rate for typical by a factor of 5 increase in detector stability, being useful for determining partial scans due to reduced detector movement time as compared to D4B. structure factors of biological solu- for D4C. For scattering angles of less than Each of the 9 microstrip detectors has an tions, for example. about 50°, the increase in counting rate over angular range of 8° in 29, which is slightly D4B is in fact a factor of about 10. Average larger than half of the 15° fixed angular statistical precision of 0.1 % (per 0.125°) can separation between detectors, allowing the To increase detection solid angle and stability, be obtained in a couple hours on D4C for 10 % execution of a complete 135.5° scan with the D4G project replaced the 2 multiwire scatterers of typical sample dimensions. sufficient overlap via a 7.5° displacement of detectors of the D4B liquids/glasses diffrac- The mechanical stability and other advan- the 9-detector ensemble on an air-suspen- tometer with 9 microstrip detectors in a fan- tages of the microstrip technology [1] permit sion (Tanzboden) surface. This reasonably like array (Fig. 1), and also made upgrades in a greatly enhanced counting stability for quick two-step scan technique allows some neutron shielding, detector collimation and D4C's detectors (Fig. 2) as compared to the real-time diffraction studies to be performed 3 multiwire technology of D4B. Previous tests electronics. Both detector types use He as at D4C. The minimum and maximum scatte- first verified the proper functioning of the detection gas and have 64 cells allowing ring angles for typical scans are about 1.5° D4C prototype detector and its associated position-sensitive detection along one dimen- and 137°. electronics [2] and then finalised the designs sion (the scattering angle 26), but the cells of The D4C detector collimation design assures for detector collimation, beamstops and neu- D4C are 100 mm high as compared to 70 mm that any point on the detection surface sees tron shielding [3]. A detector counting rate for D4B. The usual configuration for D4B the solid angle subtended by the sample volume Millennium programme and new developments 93 ched 10B4C (neutron absorption constant of 160 cm'1 for a wavelength of 0.7 A) and includes a mobile beamstop of '"E^C just before the detector. A second mobile 10B4C beamstop is placed further upstream in the sample bell jar, cutting off about half of the direct beam. This collimation+beamstops geometry prevents direct beam and parasitic peaks (e.g. from the aluminium bell jar win- dow) from striking all 9 detectors for both scan positions and for all three standard wavelengths used at D4 (0.7, 0.5 and 0.35 A). The neutron shielding is effected by 100 mm of thermalising polyethylene, followed by 10 mm of borated polymer, surrounding the 9-detec- tor ensemble. The addition of Gd sheet would be superfluous since tests [3] showed the remaining very small background count rate to result principally from non-thermalised neutrons. Figure 2: The interior of a D4C microstrip detector, showing the front face of the microstrip plate: a glass The independent instrument control systems (Schott S8900) substrate supporting the chromium metallisation of the 64 microstrip detection cells, each 2.5 mm wide and consisting of 2 anodes and 2 cathodes. A depth of 30 mm for the 3He detection gas at 15 bar pro- for D4C and D4B facilitated a rapid instru- vides a detection efficiency of about 90 % at 0.7 A and 69 % at 0.35 A neutron wavelength. Electrically polari- ment changeover prior to the D4C commis- sed guard plates at the four sides of the microstrip plate assure a uniform drift field and thus an undistorted dif- sioning tests (0.7 A wavelength neutrons fractogramme. only). Further tests at 0.5 and 0.35 A will be to be surrounded by neutron-absorbing B4G visible to both the detectors and to the made once the new hot source is installed. plates placed ring-like within the evacuated sample or direct beam, thus reducing back- On-line software at D4C now provides for collimation tubes of each detector [4], A ground counts to a minimum. To further data normalisation and regrouping into dif- series of collimation rings is the most effecti- reduce the background from direct-beam fractogrammes, the calibration of detector ve way to minimize the (partially neutron-dif- scattering at the lowest angles, the first cell efficiences, the monitor deadtime correc- fusing) collimation surface area that is detector's collimation uses isotopically enri- tion, and for most other aspects of data reduction and analysis. 0.002 Thanks and acknowledgement to participants 0.0015 in the D4C project, including: Gilbert Viande, Steve Rowe, Jacques Loppe, 0.001 Loic Pajou, Roland Gandelli, Jo Pellegrino, Othman Jeddi, Pierre Thomas, Jean-Louis 0.0005 Champon and Anthony Pascaud, Jacques Munnier, Jean-Jacques Vernier, Raymond Rouques, Bruno Guerard, Michel Gamon, Franck Cecillon, John Allibon, Marie-Claude •0.0005 ti Filhol. -0.001 -0.0015 -0.002 REFERENCES 30 40 [1] A. OED, NUCL. INSTR. AND METH. A 367 (1995) 34; time (hours) A. OED, NUCL. INSTR. AND METH. A 263 (1988) 351. Figure 3: Stability of the D4C prototype microstrip detector (upper curve) as compared to that of a D4B multi- [2] H. FISCHER, ILL ANNUAL REPORT 1997, p. 85. wire detector (lower cun'e) when counting simultaneously on a vanadium sample (counts integrated over all 64 [3] H.E. FISCHER, P. PALLEAU AND D. FELTIN, ILL REPORT cells in both cases). Taking into account the counting statistics error bars, we estimate the stability of the D4C NO. ILL98F115T, 17 NOVEMBER 1998, 99pp. prototype detector to be 2-104 over nearly 3 days, as compared to 103 at best for the D4B detector. The solid [4] H.E. FISCHER, P. PALLEAU AND D. FELTIN, PHYSICA B curves serve as guides to the eye. 276&278 (2000) 93. Millennium programme 94 and new developments USANS: Ultra Small-Angle Neutron Scattering on S18 O M. BARON, M. HAINBUCHNER, R. LOIDL, The inset of p.96 shows the types of samples that H. RAUCH (ATOMINSTITUT, WIEN) have been studied so far. The measurements with the SiC/SiCf ceramic composite material will be described in some Bonse-Hart small-angle scattering more detail. This material is considered to camera with channel-cut perfect become an alternative first wall material for crystals has been installed at S18 to fusion systems [5]. The composite material extend the accessible Q-range by at has better ductile properties than pure SiC, least two orders of magnitude. This but its gas permeability is still questioned, permits investigations of rather large because unavoidable pores cause gas pene- structures as they exist in composed tration and gas emissions. The behaviour of and micro-structured materials, in these pores under different conditions - sedimentary rocks and macromole- thermal, radiation and gas atmospheres - cular systems. A substantial improve- can be studied by USANS. Fig. 3 shows the ment of the peak/background ratio different USANS scattering patterns for two makes the new instrument applicable different SiC/SiCf samples thermally treated to many new topics. Figure 1: Sketch of the ultra small-angle neutron at 1750 and 1900 "C. The signal is related to scattering system installed at the SI 8 neutron interferometer set-up. the size of the pores. New SiC/SiC material is under development, which should be even Configured as a double Silicon crystal diffrac- the measurable Q-range by more than two more resistant against temperature, stress tometer the CRG(C) instrument SI8 (neutron orders of magnitude and permits together with and radiation effects. interferometer) can be operated as an ultra the SANS technique the measurement of the small-angle scattering instrument. In this scattering pattern over ten decades in scatte- The USANS technique probes the microsco- mode of operation a momentum transfer range ring intensities. Inhomogeneities from 5 nm to pic structure of various materials and contri- between 2.105 - 5.103 A'1 can be covered. In 30 i^m can now be investigated. Fig. 2 shows a butes to the future material developments. 2000, users from five different countries have typical combined SANS (D22) and USANS (SI8) The intensities can be converted to an abso- used this new method for the structural inves- scattering pattern of a sedimentary rock lute differential cross section. Thus, USANS tigation of minerals, composite, plastic, and sample. The two scattering curves show a clear is an adequate extension to SANS measure- micro-structured materials and cement pastes overlap in the region around 1O"3A'1 and the frac- under different sample conditions. ments and should therefore become a stan- tal scattering behaviour can be recognised Channel-cut perfect silicon crystals in a Bonse- dard method for material science. obviously over 3 orders of length scales [4]. Hart arrangement [1] are used as monochro- mator and analyser crystals. A so-called Agamalian-cut (an additional groove is cut into the long crystal plate where a cadmium absor- 10 - •— j - ber is inserted [2]) reduces parasitic reflec- -•- USANS S18 D=200|jm tions from the back-surfaces of the crystals -:> SANS D22 D=200|jm Modelfit Ds=2.68 §=9.5|jm and provides a peak to background ratio better M. 10- than 105 [3]. The use of a perfect crystal as monochromator provides a very small wave- r^^^-s. length spread leading to a low intensity of the reflected neutron beam. Therefore the peak 1 2 intensity is about 6000n/s, which corresponds ° ~ directly to the luminosity of the source becau- se nearly perfect reflecting components exist between the source and the detector. The 10" 1O'1 schematic layout of the instrument is shown in q [A'1l Fig. 1. Test measurements showed a smooth transition of the scattering pattern measured Figure 2: Combined scattering pattern of a Vosgian sandstone (200 mm) measured at D22 (SANS) and SI8 with SANS and USANS. Thus USANS extends (USANS). The fractal scattering behaviour manifests itself over more than three orders of magnitude in length scale with an upper fractal size limit of 9.5 yon. Millennium programme and new deveropmerits 95 • SiC/SiCf: composite materials: co-ope- 0s ration with L. Gobert, SEP, Bordeaux, France and with Euratom o4 2 • MHPC samples: co-operation with \ - H. Yogel, University of Freiburg, Germany f)1 N3-1 as received • Petroliferous sedimentary rocks: co- -O- N4-1 as received -O~ N4-1 1750°C60min operation with A.P. Radlinski, Geological -z- N4-1 1900°C60min Survey, Australia • \IC\F biological samples: co-operation Q[A'] with H. Amenitsch, Elettra Trieste, Italy • XbTi Superconductors and Portland Figure 3: USANS scattering pattern of thermally treated N3-l(a) and N4-(b) SiC/SiCf compounds compared to the untreated specimen. Structural changes caused by the heat treatment can clearly be obtained. This material cement pastes: co-operation with is considered as a new first wall material for fusion machines. S. Mazumder, BARC-Bombay, India • Plastic samples with silica fillers: co- operation with R. Triolo, University of REFERENCES o [ i ] u. BONSE, M. HART, APPL. PHYS. LETT. 7 (1965) 238 Palermo. Italy o [2] M. AGAMALIAN, G. D. WIGNALL, R. TRIOLO, J. APPL. • Artificially structured materials: co-ope- CRYST. 30 (1997) 345 O [3] M. HAINBUCHNER, M. VILLA, ration with Siemens, Munich, Germany G. KROUPA, G. BRUCKNER, M. BARON, H. AMENITSCH, E. SE1DL, H. RAUCH; J.APPL. CRYST. 