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PAUL SCHERRER INSTITUT ISSN 1423-7326 March 2000 —< — u
PSI • Scientific Report 1999 /Volume Condensed Matter Research with Neutrons
3 1/28 HoB6diffraction pattern measured on HRPT resolving the small