33 (2000) 851 o [4] A.P. RADLINSKI, E.Z. RADLINSKA, M. AGAMALIAN, G.D. : silicon carbide fibre material WIGNALL, P. LINDNER, O.G. RANDL, PHYS. REV. LETT. 82 ' methyl-hydroxy-propyl cellulose (1999) 3078 O [5] A.S. PEREZ RAMIREZ, A. CASO, L. GIANCARLI, N. LE BARF, G. CHAUMAT, J.F. SALAVY, J. ' mobil oil composite material SZCZEPANSKI, J. NUCL. MAT. 233-237 (1996) 1257-1261 Inelastic spin-echo on IN20: first experiments E. FARM, J. KULDA, C.M.E. ZEYEN (ILL) far from phase transitions, is a long-standing cession coils (Fig. 1) inserted between the problem. While light scattering can provide monochromator, sample and analyser tables, B. FAK (ISIS FACILITY, UK) such information on optically active phonons respectively. Their optimised field shape near the Brillouin zone center, the extraction (OFS) guarantees a negligible cross-talk bet- of similar information from neutron spectro- ween the incident and scattered beam pre- We have employed the spin-echo metry data for phonons at arbitrary positions cession fields up to their maximum values of option of the IN20 three-axis spectro- in the Brillouin zone is almost impossible. 1.4 T. Two pairs of spiral Fresnel coils are meter to measure the line-width of the Analogous problems are encountered also in used to maintain a homogeneous field inte- roton excitation in superfluid helium other fields like magnetism or physics of and to study the anharmonic beha- quantum liquids. Much improvement to this viour of a zone-boundary 'transverse situation is expected from the combination of acoustic' phonon in germanium. thermal beam three-axis spectrometers (TAS) As the present results demonstrate, with the neutron spin-echo (NSE) technique information on line-width and to access the momentum and energy transfer frequency variation in the range of ranges of several A"' and several tenths of 0.001 - 0.1 meV can be obtained for meV and to achieve an energy resolution of excitations with energies up to 10 meV. the order of 10° -10' LieV at the same time. The access to experimental information on The TASSE option of the IN20 polarised ther- weak anharmonic effects in lattice dynamics, mal beam three-axis spectrometer is one of such as thermal variation of phonon frequen- the prototypes of such a set-up [1]. It com- Figure 1: The three-axis spin-echo (TASSE) preces- cies and their natural line-widths in crystals prises two 1.6 m long superconducting pre- sion coils mounted on IN20. Millennium programme 96 and new developments gral value of up to 1 Tm across the whole 10" beam cross-section. The maximum Fourier - •? - -•_ time of 3.2 ns for neutrons with k= 2.661 A1 at;- provides for a quasi-elastic resolution about Q_ 10 neV as verified in the quasielastic HI co 0 elastic scattering experiments on a KDCO3 crystal [1]. z a. + 0.5 K \ •• • Present Results 10" * 1 K o IN10 For the first inelastic scattering tests, we v 1.1 K LK A 1.2 K have chosen the roton line-width in super- ^k '••,-. - - BPZ fluid helium, which has been studied extensi- 0.2 0.4 0.6 0.8 vely by neutron spin-echo [2] and back-scat- Fourier time (ns) tering [3] experiments. The IN20 TAS instru- 1 ment with kf= 2.662 A" has been set up for Figure 2: Decrease of the NSE contrast as a function of Fourier time for elastic scattering and for inelastic roton 4 the momentum (Q = 1.925 A'1) and energy signal in superfluid He (a) and the resulting roton linevvidth (b) compared to the previous INIO results [3] and to the theoretical predictions. {AE= 0.743 meV) transfers corresponding to the roton minimum in the elementary excita- 157 (15) (xeV and 266 (5) ^eV, respectively. ted by the popular quasiharmonic model. The tion dispersion curve. The amplitude and In absence of a more involved analysis we fit of our low-temperature data in Fig. 3 phase of the NSE signal have then been mea- may assume that the low-temperature intrin- results in the same parameter values as the sured as a function of the Fourier time for sic line-width is negligible and that the fit of the high-temperature (300 -1000 K) several temperatures [4]. Fig. 2 displays the observed value is entirely due to the non- data in Ref. [7] and proves that, even at low results concerning the line-width being in linear terms beyond the condition (1). The temperatures, the quasiharmonic model is excellent agreement with the previous work [3]. room-temperature intrinsic phonon line- not applicable. That finite width observed for the roton at width would then amount to the difference T = 0.5 K is of instrumental origin and goes 110 (20) |ieV of the two observed values. back to the condition to obtain an inelastic echo signal, which is not given by equal spin Fig. 3 displays the temperature dependence precession phases in the two coils, but rather by a phase difference stationary with respect to the energy transfer [5]. In the linear approximation the latter condition leads to where Millennium programme and new deveropmSnts 97 Spin-dependent neutron scattering length of 3He: a precise determination on INI 5 O. ZlMMER, W. KETTER poor precision. Its real part, Re(/w), in the sample as a consequence of the spin-depen- (UNIVERSITY OF MAINZ) past could not be determined to better than dent strong neutron-nucleus forward scatte- C G. EHLERS, B. FARAGO, H. HUMBLOT 25% (fev denotes the difference of the triplet ring. Its observable effect is a precession of (ILL) and the singlet scattering length). An impro- the neutron spin around the axis of nuclear 0 R. SCHERM (PTB BRAUNSCHWEIG) ved knowledge of this value would be of great polarisation on propagation through the help for the interpretation of existing inelas- sample [1]. We have applied a new method to tic neutron scattering data, in order to reveal The pseudomagnetic precession angle jpm is measure the poorly known spin- to which extent observed excitations in the related to Re(b,\) via dependent neutron scattering length quantum liquid originate from spin or density of 3He. The obtained result is of impor- fluctuations. A second motivation to measure tance for the interpretation of the this quantity is to test nuclear where ~k is the neutron wavelength, P the quantum liquid 3He, and for the few theory. In fact, the nuclear four-body problem nuclear polarisation, N the number density of body theory in nuclear physics. The can be described with a relatively simple the polarised 3He nuclei, and d is the method relies only on relative measu- model, whose predictions can be tested thickness of the sample. rements of neutron intensities without through experimental results. any absolute calibration. It employs A second type of experiment serves to deter- polarised gaseous 3He and the uncon- 3He is a strongly spin-dependent neutron mine the proportionality factor between q>pm ventional use of neutron spin echo absorber with a huge capture cross section and Re(ftv). Here the trick is to obtain the spectrometry. for slow neutrons. At first glance a precise whole product XPNd from relative measure- measurement of Re(ft,v) thus seems ambi- ments, rather than from absolute measure- tious. However, through the new method des- ments of its four individual factors. cribed below, the well known cross section for :! Compilations of neutron scattering lengths neutron capture of He turns out to be the Using non-polarised neutrons, we can for the different isotopes are very useful for only required information needed to measure the transmission of the polarised anyone who employs neutron scattering to determine Re(fov). sample, T(P), and of the non-polarised study structure and dynamics of condensed sample, T(0). Their ratio is given by matter. So far, scientists who study excita- Our method employs two types of neutron R = J\P)_ = CQshrK _ xpNcf[ tions in liquid :!He had to live with the disap- transmission experiments. The first one is a T(0) pointing fact that the spin-dependent measurement of the "pseudomagnetic field" Kck = oc is the cross section for neutron :! : scattering length of He was only known with which can be attributed to any spin polarised capture by TIe. Kc= 2.963(5) fm is constant Precession coil Precession coil 2 Sample beam Polariser Analyser \ poiorised 3He sampie Figure 1: Beam configuration and sample position within the spin echo spectrometer INI 5. The dotted lines indicate the two iron shims used to depolarise the beam for the measurement of R. In all measurements we related the neutron intensity through the sample to a "reference beam" passing by the sample. Common drifts in both beams thus are strongly suppressed (this two-beam technique was also employed in a prior measurement of the scattering length densities of SiBe, quartz and graphite). Millennium programme 98 and new developments signals obtained with and without nuclear -— beam through sample polarisation respectively. In the example '— reference beam 30000 shown, q> is about 50 degree. The measu- polarised sample pm R ft ring time for each pair of signals was 20 min. 20000 f\ h This should be compared to the drift of the phase difference between the two beams 10000 (less than 0.05 degree during 12 hours), and to the spin relaxation time of the 3He samples V I " (120 hours). Note that the spin-dependent neutron absorption only reduces the amplitu- 30000 - non-polarised sample de of the spin echo signals but does not affect cppm- o O ft h 20000 Fig. 3 shows preliminary results for Re(£w), measured with four 3He samples of different gas pressure and at two different neutron 10000 wavelengths. For the final result, which will be published by the time of appearance of 0 this report, we expect a precision of better than 0.3 %. AB, (arb. units) Figure 2 : Spin echo signals with the sample polarised, respectively, non-polarised. ABi indicates the sweep of We note that pseudomagnetic precession in the magnetic field in the first precession coil (see Fig.l) which results in the intensity oscillations. The pseudo- the past has been employed to determine magnetic phase shift, caused by the nuclear polarisation of the 3He, is visible in comparing the upper and the lower figure. Re(£w) of many isotopes [3]. In our present study we have used for the first time a spin because for slow neutrons ac is proportional sample, which completely depolarised the echo spectrometer for this purpose. The to X. Obviously, it is the only absolute value neutrons. Fig. 2 shows the first signals mea- combination with non-polarised neutron one has to know. Its relative uncertainty of sured with polarised neutrons (shims taken transmission results in a value of Re(£w) on only 0.17 % limits the precision with which out). According to the spin echo principle [2], an excellent level of precision. The quality of one can determine Re(£w) using the two the lowest intensity minimum intensity mini- the data obtained is due to the stability of IN 15, equations. Of course, we felt challenged to mum corresponds to equal numbers of neu- the quality of the polarised 3He cells of the ILL, and a method which avoids any absolute calibrations. Ii§ , --5.60- Acknowledgement: We are very grateful to the Directors of the -5-55" ILL for giving beamtime on their discretion to this experiment. One of us (W.K.) was sup- ported by a DFG grant (contract number OT > -5.50- 33-17-1). 1st sample 2nd sample -5.45- 3rd sample 4th sample 0.5 1.0 1.5 2.0 2.5 Sample opacity Figure 3: Preliminary values for Re(MV). The abscissa is the so-called sample opacity, defined as ocPNd. The nuclear polarisation can easily be reduced in steps, applying a depolarising magnetic field gradient for a short 1 moment. Thus we could obtain several values with a single sample. The independence of the values of ReffiA ), REFERENCES measured under various conditions, demonstrates the reliability of our method. 0 [1] A. ABRAGAM AND M. GOLDMAN, NUCLEAR MAGNETISM: ORDER AND DISORDER, CLARENDON PRESS, OXFORD 1982, approach this limit in real measurements. tron spin precessions in the first and the CHAPTER 7 6 [2] F. MEZEI, NEUTRON SPIN ECHO, LECTURE Fig. 1 shows the experimental set-up. The second coil. The polarisation of the sample NOTES IN PHYSICS 128, SPRINGER, BERLIN 1980 9 [3] H. GLATTLI AND M. GOLDMAN, METHODS OF EXPERI- measurements of i?were performed with thin adds the pseudomagnetic field to B2. We can MENTAL PHYSICS, VOL. 23, PART c, ACADEMIC PRESS, NEW iron shims inserted before and behind the extract cppm as a phase shift, comparing the YORK 1987 Millennium programme and new deveropmSnts 99 Spin precession optics on the INI 5 spin-echo spectrometer A. FRANK, A.V. KOZLOV, I.V. BONDARENKO (FLNP, DUBNA) G. EHLERS, P. H0GH0J (ILL) The Larmor precession of neutron spins has been used as a clock to mea- sure the delay in neutron time-of-flight caused by different neutron transit velocities in vacuum and in matter, res- pectively. The bound coherent scatte- ring length density has been measured for a number of materials with an accu- racy of a few percents. The absolute accuracy of the measurement of the neutron's delay time in matter achieved Sq> =— (n-l)coL {3 - 5) x 1Ol0s, which will make it pos- v sible in the future to perform many new experiments on the interaction time of the neutron with different quantum objects. It was found that the diama- gnetic properties of the samples, espe- Figure 1: Scheme of the experiment. 1-velocity selector 2- polariser, 3- it/2 flipper, 4- precession coils, 5- it flip- cially pyrolytic graphite, affect the per, 6- analyser, 7- position sensitive detector, 8- multi-layer monochromator, 9- position of the diaphragm with results remarkably. the sample holder (shown in the lower part). Am Aq> Acp during the interaction itself, i. e. the total time- For A{ we can write At = — = n—— = %-—, of-flight also changes due to this interaction. (Dh 2/JB At Neutron spin-echo spectrometry is based on This delay time can be measured by the Larmor where A£ = 2\iB is the Zeeman energy split- the comparison of neutron velocities before clock in slightly modified experimental condi- ting. This may be compared to the Bohm- and after scattering by the sample under inves- tions, by placing the sample inside the preces- Wigner formula At=h—^ , obtained from wave tigation. On its way to the sample, the neutron sion path. Then the neutron will precess during packet arguments [1,2]. This is the approach passes through a precession coil that provides the interaction at the Larmor frequency used for theoretical investigations of neutron a permanent magnetic field which is orthogo- mL = —— and a delay time At will introduce an optical phenomena in the presence of a magne- nal to the spin direction. Another identical coil additional precession phase Aq> = At •a>Lwhich tic field. The effect of multiple frequency neu- is located behind the sample. The change of the can be measured in the experiment. Here ,u is tron spin precession in Bragg diffraction [3] neutron velocity due to inelastic scattering in the neutron magnetic moment and B is the and an additional spin precession angle in neu- the sample results in a different time-of-flight magnetic induction. tron wave refraction were theoretically predic- through the second precession coil and hence In terms of stationary quantum mechanics the ted long ago [4,5]. causes a difference of the total precession state of a precessing neutron is interpreted as Experimental activity in neutron spin preces- phases. In fact, the Larmor precession of the the interference of two coherent waves with sion optics started some years ago [6]. In 1999 neutron spin is used as a clock to measure the different spin projection onto the magnetic field we demonstrated that the unique characteris- neutron velocity. If the neutron-sample inter- vector. Since in a magnetic field the two spinor tics of the INI5 spin-echo spectrometer can action is purely coherent and elastic, as in all components have different wave vector contribute to this kind of experiment [7]. Here kinds of neutron optical phenomena, only the 2 we report on some recent results obtained K =/co(i+—) , with ko the neutron wave direction of the neutron's A-vector changes but vector in the'absence of a field and E its kinetic during a second experiment [8]. not its absolute value. It might appear therefo- energy, a neutron-sample interaction will result For a neutron passing through a samplcessie witoh re that in this case there is nothing to measu- in different phase variations for both waves. phasrefractive mae y indebe expressex n the dadditiona as l precession re by spin-echo. But this is not exactly correct. This gives an extra precession angle Acp which where rfis is the sample thickness and vis the The point is that the neutron spends some time can be measured in the experiment. neutron velocity. This leads too «t, because Millennium programme 100 and new developments B: pb = 7.2x10"6 A~2, which has to be compared to the table value pb = 7.5x10'6 A~2. Actually, even for silicon and beryllium (much smaller susceptibility) a term linear in wavelength can be observed. CD T3, The future perspectives of Larmor clock appli- cations should be discussed from (at least) two points of view. As noted above, the additional CD CO phase in refraction is proportional to {1-ri)d, CO which is actually the difference between the geometrical and optical paths. This could be used for the development of a phase contrast imaging technique in neutron optics [10]. Due to the A,3 dependence of the phase shift it is prefe- rable to use very slow neutrons. Note that we 0 4 8 12 16 0 10 20 30 40 50 were able to measure the very small phase shift introduced by a 4 mm thick quartz sample using Sample Thickness (mm) Sample Thickness (mm) 17 A neutrons: « A. law. We explain this effect by © [3] V.G. BARYSHEVSKY, JETP LETT, 33 (1981) 74 © [4] A.I. FRANK, NUCL. INST. METH. A284 (1989)161 the large and anisotropic diamagnetic suscep- was measured in a magnetometer at tibility of pyrolytic graphite, which has been © [5] V.G. BARYSHEVSKY, S.V. CHEREPITSA, A.I. FRANK, Laboratoire Louis Neel, Grenoble, using a PHYS.LETT. A1S3 (1991) 229 © [6] N. ACHIVA ET documented in the literature [9]. The magnetic small sample that was cut from the one used in AL., J.PHYS. SOC. JPN. 65 SUPPL. A (1996) 183 induction B and, consequently, the Larmor 8 [7] A.I. FRANK, ET AL. PROCEEDING OF THE ISINN7 the neutron experiment. This resulted in CONFERENCE, DUNBA, 1 999, PP. 126-128. S [8] A.I. FRANK, frequency inside the graphite is significantly 3 X = (-0.574±0.003) x 10- for the susceptibility ET AL. PROCEEDING OF THE PNCM-2000 CONFERENCE, smaller than outside. This gives a negative measured with the external field parallel to the GATCHINA, JUNE 2000. To BE PUBLISHED IN PHYSICA B contribution Odia to the phase difference which © [9] LANDOLT-BORNSTEIN NUMERICAL DATA AND c axis, and a huge anisotropy: x± = X»/23 is proportional to the time the neutron spends FUNCTIONAL RELATIONSHIPS IN SCIENCE AND TECHNOLOGY, (field perpendicular to c), thus giving a reaso- GROUP II, VOL. 16, EDITED BY K.-H. HELLWEGE AND A. M. in the sample and therefore proportional to nable agreement with the INI 5 experiment. HELLWEGE, SPRINGER, BERLIN, NEW YORK, CHAPTER 9.1, wavelength: Millennium programme and new developments l101 Magic mirrors demonstration on IN10, during the Open Days Remembering the good old days: Diana Dijoux with John White, in Mav 2000. the British ILL director from 1975 to 1980. Ross Stewart (left) and \ndie\\ Wildes (light) singing goodbve to From the left: Pierre Comcrt, Michel Molher and Olmer Isnard Heima Buttnei on the da\ of hei Ieaung pot in Januaij 2000. enjoying the break during the Journee D4, in September 2000. Workshops 102 Workshops Workshops in which ILL was a major player in the year 2000: Dynamics in Confinement organised by Bernhard Frick (ILL), ILL 26-29 January Reiner Zorn (Jiilich) and Herma Biittner (ILL) Monte Carlo Simulation of organised by Ian Anderson, ILL 31 January Neutron Spectrometers (SINS) Ron Ghosh and Emmanuel Farhi (ILL) European Neutron Polarisation organised by Francis Tasset ILL 24-25 February Initiative (ENPI) and Trevor Forsyth (ILL) Neutron characterisation organised by Peter Lindner ILL June of Industrial Materials and Albert Wright (ILL) Journee D4 organised by Pierre Palleau ILL 15 September and Gabriel Cuello (ILL) Workshop on Correlated organised by Christian Vettier (ILL) ILL 30-31 October Fermions and Yves Petroff (ESRF) Ultracold Neutron Anomalies organised by Peter Geltenbort, ILL 25 November James Butterworth and Thomas Brenner (ILL) Workshops 103 International Workshop on very different methods and different types of Monte Carlo Simulation of 'Dynamics in Confinement' samples. Furthermore, it provided a complete Neutron Spectrometers review of the experimental and theoretical (SINS) More than one hundred participants, at least situation. The organisers of the workshop have encouraged some participants to consider half of them not being neutron scatterers, The SINS workshop reviewing Monte Carlo inelastic neutron scattering methods as an attended the 4-day international workshop on (MC) methods under development in Europe appropriate investigation tool. Dvnamics in Confinement on 26 January 2000. was held at the ILL on 31 January 2000. The aim was to bring together groups developing different simulation packages, potential users and component developers. About thirty-five scientists from the ILL-ESRF joint site atten- ded the meeting. Recent advances have shown that the accurate simulation of neutron instru- ments enables the optimisation of the instru- ment configuration: accurate determination of good resolution functions is vital for data ana- lysis. Furthermore, it is vital in studies for ins- trument improvements or new designs. The meeting was opened by a general intro- duction and an overview of the important points to be considered in a MC neutron ray- tracing programmes. The rest of the meeting was then devoted to talks describing the diffe- rent packages available: - the VITESS programme (HMI, Berlin), initial- ly designed to simulate spallation source in- Reiner Zorn, Herma Biittner and Martine Espitallier at the poster session of the workshop 'Dynamics in struments is now available for various opera- Confinement'. ting systems. The presentation included the demonstration of an attractive Graphical User The meeting - held in the Chadwick amphi- theatre of the ILL - focussed on the changes of dynamics due to confinement in relation to bulk properties, especially in supercooled liquids and glass transitions. The scope of the work- shop has been broadened by adding classical topics such as small molecule dynamics in confinement, adsorbates and fringe topics where the importance of confinement is less obvious. The workshop was structured into 7 sessions: "Theory and simulation", "Liquids and Glasses in Confinement" , "Adsorbates, Atoms & Molecules in Confinement", "Quantum Liquids in Confinement", "Water in Confinement", "Biological, Colloidal and Polymeric Confined Systems" and "Confinement in Thin Films". Besides the 13 invited speakers a further 26 speakers from different countries presented results from experiments, simulations and theory. There were also two poster sessions, where an addi- tional 42 contributions were presented. The sessions were well attended and lively dis- cussions took place. The meeting brought toge- ther many scientists working in this field using Jan Saroun and Thomas Hansen debating the merits of ResTrax at the SINS practical demonstration session. Workshops 104 Interface for both simulation design and usage; - the McStas package (Ris0) has a source code, including instrument design and visualisation tools, now available for most platforms. As an example of application, a single crystal diffrac- tion pattern simulation was highlighted; - the ResTrax (NPI and ILL) programme, opti- mised for simulating triple-axis spectrome- ters, has recently also proved very useful for simpler two-axis problems. However, at pre- sent ResTrax only runs on a limited number of systems; - lastly, the possibility to adapt the Shadow X-ray tracing simulation program (ESRF) to neutrons was presented. This evolution seems quite promising, but still requires substantial effort. The meeting concluded with most of the parti- cipants having an opportunity to debate the various merits of each programme with the authors during a practical demonstration ses- Thomas Briickel (Ju'lich) and Roger Eccleston (ISIS) in a friendly discussion during ENPI meeting. sion. Despite the limited time available the workshop proved really useful. Further infor- The meeting started with the scientific session, meeting was to provide a forum for all those mation on the workshop as well as some biblio- opened by an introduction to the network and concerned to meet and to discuss how joint pro- graphic references can be found at the follo- which was then devoted to talks from each of jects using these advanced resources could be wing address: the partner laboratories on their current roles launched. The industrial and public sectors http://ww.ill.fr/tas/mcstas/workshop0100.html. and future projects within the network. The were equally represented both in terms of spea- day was rounded off with a very pleasant work- kers and participants. ing dinner at the Chateau de Sassenage. The The first morning session was devoted to a brief European Neutron second day was devoted to the more manage- introduction of the ILL facilities and to the pre- Polarisation Initiative rial aspects of the network. The organisation of sentation of experimental techniques relevant (ENPI) the project was discussed and the programme to industrial research, such as small-angle neu- until the next meeting in June at PNCMI'2000 tron scattering, neutron diffraction and neutron was decided. The co-ordinators were then The first ENPI meeting was held at the ILL on reflectometry. The second morning and the given guided tours of the Neutron Optics 24 and 25 February 2000. ENPI is a network of afternoon sessions turned to contributions from Laboratories before the meeting ended. For eight laboratories co-ordinated by the ILL. It is ILL users on specific research applied to real further information refer to the ENPI web-site funded over three years to the tune of 0.9 M € industrial materials. Speakers outlined the use- http://enpi.vitamib.com. by the European Commission FP5 programme. fulness of the small-angle neutron scattering The primary objectives of ENPI are: technique in such fields as surfactant science for the development of detergents, the study of • to improve and make widely available tools Neutron characterisation polymer blends, fuel additives and latex disper- for neutron polarisation; of Industrial Materials sions, the study of filtration processes and food • to equip instruments at European facilities science. Further contributions underlined the with such tools; A one-day workshop on the Characterisation of use of neutron diffraction in the characterisa- • to train young scientists and technicians in Industrial Materials using Neutrons was held at tion of natural and industrial fibres, in the using them. the ILL on 29 June. One goal of this workshop observation of hydrogen bonding in water-sen- was to bring together research scientists from sitive fibres such as cellulose, and in the appli- The ENPI network has stemmed from the highly publicly-funded applied research laboratories cation of neutron diffraction for studying active successful XENNI programme for neutron instru- and those from private industrial laboratories. porous materials such as zeolites and catalysts. mentation and is one of several EU neutron scatte- Participants were shown how neutron tech- Finally, the neutron strain scanning technique ring initiatives in FP5. These include the PECNO, niques are currently being used in pre-compe- was presented as an ideal method for the control of linear friction welding techniques TECHNI, SCANS, Vesuvio and Cool Neutrons titive applied research and confidential pro- now being developed programmes and the Neutron Round Table. prietory research. The other purpose of this Workshops 105 of the detection stability by a factor of 5, to become 2x10"' over several days. The project was completed within budget, in time and to specification. A half-day workshop, the "Journee D4", was held on 15 September to celebrate this suc- cess. This inauguration brought together the previous D4 instrument responsibles, as well as a number of "regular" D4 users. The work- shop was divided into two sessions, the first being opened by Pierre Chieux, who presented the history of the D4 diffractomeler, followed by Pierre Palleau and Dominique Feltin, who described the technical and instrumental aspects of the diffractometer up-grading (neu- tron shielding, detector collimation, microstrip detector design and operation). The second session was devoted to the presentation of the results from the D4 tests (Henry Fischer, LURE) and the results of the first scheduled experiments (Frangoise Leclercq, Lille, and Adrian Barnes, Bristol). A big hurrah to the D4-team! Roberto Coppola (Frascati) and Albert Wright discussing the use of neutrons in materials for the future: plastic! for the production of the next generation of tur- bine blades for air engines. The examples from applied research presented during the meeting are indicative of the broad range of applica- tions for which neutrons are now used to cha- racterise industrial materials. The workshop provided a forum for all those concerned to meet and discuss how to set up joint projects using the unique facilities available at ILL. Journee D4 We are happy to announce to the whole neu- tron community, but especially to those wor- king with liquids and amorphous systems, that the upgrade of the D4 diffractometer was achieved with success and within the planned time scale, and that the instrument is now available and awaiting beamtime proposals. The results of the commissioning tests and of the first scheduled experiments showed that all the objectives of the project have been rea- ched and, notably, the increase of the overall Anton Oed explaining to Patrick Van Esch the avalanche mechanism in a microstrip detector. counting rate by a factor of 5 and the increase Workshops 106 Workshop on Correlated ted of invited presentations related to the cen- Ultracold Neutron Fermions tral theme. A wide range of topics was covered Anomalies and a significant amount of time was left open Where Do We Stand? Philippe Nozieres has been leading the Theory for discussions and exchanges. Further infor- mation concerning the programme and the Group at the ILL for 28 years. He has stepped A "mini-workshop" was organised to review participants can be found at: down from his position while remaining a visi- the latest results on ultra-cold neutron storage http://www.ill.fr/Events/nozieres.html. ting scientist at the ILL. Philippe's pivotal role and spectroscopy. This work was initiated at ILL several years ago to shed more light on recurrent reports indicating that the lifetimes measured for ultra-cold neutron (UCN) stored in traps with material walls are always shorter than expected, presumably due to enhanced wall reflection losses. Long storage lifetimes are especially needed for precise measure- ments of the neutron lifetime for beta-decay. About 25 participants vividly discussed the contributions which dealt with: • depolarisation and UCN spectroscopy; neu- tron-gamma data, storage lifetimes and spec- troscopy; • analysis of quasi-elastic processes; • theory of wall losses; scrutiny of the neutron optical model; • reflectivity of VCN - very cold neutrons and spectroscopy; • UCN efflux from the trap It became clear that no commonly accepted explanation of the "anomalies" has been rea- ched so far. Among other proposals, a new type of "low-temperature Fomblin" material (per- fluorinated poly-formaldehydes with low mel- ting temperatures) should be studied as a pos- sible low-loss wall coating, and the role of quasi-elastic processes and of surface cleanli- ness should be further investigated - although ultra-high-purity surface conditions are diffi- cult to achieve in the large and complex UCN traps needed in UCN storage experiments. A detailed report of the meeting can be obtained from Peter Geltenbort ([email protected]). Philippe Nozieres during the Workshop on Correlated Fermions, organised by the ILL and ESRF to mark his retirement. in the field of condensed matter physics and Philippe Nozieres is still present at the ILL, his personality have attracted many theoreti- sharing his free time between CNRS LEPES cians and have contributed considerably to the Laboratory (Laboratoire d'etudes des proprie- development of theoretical physics. The ILL tes electroniques des solides) and the ILL and the ESRF organised a lively and friendly Theory Group. The ILL is very grateful to meeting on 30 and 31 October, which focused Philippe Nozieres for the activity he has deve- on one of Philippe's favourite subjects of inter- loped to make the Theory Group so attractive est: Correlated Fermions. The meeting consis- and visible. Workshops 107 Roger J. Stewart (right), Reading University, and Jessica Cheung Jonathan Goff (right), Liverpool, back on DIO again with his student prepare their experiment on D22. Simon Lee (left). Winfried Petry (Tl Munehen) happ\ user on the upgraded IM3 The up-graded D4 welcomes its first users: from the left, Fabrice From the left, Claude Pfistei, Fredenc Marchal, Alessandro Cavillon, Pierre Damay and Frangoise Leclercq, Lille. Paciaroni, Winfried Petry and Jerome Locatelli. Experimental 108 programme Experimental programme Reactor operation Instrument list Beam-time allocation Instrument performance Industrial use of neutrons User forum User satisfaction Experimental programme 109 L operational, with EMB h operational, with FZjiilic CRG-B operational potential CRG CRG-A operational CRG-B operational CRG-B commissioning CRG-B operational CRG-B operational CRG-A operational CRG-B commissioning , with EMBL operational and HMI Berlin t CRG-B under developmen CRG-C operational TEST BEAMS CRG INSTRUME FUNDED INS1 JOINTLY reflectometer reflectometer r Brillouin spectromete powder diffractometer powder diffractometer single-crystal diffractometer single-crystal diffractometer backscattering spectrometer interferometer Laue-crystal alignment facility LAUE diffractometer three-axis spectrometer three-axis spectrometer detector test facilities neutron optics test facility monochromator test facility r spin-echo spectromete cold neutron test facility l diffractomete r single-crysta DIB DI5 D23 EVA IN 12 IN22 BRISP DIA ADAM CT1, CT2 SI8 S42 T3 TI3A.C TI7 LADI (50%) ) DB2I (50% INI5 2000 ) (Dec. operational operational operational operational operational operational under reconstruction 'N' •* operational operational operational operational operational operational operational operational operational operational operational operational operational operational operational operational operational commissioning operational operational operational under construction under construction s f instrument List o S ILL INSTRUMENT diffractometer small-angle scattering reflectometer powder diffractometer r powder diffractomete r powder diffractomete r liquids diffractomete r diffuse-scattering spectromete r single-crystal diffractomete r diffractometer small momentum-transfe single-crystal diffractometer small-angle scattering diffractometer three-axis spectrometer r single-crystal diffractomete r single-crystal diffractomete backscattering spectrometer spin-echo spectrometer three-axis spectrometer backscattering spectrometer neutron beam for fundamental physics three-axis spectrometer time-of-flight spectrometer three-axis spectrometer three-axis spectrometer l physics ultracold neutron source for fundamenta time-of-flight spectrometer time-of-flight spectrometer fission product mass-spectrometer gamma-ray spectrometer thermal neutron Laue diffractometer ; hot neutron instruments DI9 IN3 D4 (50% with IN If D7 DIO DM DIA(50%) D2B IN4 IN5 IN6 INN DI7 D20 D22 INI (50%withD4f D9* DI6 D3* PN3 INI4 IN 16 IN20 PFI PNI IN8 INIO VIVALDI PF2 Strain Imager Experimen programme Guide hail 2 (ILL 22) Reactor hall Level D Guide hall I (ILL 7) INIO Dll TI7 „ _ D CTI TI3C TI3A D M ViVALD, V SI8 IN3 A D D23 A INI3 DIO IN22 CT2 S42 ILL instruments [filled in : operational A Three-axis group jointly funded instruments i open : commissioning or D Diffraction group m x CRG instruments [ under construction -^3 Large-scale structure group •a Reactor hall o O Reactor core Level C X Time-of-flight/high-resolution group it —®—Hot neutrons O Nuclear and particle physics group Thermal neutrons eras • Test and other beam positions 35. -#-Cold neutrons 'A. Filhol 2000 Reactor operation The second operation concerned the treat- Cycle starting date finishing date days days unscheduled scheduled of operation shutdowns ment of 2 m:! of water, to reduce its activity to about 10 mCi/1. This water with a very low 121-2 | 15/02/00 02/03/00 16 16 0 tritium level will be used for rinsing the 122 14/03/00 04/05/00 50 51 0 unloading flasks, in order to reduce the levels 123 ! 23/05/00 12/07/00 50 50 0 of liquid waste produced during these opera- 124 : 17/08/00 06/10/00 50 50 1 tions. Following these detritiation opera- 125 | 18/10/00 08/12/00 50 49 1 tions, the equivalent of approximately 15 gm of tritium were sent to the CEA. Prior to the Total 216 216 2 operations the tritium letdown equipment Table 1: Reactor operation in 2000. had been modified and requalified, to adapt it to the new transport flasks. work will increase the The hot source was removed from the reac- instrument PNl's per- tor to the storage pool in December 1999, formance and will and the reactor has operated without hot allow users to alterna- source during the year 2000. The design of te between periods of the new source has been completed and the irradiation and moni- construction will be subcontracted to an toring the count-rate external manufacturing company. The new on a particular sample. hot source is expected to arrive before the Two detritiation opera- end of 2001. In the meantime, the instru- tions were performed ments D3, D4, D9 and INI will continue ope- in the second half of rating with thermal neutrons. the year. In the first operation a little over More information about the instrument per- :i 60 m of water from formance can be found on the ILL web under the reactor were trea- htlp://www.ill.fr/hotsource.html. ted, with tritium acti- vity in the reactor water being reduced by a factor of two. Replacing the H10 guide: Jacques Italia checks the dose rate before starting operations. The 216-day programme of operations plan- Preparing ihc HIO guide for replacement (N. Laurens, M. Samuel and N. Della-Gatta). ned for scientific work in the year 2000 was completed successfully. Any time lost due to the unscheduled shutdowns noted above was compensated by an extension of cycles 122 and 125. The first unscheduled shutdown involved a suspension of the cycle 124 for about one hour as a result of technical problems with a compressor used on the refrigeration circuit for the cold sources. A longer shutdown during the cycle 125 was decided by the Management following industrial action. The maintenance programme, since the replacing of the reactor block six years ago, is proceeding smoothly and this year work was carried out as scheduled to replace the safety rod absorbers on the H9 and HIO guides. Amongst the new projects, the "source changer" tube for H9 has been transformed, as well as its associated control system. This Experimental 112 programme Instruments Beam-time allocation The instrumental facilities at the ILL are for 2000 shown in the schematic diagram on p.lll. Besides the 30 ILL instruments there are 9 Overall the Subcommittees of the Scientific Council (meetings in October 1999 and April 2000) GRG-instruments, which are operated by scrutinised 924 proposals, out of which 677 proposals received beam time, allocating 4450 days external Collaborating Research Groups. There of beam time on the different instruments. About 750 experiments were carried out. Table 2 shows are currently three different categories of GRG the request and allocation of beam time per instrument. instruments. • CRG-A in which the external group leases an Note that D4 and INI share a beam and that the CRG instruments offer a reduced number of days instrument owned by ILL. They have 50% of for ILL users. For PF2 several experiments share the beam taking neutrons alternatively, so the the beam time at their disposal and for the table contains the beam-days allocated but gives the total number of experiments running simul- taneously. other 50% they support ILL's scientific user programme. • The CRG-B category owns their instrument and have 70% of the beam time, supporting Instrument days days number of days used" days lost the ILL programme for the other 30%. requested allocated* experiments • Finally, CRG-C instruments are used full time AD.W // 186 for specific research programmes by the exter- 79 55 6' D10 268 177 19 187 4 nal group who has exclusive use of the beam. Dll 242 163 64 165 10 1)15 33 36 5 III 80 DB21, LADI and INI5 have a special status, D16 231 160 17 162 4 D17 * 74 7 1 since they are a joint venture of ILL with other D19 302 190 17 196 12 laboratories: in the case of DB21/LADI with D1A 156 122 32 123 9 EMBL and for IN 15 with FZ Jiilich and HMI DIB 168 87 42 197 8 D20 105 91 35 41 0 Berlin. D22 333 163 69 150 25 1)23 9 34 3 185 15 The list of instruments as of December 2000 is D2B 361 168 75 190 9 162 summarised below (CRG instruments are mar- D3 280 160 17 33 D4 142 57 14 64 8 ked with an asterisk *): D7 341 180 17 185 5 • powder diffractometers: Dl A, Dl B*, D2B, D20 D9 310 179 31 185 6 • liquids diffractometer: D4 DB 2131 25 2 59 0 /i'l.'-t 74 44 4 208 2 • single-crystal diffractometers: D3, D9, D10, INI 133 56 9 68 4 D15*. D19, DB21.D23*. LADI IN10 103 139 16 156 4 • small-angle scattering: Dll, D22 IN11 230 173 19 170 8 1X12 93 52 6 205 o • low momentum-transfer diffractometer: D16 IN 13 232 82 11 199 0 • reflectometers: ADAM*, D17, EVA* IN14 362 167 21 177 14 • diffuse-scattering and polarisation analysis INI 5 162 59 8 123 30 spectrometer: D7 INI 6 321 160 22 184 12 IN20 244 164 18 170 8 • three-axis spectrometers: INI, IN3, (IN8 1X22 100 61 7 198 8 under reconstruction), IN12*. IN14, IN20, IN22* IN3 32 57 7 109 0 • time-of-flight spectrometers: IN4, IN5, IN6, IN4* 19 30 7 IN5 208 148 30 193 7 • backscattering and spin-echo spectrometers: IN6 376 168 40 182 17 IN10, IN11.IN13*, IN15, IN16 LADI 220 205 11 145 3 • nuclear physics instruments: PN1, PN3 PF1 565 200 6 216 0 PF2 371 225** 16 215 1 • fundamental physics instruments: PF1, PF2 PN1 239 180 11 131 58 PN3 377 141 7 189 4 Details of the instruments can be found on the web under http://www.ill.fr. Total 7555 4450 747 5790 415 S18*, an interferometer, is a CRG-C instru- ment and is normally not available as a 'user' Table 2: Beam-time request / allocation by instrument and instrument performance. ILL use of the CRG instruments (italic and blue) corresponds to 30 or 50% of the total beam time used. instrument. Some beam time was made avai- f 'days allocated' refers to only those days reviewed by the subcommittees (i.e., excluding CRG days) lable for prototype tests of USANS. " 'days used' refers to the total number of days delivered (i.e., inxcluding CRG days) (* Dl 7 and IN4 were not officially scheduled in 2000; **PF2 consists of several long-term experi- ments so the number of days scheduled is given as an estimation). Experimental programme 113 In 2000, the member countries of the ILL Country Requested (lays Requested % Allocated days Allocated % were as follows: France, Germany, UK, Spain, Switzerland, Austria, Italy, the Czech Republic and Russia. AUT + CZ 118.9 1.5% 60.2 1.4% CH 254.4 3.3% 180.5 4.0% In calculating the statistics of beam-time per D 2154.5 27.8% 1313.2 29.5% country, shown in Table 3, the attribution is E 363.7 4.7% 190.7 4.3% based on the location of the laboratory of the proposers, not their individual nationality. ¥ 2108.3 27.2% 1257.2 28.2% For a proposal involving laboratories from GB 1971.2 25.4% 1078.8 24.2% more than one member country, the total [ 307.0 3.9% 148.3 3.3% number of days is divided equally amongst the collaborating countries. When a proposal RUS 476.1 6.1% 229.8 5.2% involves a collaboration with a non-member country, the allocated time is attributed en- Total 7754.1 100% 4450.6 100% tirely to the collaborating member country (or countries). When ILL scientists are pro- Table 3: Distribution of beam-time request and allocation amongst the member and scientific-member countries. posers or co-proposers, the allocated ILL time' is attributed amongst the member K 5% countries according to their financial contri- butions to ILL. Local contacts are not coun- I) 19 ted as proposers. CZ 1»/i The ILL welcomed 1207 users in 2000. Approximately 80% came from the member countries including 290 from France, 225 GB 20% from Germany and 236 from the UK (Fig.l). : OTHKRS15% Many of our visitors w ere received more than RUS 6% once. There were thus almost 1900 visits in Figure 1: Nationality of the users who came to the ILL in the year 2000. which about 750 experiments were carried out. College 9 College 3 College 4 The distribution of beam time for these expe- 15% riments amongst the different 'Colleges' is College 8 shown in Fig.2: 7% of the days were alloca- 10% ted to nuclear and fundamental physics (col- College 5a lege 3), 10% to structural and magnetic exci- tations (college 4), 40% to crystal and College 7 11% magnetic structures (college 5a + 5b), 7% to structure and dynamics of liquids and glasses (college 6), 11% to materials science, sur- College ab 21% faces and spectroscopy (college 7), 10% to biology (college 8) and 15% to structure and Figure 2: Beam-time allocation in 2000: distribution amongst the different colleges. dynamics of soft-condensed matter (college 9). Instrument performance Table 2 on page 113 gives a summary of instrument • D3 had various problems with bad connectors failure of the target changing mechanism, which performance for 2000. For each cycle a record is kept involving polarisation instabilities and with a cryo- eventually was totally reconstructed by the reac- of any time lost from the total available beam-time, and pad malfunction due to a leak between the bath tor department. the reasons for the lost time are analysed for all the and the vacuum. In addition, beam time could not instruments. The table gives a global summary for the be used efficiently due to problems on the user year. In 2000,415 days were lost due to various mal- side (bad sample); it proved impossible to find Reasons for beam time loss in 2000 functions, which represents about 7% of the total avai- replacement experiments at short notice. lable beam-time (lost time in 1999 was 11.2%). • D22 suffered time loss due to a magnetic field However, the majority of this time is returned to users interference with the neighbourhood instrument because usually time for minor breakdowns, tests, cali- INI 5. Solutions to avoid this kind of problem in brations, scheduling difficulties and director's discre- future are under investigation. tion time is allowed for in the scheduling process. • INI5 struggled with the mechanics and electro- Mechan .mple Env. Detailed comments on the larger beam-time losses nics of its chopper for the time-of-flight option. (20 days or more) are as follows: • A longer shut down period on PN1 was due to a Experimental 114 programme Industrial use of neutrons The year 2000 has been a remarkably good The role of the year for relations with industry, both in terms of Industrial Liaison completed neutron experiments and in terms of Group (ILG) is to sti- new contracts with high profile industrial com- mulate a continuous panies. Most of the experiments were perfor- programme of com- med in the field of small-angle neutron scatte- munications and ring (instruments Dll & D22), while other exchanges. In 2000, measurements required the use of neutron the ILG organised a strain imaging (instrument D1A). Furthermore, wide programme of D20 was used for industrial tests. meetings and pre- The sales figures for 2000 were as follows: sentations of neu- Johannes Zipfel mounting the high-temperature sample changer on Dll. in 2000 in 1999 tron-related indus- Number of customers 9 4 trial work. In April 2000, ILL experts presented For 2001, it is planned to establish a forum for Number of new customers 5 1 neutron techniques to Engineering Schools and industrial engineers and applied scientists, inter- Number of instrument days 17 8.5 the University Claude Bernard in Lyon which ested in the use of neutron methods. Our aim is resulted in a new user for strain measurements to create a network to promote the industrial Bearing in mind that it takes from 6 to 12 with close links to industry [1]. applications of neutrons in different fields of months to convert a serious contact into an A one-day workshop was held on 29 June 2000, interest. ILL will animate seminars and meetings industrial experiment, these figures speak for dedicated to the "Characterisation of industrial for this forum during the year to come in order to themselves as a reflection of the combined materials using neutrons", to enable industry build exchange and to develop a strategy for neu- effort made by all members of the Industrial and applied research laboratories to learn more tron applications for the benefit of all parties. Liaison Group (Pierre Convert, Peter Lindner, about what neutrons can be used for [2]. * Organised by Werkstoffwoche-Partnerschaft GBRmbH, Thilo Pirling and Albert Wright). Building up a In September 2000, the ILL presented two pos- Frankfurt. continuous and effective industrial activity is a ters at the "Materials Week"* in Munich. This long-term effort. also provided the opportunity to make useful REFERENCES contacts with many German-based companies at • [1] ILL NEWS FOR REACTOR USERS, N° 33 (JUNE 2000) P. 3 «[2] THIS ANNUAL REPORT (EDITION 2000), WORKSHOP SECTION, P. 106 the associated trade fair. User forum User satisfaction The ILL values feed-back from the users as an User satisfaction has been tested since July 2000 via a questionnaire - the User Satisfaction Form - indicator of how well our facility is fulfilling received by all users upon arrival at the ILL. The results of the survey - as can be seen in Fig.4 - show- their needs and to initiate actions when we are ed in most cases a very high satisfaction (higher than 90%) with the staff support, ILL equipment and not. Therefore this year for the first time we human comfort. From the comments on the sheets we are pleased to see that ILL is considered as an have instigated the idea of a User Forum. Those excellent facility to carry out research and a great place to come for experiments. We will do our best to users already on site for scheduled experiments keep up the good work! were invited to an informal meeting with the User satisfaction with .... 1 1 1 1 t ILL directors, the physicists' representatives, Scientific support ; • '. V-'- i 1 and the Group Leaders (DS and DPT) and were Breakdown service given the opportunity to comment on several User office support points of interest such as instruments and ser- Reception vice at the ILL. We have organised two User Forums in 2000. The first one - held in March - Instrumentation was already considered to be a success, but the Simple environment second one in October was even more Instrument control 1 j constructive. Clearly we are on a positive lear- Data reduction programmes 3 ning curve here! Many positive suggestions Electronic proposal submission ii were made ranging from instrument up-grading and new sample environments to network Accommodation . .,....«!.1 1* connections in the Guest House rooms. We very Restaurant *• i ' ^ j much appreciated the enthusiasm and co-ope- Library . I ration of our users in giving their views, and every effort will be made to implement sugges- Source reliability* f tions. To date, our electronic proposal submis- 20 30 40 50 60 70 80 90 100 sion has already been further improved follow- Satisfaction m % ing your initial comments, and the possibility of Figure 4: User satisfaction survey results (July 2000 - December 2000). strengthening the technical support outside * The source reliability has been calculated from Tab.1. working hours is being looked into. Experimental programme 115 3. Florian Nettesheim (University of Kiel) loading a sample into the Anton Ocd checks a microstrip plate before mounting. Bohlin rheometer. Thilo Piriing fits out the area for the future Strain Scanner. David Wcddlc (left) and Roger Chung mount the new low-angle multidetector on IN4. Jane Brown and Jens-Boie Suck (Chemnitz University) during the Alan llewat and Maria Teresa Fernandez Diaz enjoy the first ILL User Forum, held in March 2000. Subcommittee lunch. Facts 116 and figures and figures Facts and Figures for 2000 Name Institut Max von Laue - Paul Langevin (ILL) Founded 1967 Associates France: Commissariat a l'energie atomique (GEA) Centre national de la recherche scientifique (CNRS) Federal Republic of Germany: Forschungszentrum Ju'lich United Kingdom: Engineering and Physical Sciences Research Council (EPSRC) Countries with Scientific membership Spain Ministerio de Ciencia y Tecnologia (MCYT) Switzerland Schweizer Bundesamt fur Bildung und Wissenschaft (SBBW) Italy Istituto Nazionale per la Fisica della Materia (INFM) Russia MINATOM MENI (Middle European Neutron Initiative) Consortium, composed of Austria: Osterreichische Akademie der Wissenschaften Czech Republic: Charles University of Prague Staff 409 people including 52 experimentalists in the scientific sector 259 French, 52 German, 56 British, 42 others. French 259,5 63,37% British 56 13,68% German 52 12,70% • Others 42 10,26% Facts and figures \ \ 7 a DIRECTOR D. Dubbers DIRECTOR'S SERVICES HEAD OF HEALTH PHYSICS SAFETY ENGINEER MEDICAL ASSISTANT COMMUNICATION J.Tribolet H. Schweitzer Dr.J.F.Trzmiel TO THE DIRECTION OFFICER B. Dorner A.LeNoel SCIENCE DIVISION PROJECTS & TECHNIQUES REACTOR ADMINISTRATION DIVISION DIVISION DIVISION C. Vettier CCarlile E.Bauer S. Lettow Associate Director Associate Director Deputy A. Heidemann Deputy I.M.Astruc . Assistant: M.Dorn . SHIFTTEAMS, NEUTRON DISTRIBUTION, SITE SECURITY NUCLEAR LBrayer THEORY SCIENTIFIC SUPPORT MECHANICS FINANCE E. Kats G. Cicognani W. Kaiser Ph.Guerin PARTICLE PHYSICS H.Borner ELECTRICITY INSTRUMENT & ELECTRONICS SENIOR ILL FELLOWS CONTROL M. Kramer SCIENTIFIC R. Gahler DIFFRACTION F. Descamps PURCHASING i) COORDINATION F.Tasset GROUP B. Donnelly \i A. Hewat MECHANICAL COMPUTING SERVICE J.Bazin SCIENTIFIC JOINT ILUESRF A. Barthelemy PERSONNEL COMPUTING LIBRARY LARGE SCALE & HUMAN RESOURCES M. Johnson V.Teissier STRUCTURE GROUP R.Mulot RTimmins NEUTRON-OPTICS BIOLOGY/CHEMISTRY I. Anderson INDUSTRIAL LIAISON & CONSULTANCY LABORATORIES BUILDING R Lindner & MAINTENANCE C.Bouton AssistantT. Pirling DETECTORS THREE-AXIS B. Guerard SPECTROMETER GROUP R. Currat MANAGEMENT !j SAMPLE INFORMATION SYSTEMS || ENVIRONMENT M.Grevaz II COLLEGE SECTOR TIME-OF-FLIGHT, S. Pujol I - Instruments & Techniques HIGH-RESOLUTION GROUP 2-Theory AJ. Dianoux 3 - Nuclear & Particle Physics 4 - Structural & Magnetic Excitations 5 - Crystallography & Magnetic Structures 6 - Structure and Dynamics of Liquids and Glasses 7 - Materials Science, Surfaces and Spectroscopy 8 - Biology 9 - Structure and Dynamics of Soft-Condensed Matter Distribution of ILL purchases Income (MF) Expenditure (MF) (MF, excluding taxes) Income from Associates 315,8 84,00% Staff costs 187,4 49,84% France 52,1 66,88% (Divided: F 37,87%: D 37,113%: UK 25%) II Income from 53,4 14,20% Operating costs 70,8 18,83% Germany 7,5 9,64% scientific members • • Own Income 6,8 1,80% Investment costs 43,4 11,54% United Kingdom 4,5 5,79% 17,72% Total 376 100% Fuel cycle 74,4 19,79% Others 13,8 Total 376 100% Total 77,9 100% Budget 376 MF (excluding taxes) Bodies Steering Committee, meeting twice a year Scientific Council with 8 Subcommittees, meeting twice a year Management Board, meeting weekly Reactor 58 MW, running 4,5 cycles in 2000 (with cycles of 50 days) Experimental Programme 747 experiments (allocated by subcommittees) on 25 ILL-funded and 8 CRG instruments 1207 visitors coming from 33 countries 924 proposals submitted and 677 accepted Experiment Selection by the Scientific Council via its 8 subcommittees Nuclear and fundamental physics (college 3) Structural and magnetic excitations (college 4) Crystallographic structures (college 5a) Magnetic structures (college 5b) Structure and dynamics of liquids and glasses (college 6) Materials science, surfaces and spectroscopy (college 7) Structure and dynamics of biological systems (college 8) Structure and dynamics of soft-condensed matter (college 9) Scientific Life based on 10 colleges 8 of which map on to the subcommittees plus two others: instruments and techniques (college 1); theory (college 2) Facts ana figures 119 Ralph Siebrecht can't believe how lucky he is, surrounded by this Stephanie Pouget (right) explaining the principle of the spm-echo smiling bunch of ILL young scientists: from the left, Isabelle Grillo, technique. Laurence Perino, Giovanna Fragneto and Claudia Mondclli. • • IT Jean Paul Gonzales checks the inside of the 6mK dilution refrigerator. Sending out the Annual Report! Publications 120 Publications In 2000, the ILL received notice of 475 publications by ILL staff and users of which 285 were published as journal articles, 187 as conference pro- ceedings in journals, books or reports and 3 as book chapters. The distri- bution by subject is as follows: 41 in instruments and methods, 11 in theory, 64 in fundamental and nuclear physics, 42 in structural and magnetic excitations, 77 in crystallographic structures, 79 in magnetic structures, 45 in structure and dynamics of liquids and glasses, 54 in mate- rials science, surfaces and spectroscopy, 31 in biology, 31 in structure and dynamics of soft-condensed matter. Publications 121 This list gives publications received during 2000 resulting from research carried out at the ILL. It was generated from the library database LORIS-DORIS and can be consulted on the Web at http://www.ill.fr Papers published in scientific books and conference proceedings ABELE H.. BOUCHER A., GELTENBORT P., KLEIN ALONSO J.A., MARTINEZ-LOPE M.J., CASAIS M.T., ANDERSEN K.H., BOURGEAT-LAMI E., DREYER XL. SCHMIDT L\. STELLMACH C. Radio frequency- GARCIA-MUNOZ J.L., FERNANDEZ-DIAZ M.T. J., HEIL W, HOFMANN D., HUMBLOT E, IVANOV induce:; pola:iza;:on of u!tr?.-coki neutrons or how- to Room-temperature monoclinic distortion due to S., KULDA J., LELIEVRE-BERNA E., PETOUKHOV pump a two-levei svslcm. charge disproportionalion in /A1O, pcrovskitcs with A., PUJOL S., REGNAULT L.P., ROBERTS T.W., STE- .Nuclear Instruments and Methods in Physics small rare-earth cations (R= I lo, Y Er. Tm. Yb. and Lu): WART J.R, TASSET E, THOMAS M., WILDES AR Research A 440, 760-763 (2000) A neutron diffraction study. Recent news on ILL polarised 'I Ic developments. Physical Review B 61,1756-1763 (2000) Physica B 276-278,65-66 (2000) ACLT \L ROESSEL T. WASSERMANN E.E, ANDER- SEN K.H.. KULDA J.. MURANI AR, WILDES A. ALONSO J.A., MARTINEZ-LOPE M.J., CASAIS M.T., ANDERSON C.R., STIRLING W.G., ANDERSEN Moment flucluaLions and \i.)lume enhancements in MARTINEZ J.L., DEMAZEAU G., LARGETEAU A, K.H., SOKOL RE., DIMEO R.M. Two-dimensional anti-Invar. GARCIA-MUNOZ J.L., MUNOZ A., FERNANDEZ- excitations of super-fluid :I Ic confined in 72% poro- Journal of the Physical Society of Japan Supplement A DIAZ M.T. High-pressure preparation, crystal struc- sity xerogel. 69.108-111 (2000) ture, magnetic properties, and phase transitions in Physica B 276-278,820-821 (2000) GdNiO., and Dy.XiO., perovskites. ACETf M., WASSERMANN E.F., ANDERSEN K.H., Chemistry of Materials 11,2463-2469 (1999) ANDERSON I.S., HAMELIN B., H0GH0J P., KLLDA J., MURANI AR, WILDES A Large spin fluctua- COURTOIS P., HUMBLOT H. Novel trends in tions and moment-volume coupling in l-'c in an FCC en\l- ALONSO J.A., MARTINEZ-LOPE M.J., CASAIS M.T., neutron optics. ronment. MARTINEZ J.L., FERNANDEZ-DIAZ M.T. Large In: 'Frontiers of Neutron Scattering. Proceedings of Physica B 276-278, 728-729 (2000) increase in magnetoresistancc and cluster-glass the Seventh Summer School on Neutron behavior in defect Tl.^Mn/L pyrochlores. Scattering, FURRER A. (Ed.) (World Scientific, ACET M., WASSERMANN E.E, PEPPERHOFF W Chemistry of Materials 12 J127-l'l 33 (2000) 2000) pp. 44-71 Kele\ance of magnetic instabilities to the phase stabi- lities of Fe alloys. ALONSO J.A., MARTINEZ-LOPE M.J., CASAIS ANDREICA D., GYGAX F.N., PINKPANK M., Philosophical Magazine B 80,127-139 (2000) M.T., VELASCO P., MARTINEZ J.L., FERNANDEZ- SGHENCK A., CHATTERJI T., SURYANARAYANAN DIAZ M.T., DE PAOLI J.M. Enhancement of ferro- R, DHALENNE G., REVCOLEVSCHI A. ADAMS S. CDW superstructures in hydrogen mol\l> magnetic coupling in Sb-substitutcd TI2\ln207 Charge/orbital and antifcrromagnctic ordering in denum bronzes H,MoO. . pyrochlores (Rapid communications). LaSr.,Mn.,O7. Journal of Solid State Chemistry 149,75-87 (2000) Physical Review B 60, R15024-R15027 (1999) Physica B 289-290, 65-68 (2000) ADELMANN P., RENKER B., SCHOBER H., BRADEN ALONSO J.A., SANZ J., SANTAMARIA J., LEON C, ANTONOVV.E., CORNELL K., DORNERB., FEDO- M., FERNANDEZ-DIAZ F. Lattice dynamics of Li- VAREZ A., FERNANDEZ-DIAZ M.T. On the location TOVVK., GROSSE G., KOLESNIKOVAL, WAGNER ZrNCl: An electron doped laxcred suixstoncluctor. of Li cations in the fast Li-cation conductor F.E., WIPF H. Neutron spectroscopy of y manga- Journal of Low-Temperature Physics 117,449-453 La()J J(rTi(), perovskitc. nese hydride. (1999) Angewandte Chemie International Edition English Solid State Communications 113,569-572 (2000) 39,619-621(2000) AGTERBERG D.F., HEEB R, KEALEY P.G., RISEMAN ANTONOV VE., FEDOTOV VK, GNESIN B.A., T.M., FORGAN E.M., MACKENZIE A.P., GALVIN L.M., ALONSO J.A., VELASCO P., MARTINEZ-LOPE M.J., GROSSE G., IVANOV AS., KOLESNIKOV A.I., WAG- PERRY R.S., LEE S.L., PAUL D..VICK, CUBfTT R. MAO CASAIS M.T., MARTINEZ J.L., FERNANDEZ-DIAZ NER F.E. Anisotropy in the inelastic neutron scat- Z.Q., AMMA S., MAENO Y \ortex lattice structures M.T., DE PAOLI J.M. Unprecedented magncloresis- tering from fee Nil I. and pairing symmetry in Sr.JRuO.. lance in Cd-substituted TI,Mn,07 pyrochlores. Europhysics Letters 51,140-146 (2000) Physica C 341-348,1643-1646 (2000) Applied Physics Letters 76,3274~-3276 (2000) APRAHAMIAN A, DE HAAN R.C., BORNER H.G., AKSEN0Y VL. KOZHEVXIKOV S.V, NTKITENKO Y.V, ALVAREZ L, RIGHI A., ROLS S., ANGLARET E., LEHMANN E, JENTSCHEL M. The nature of LAUTERH.J. Reflection and refraction of spin-flip neu- SAUVAJOL J.L. Excitation energy dependence of the I Publications 122 ARZEL L, HEHLEN B., CURRAT R, HENNION B., BARNES A.C., HAMILTON M.A., BECK U., BIENFAIT M., ASMUSSEN B., JOHNSON M., ZEP- SAINT-PAUL M., COURTENS E. The effect of domains FISCHER H.E. A determination of the structure of PENFELD P. Methane mobility in carbon on spectral anomalies of SrTiO,; below liquid Ag,Te using neutron diffraction and isotopic nanolubes. the structural transilion. substitution and its comparison to Ag,Sc. Surface Science 460, 243-248 (2000) Ferroelectrics 236,81-92 (2000) Journal of Physics Condensed Matter 12, 7311- 7322 (2000) BIOTTEAU G., MOUSSA F., HENNION M., RODRÍ- ARZUMANOVS., BONDARENKO L, CHERNYAVS- GUEZ-CARVAJAL J., WILDES A., PINSARD L., KY S., DREXEL W, FOMIN A., GELTENBORT P., BASTIE P., HAMELIN B., COURTOIS P. Méthode de REVCOLEVSCHI A. Strong change in spin dyna- MOROZOV V., PAN1N Y., PENDLEBURY J., I ,auc rcfocaliscc à haute énergie : intérêts cl apports mics close to percolation in LalxCavMnOr SCHRECKENBACH K. Neutron lifetime measured de la spectrométrie des faisceaux diffractés. Physica B 276-278, 562-563 (2000) " by monitored storing of ultra-cold neutrons. Journal de Physique IV10, PrlO-21-PrlO-26 (2000) Nuclear Instruments and Methods in Physics BLASIE J.K., TIMMINS P.A. Neutron scattering in Research A 440, 511-516 (2000) BECVAR F, KRTICKA M., JENTSCHEL M. structural biology and biomolccular materials. Simulations of gamma cascades and modelling Materials Research Bulletin 24,40-47 (1999) ARZUMANOV S., BONDARENKO L, CHERNYAVS- atomic collision chains. KY S., DREXEL W, FOMIN A., GELTENBORT P., Journal of Research of the National Institute of BOMBARDI A., BOURDAROT F., BURLET P., SAN- MOROZOV V, PANIN Y., PENDLEBURY J., Standards and Technology 105,113-123 (2000) CHEZ J.P., VULLIET P., COLINEAU E., REBIZANT SCHRECKENBACH K. Neutron life lime value J., WASTIN F, LANDER G.H., VOGT 0., MATTEN- measured by storing ullracold neutrons with BENITES G.M., FERNANDEZ GUILLERMET A., BERGER K. Involution of magnetic structures in delecta of inclaslically scattered neutrons. CUELLO G.J., CAMPO J. Structural properties of NpAs, xSex solid solutions. Physics Letters B 483,15-22 (2000) melasiable phases in Zr-Nb alloys. 1. Neutron dif- Physical Review B 62,14920-14927 (2000) fraction study and analysis of lattice parameters. ARZUMANOV S., BONDARENKO L, GELTEN- Journal of Alloys and Compounds 299, 183-188 BOMBARDI A., GRENIER B., BURLET P., LANDER BORT P., KOROBKINA E., MOROZOV V, PANIN Y, (2000) G.H., REGNAULT LP, MATTENBERGER K, VOGT 0. FOMIN A., GHERNJAVSKY S. Investigation of the Observation of ferromagnetic correlation in diluted LTe. radiative capture of UGN at the matter surface. BERMEJO F.J., FÂK B., BENNINGTON S.M., FER- Physica B 276-278,606-607 (2000) Nuclear Instruments and Methods in Physics NANDEZ-PEREA R., GABRILLO C, DAWIDOWSKI Research A 440,690-694 (2000) J., FERNANDEZ-DIAZ M.T., VERKERK P. BONDARENKO I.V., BALASHOV S.N., CIMMINO Microscopic collective dynamics in liquid para-H,. A., GELTENBORT P., FRANK A.I., H0GH0J P., ASO N„ ROESSLI B., BERNHOEFT N., CALEMC- Physical Review B 60,15154-15162(1999) KLEIN A.G., MASALOVIGH S.V, NOSOV YG. UCN ZUK R., SATO N.K., ENDOH Y, KOMATSUBARA T., gravity spectrometry using neutron interference HIESS A., LANDER G.H., KADOWAM H. Neutron- BERMEJO F.J., JIMENEZ-RUIZ M., CRIADO A., filters for fundamental investigations in neutron scallcring study of the magnetic response in the CUELLO G.J., CABRILLO C, TROUW ER., FER- optics. heavy-fermion superconductor UN^Al., (Rapid NANDEZ-PEREA R., LÖWEN H., FISCHER H.E. Nuclear Instruments and Methods in Physics communications). Rotational freezing in plastic crystals: A model sys- Research A 440, 591-596 (2000) Physical Review B 61, RI 1867-R11870 (2000) tem for investigating the dynamics of the glass transition. BONETTI E., PASQUINI L., SAMPAOLESI E., BAFILE U., BAROGCHI F., CILLOCO F., HOCHGE- Journal of Physics Condensed Matter 12, A391- DERIU A., CICOGNANI G. Vïbrational density of SAND K., WINTER R., FISCHER H.E. The micro- A397 (2000) states of nanocrystalline iron and nickel. scopic structure of liquid mercury from neulron Journal of Applied Physics 88,4571-4575 (2000) and X-ray diffraction. BERMEJO F.J., KINUGAWA K., CABRILLO C, Physica B 276-278,452-453 (2000) BENNINGTON S.M., FÁK B., FERNANDEZ-DIAZ BORDET P., BOUGEROL CHAILLOUT M.T., VERKERK P., DAWIDOWSKI J., FERNAN- C, GREY I.E., HODEAU J.L., ISNARD 0. Structural BALAGUROV A.M., SHEPTYAKOV D.V, AKSENOV DEZ-PEREA R. Quantum effects on liquid dyna- characterization of the Engineered Scavenger YL, ANTIPOV E.V, PUTILIN S.N., RADAELLI P.G., mics as evidenced by the presence of well-defined Compound, U-U,¿ñ.pr MAREZIOM. Structure of HgBa2CuO,M(0.06<&<0.19) collective excitations in liquid para-hydrogen. Journal of Solid State Chemistiy 152,546-553(2000) at ambient and high pressure. Physical Review Letters 84, 5359-5362 (2000) PhysicalReviewB 59, 7209-7215 (1999) BORISOV Y, HEIL W, LEDUC M., LOBASHEV V, BERMEJO F.J., SABOUNGI M.L., PRICE D.L., OTTEN E.W., SOBOLEV Y. Feasibility study of a BALDINOZZI G., GALVARIN G., SCIAU P., GRE- ALVAREZ M., ROESSLI B., CABRILLO C., IVANOV :!l Ic-magnctometer for neutron electric dipolc BILLE D., SUARD E. Neutron Rictvcld refinement A.S. Persistence of well-defined collective excita- moment experiments. of the incommensurate phase of the ordered per- tions in a molten transition metal. Nuclear Instruments and Methods in Physics ovskile Pb,CoWO(j. Physical Review Letters 85,106-109 (2000) Research A 440,483-488 (2000) Acta Crystallographica B 56, 570-576 (2000) BERNE B.J., CICCOTTI C, COKER D.F. Book BORNER H.G., JENTSGHEL M., LEHMANN H., BÄR NX, ERNST R, JOBIC R, KÄRGER J. Review: Classical and quantum dynamics in KESSLER E.G. Gamma ray spcclroscopy with Combined quasi-elastic neutron scattering and NMR condensed phase simulations. ppm resolving power. study of hydrogen diffusion in zeolites. Journal of Statistical Physics 97,419-420 (1999) AIP Conference Proceedings 529, 35-42 (2000) Magnetic Resonance in Chemistry 37, S79-S83 (1999) BERNHOEFT N., ROESSLI B., SATO N., ASO N., BORNER H.G., JENTSCHEL M., LEHMANN H., KESS- BÄRN.K., JOBIC H., KÄRGER J. Diffusion of hydro- HIESS A., LANDER G.H., ENDOH Y, KOMATSU- LERE.G. Ultra-high precision gamma-ray spectroscopy. gen in various zeolites studied by pulsed-field gra- BARA T. Order parameter symmetry in UPd2Alr BgNS Transactions 5,13-17 (2000) dient NMR and quasi-clastic neutron scattering Physica B 281-282, 993-995 (2000) techniques. BOTT1 S., CICOGNANI G., COPPOLA R, LAPP A., In: '12. International Zeolite Conference' (MRS, BICOUT D.J. Incoherent neutron scattering MAGNANIM. Particle size and optical performances of 1999) pp. 77-84 functions for diffusion inside two concentric photoluminescent laser synthesized Si nanopowders. spheres. Physica B 276-278,860-861 (2000) Physical Review E 62,261-271 (2000) Publications 123 BOUCHERLE J.X., GI\"ORD F., SCHWEIZER J., Xeutron-diffraclion study of the mctamagnctic CICOGNANI G., MUTKA H., SACCHETTI F. The GUKASOV A.. MIGNOT J.M., LEL1EVRE-BERXA phases in IloXi.,BvC. thermal neutron time-of-llight spectrometer IM4C. E., AOfd H., OCHLAI A. Polarized neutron investi- Physical Review B 61, 5872-5875 (2000) Physica B 276-278, 83-84 (2000) gation in the mixed-valence compound Smje. at different temperatures. CAPOGNA L, RADAELLI PG., CHEONG S.W, MARE- CICOGXANIG., MÜTKA H., WEDDLE D., HAiMELIN B., Physica B 281-282,139-140 (2000) ZIO Yl. Charge, orbital and magnetic ordering in MALBERT R, SACCHETTIE, PETRILLO C, BABUCCIE. La,,.,.,.,Ca(i,,.ï\InO,. The double focussing monochromator of IX4C. BOURGES P.. SIDIS Y, FONG H.E, REGNAÜCT Materials Science Forum 321-324,818-822 (2000) Physica B 276-278, 85-86 (2000) L.P., BOSSY J.. IYAXOY A.S., KEIMER B. The spin exci:a'.;on spectrum in superconducting CARSUGHIE, MAY R.P., PLENTEDA R., SAROUN J. COAD S., HIESS A., MCMORROW D.F., LANDER YBa Cu O , -. Sample geometry effects on incoherent small- G.H., AEPPLIG., FISKZ., STEWART G.R., HAYDEN Science 288,1234-1237 (2000) angle scattering of light water. S.iVL, M00KH.A. Magnetic response in UBcn. Journal of Applied Crystallography 33, 112-117 Physica B 276-278, 764-765 (2000) BOUZERAR G.. LEGEZA Ö.. ZIMAX T. Minimal (2000) me::-! to describe the magnetism oiCuGeO,. COAD S., HIESS A., PAOLASINIL, BERNHOEFT N., Physical Re\ie\v B 60,15278-15284 (1999) CASALTA H. Les spcclromètres à temps de vol. DERVENAGAS P., KACZOROWSK1 D., CZOPNIK A., Journal de Physique IV10, Prl-15-Prl-26 (2000) TROC R, LANDER G.H. Magnetic excitations in UGaT BOYE LE.. SACCHETTI F., PETR1LLO C, DORXERB. Physica B 281-282, 200-201 (2000) Neuron investigation of collective excitations in CASALTA H., SCHLEGER P., BELLOUARD C, liquid K-Cs alloy s: The role of the electron density HENNION M., MIREBEAU I., EHLERS G., FARA- COLE J.M., WILSON C.C., HOWARD J.A.K., Physical Review Letters 85, 5352-5355 (2000) GO B. 1 ,o\v Q measurement of super-paramagnetic CRUICKSHANK F.R. Quantitative analysis of hydro- fluctuations in monodomain Fe particles. gen bonding and atomic thermal motion in the BRAXD R.A.. DIAXOUX A.J., CALYAYRAC Y Physica B 276-278, 664-665 (2000) organic non-linear optical material DCXP using i ' i •- ht Mil i he archetypical ico- X-ray and neutron diffraction. -ii if u,--Fe....: Neutron GAVADIN1N., HE1GOLD G., HENGGELER W, FUR- Acta Crystallographica B 56,1085-1093 (2000) ULI - ' - RER A., GÜDEL H.U., KRAMER K., MUTKA H. Physical Review B 62.8849-8861 (2000) Quantum magnetic interactions in S= 1/2 KCuCl,. COLMENERO J., MORENO A.J., ALEGRÍA A., Journal of Physics Condensed Matter 12, ALVAREZ F., MUKHOPADHYAYR., FRICKB. Methyl BRAZOYSKI S.. KIROVÄ X., REQUARDT H., XAD 5463-5472 (2000) group dynamics in glassy polymers by neutron F.Y. MONCEAU P., CURRAT R., LOREXZO J.E., scattering: From classical to quantum motions. GRÜBEI G \ETTIERC P'aMif -Ii;iing of charge CAVADINI X., HENGGELER W., FURRER A., Physica B 276-278, 322-325 (2000) 1 -- - \ -i i < i -• À ( (1-sludies ver- GCDEL H.U., KRÄMER K., MUTKA H. Magnetic ^ *• i I I L r 1 I M< i excitations and spin gap in KCuCl,. COMBET J., FRICKB., LOSSERAND 0., GAMÓN M., Physical Review B 61, 10640-10650 (2000) Physica B 276-278. 540-542 (2000) GUERARD B. Simultaneous diffraction and inelastic scattering on Ihc backscallcring instrument 1X16. BREXXER T., BUTTERWORTH J., GELTEXBORT P., CHACON C, ISNARD 0. Determination of the sub- Physica B 283, 380-385 (2000) HIXO M., MALIK S.S., OKUMURA K., S.ARKISOV D., stitution scheme of gallium and magnetic features STEYERL A., LTSURO M. Looking for surface stales of\cl.,i'e.lAGaxB. COMBET J., JOHNSON M., GABRIEL A., VOGL G., of ultra-cold neutrons. Journal of Applied Physics 88,3570-3577 (2000) PETRY W. New position-sensitive detector Xuclear Instruments and Methods in Physics for the 1X10 backscatfering spectrometer. Research A 440, 722-728 (2000) CHACON C, ISNARD 0. Structure of Physica B 276-278, 154-155 (2000) Xd2Fe.,vSiNBH, measured by neutron diffraclion. BROWS P.J., NEUMANN K.U., WEBSTER P.J., ZIE- Journal of Applied Physics 88,2342-2348 (2000) CONVERT P., HANSEN T., TORREGROSSA J. The high BECK KRA The magnetization distributions in some intensity two axis neutron diffractomcler I lousier alloys proposed as half-meLalllc ierromagnets. CHACON C, ISNARD 0. Study of the Y(Co.Fc) ,ß phases. 1)20 rirst results. Journal of Physics Condensed Matter 12,1827-1835 Physica B 276-278, 652-653 (2000) Materials Science Forum 321-324,314-319 (2000) (2000) CHAPPEL E., NUNEZ-REGUEIRO M.D., CHOU- COPLEY J.R.D., HAYES C, LARTIGUE C. BLDAYOYVSPAXO M., LAFOXT S., ASTIER J.R, TEAU G., ISNARD 0., DARIE C. Study of the, ierro- 1 'erformance comparisons for the ILL neutron spin EBEL C, VEESLER S. Comparison of solubility and distorsive orbital ordering in XaXiO., by neutron echo spectrometer 1X15 in its standard and focu- interactions ofaprotinin iBPI'l) solutions in 11,0 and 11,0. diffraclion and submillimcler wave ESR. sing mirror configurations. JournalofCrystal Growth 217,311-319^(2000) " European Physical Journal B 17,615-622 (2000) Proceedings SPIE 3767, 296-310 (1999) BURDIN S., GEORGES A., GREMPEL D.R. CHATTERJI T., FRICKB. Xuclear spin excitations in COPPOLA R, MAGNANI M., MAY R.P., MOES- Coherence scale oí the Kondo lattice. Xd;CuC\. LANG A. lie bubble growth in a martensitic steel Physical Review Letters 85,1048-1051 (2000) Physica B 276-278, 252-253 (2000) for fusion reactors. Journal of Applied Crystallography 33,469-472 (2000) CACIUFFO R., RIXALDID., BARUCCA G., MIRA J., CHATTERJI T., MCIXTYRE G.J., CALIEBE W, RT\AS J., SEXARIS-RODRIGUEZ M.A., RADAELLI SURYAXARAYANAN R, DHALEXXE G., REVCO- CORBEL G., SUARD E., EMERY J., LEBLANC M. P.G.. FIORANI D., GOODEXOUGH J.B. Structural LEVSCHI A. Reentrant behavior of the charge and 01 I-I1' disorder in non-centrosymmctric details and magnetic order of La. Sr.CoO., fx<0.3). orbital ordering and anliferromagnciism in 7,n,(ßO.,)(()il)07_]-'().^_: Ab initie slnicture determi- Physical Review B 59,1068-1078 (1999) LaSr,Mn.,O;. nation and NMR study: comparison with tridymitc Physical Review B 61, 570-574 (2000) and fluoride borates. CAMPBELL A.J.. PAUL D.MCK., MCIXTYRE G.J. 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Dynamics of proteins, IQENS neutron scattering as a function of temperature: A Mémoire présenté pour un diplôme d'habilitation à study on D2O hydrate Crambin. fast way to characterise in-situ biological dynamics diriger les recherches, Université Joseph Fourier - Physica B 276-278, 522-523 (2000) in complex solutions. Grenoble I.France (1999) Journal de Physique IV10, Pr7-283-Pr7-287 (2000) WANDERLINGH U.N., GIORDANO R., SCIORTINO MONDELLI C. Fluctuations, frustration and dilution in M.T., D1ANOUX A.J. Protein and solvent dynamics ZEYEN C.M.E., OTAKE Y. Neutron EDM search using the SrCrxGaI2_xO]S) compounds. Neutron scattering in hydrated Crambin. crystal techniques. investigation in a broad dynamical range. Journal de Physique IV10, Pr7-325-Pr7-328 (2000) Nuclear Instruments and Methods in Physics Thèse présentée pour obtenir le grade de Docteur de Research A 440,489-490 (2000) l'Université Joseph Foumier - Grenoble I, France (2000) WASSE J.C., HAYAMAS., SMPPERN.T, FISCHERH.E. Structure of a metallic solution of lithium in ammonia. ZHELUDEV A., MASLOV S., SHIRANE G., TSUKA- PLAZANET M. Structure et dynamique quantique Physical Review B 61,11993-11997 (2000) DA I., MASUDA T., UCHINOKURA K., ZALIZNYAK I., de cristaux moléculaires: Spectroscopic et modéli- ERWIN R., REGNAULT L.P. Magnetic anisotropy sation numérique. WECHSLER D., ZSIGMOND G., STREFFER F., and low-energy spin waves in the Dzyaloshinskii- Thèse présentée pour obtenir le grade de Docteur MEZEI F. VITESS: Virtual instrumentation tool for Moriya spiral magnet Ba2CuGe207. de l'Université Joseph Fourier - Grenoble I, France pulsed and continuous sources. Physical Review B 59,1Ï432-11444 (1999) (2000) Neutron News 11,25-28 (2000) ZIMMER 0. A method of magnetic storage of ultra- WIMPORY R.C. Aspects of neutron residual stress WEITKAMP J., NEUEFEIND J., FISCHER H.E., cold neutrons for a precise measurement of the analysis. ZEIDLER M.D. Hydrogen bonding in liquid metha- neutron lifetime. A doctoral thesis submitted in partial fulfilment of nol at ambient conditions and at high pressure. Journal of Physics G: Nuclear and Particle Physics the requirements for the award of the degree Molecular Physics 98,125-134 (2000) 26,67-77 (2000) Doctor of Philosophy of Loughborough University, Great Britain (1999) WILDES A.R., COWLAM N., AL-HENITI S., KISS ZIMMER 0., BYRNE J., VAN DER GRINTEN L.F., KEMENY T. A polarised neutron scattering M.G.D., HEIL W, GLÜCK F. "aspect" - A new spec- study of two samples of Fe00ZrI0. trometer for the measurement of the angular cor- Physica B 276-278, 712-713 (2000) relation coefficient a in neutron beta decay. Publications 135 Editors: Giovanna Gicognani, Christian Vettier Design and Typesetting: PARALLELES - [email protected] Photography by S. Claisse (ILL) and Artechnique - [email protected] Photo DNA molecule page 56: © GNRS/Universite de Montpellier 2 Printing: Imprimerie du Pont-de-Glaix April 2001 Acknowledgement We would like to thank all the people who helped make this report. Further copies can be obtained from: Institut Laue-Langevin Scientific Coordination Office (SCO) BP 156 - F-38042 Grenoble Cedex 9 (France) Tel: +33 (0)4 76 20 72 40 - Fax: +33 (0)4 76 48 39 06 email: [email protected] or [email protected] WEB: www.ill.fr NEUTRONS FOR SCIENCE ill NEUTRONS FOR SCIENCE 6, RUE JULES HOROWITZ - BP 156 - 38042 GRENOBLE CEDEX 9 - FRANCE Web site: www.ill.fr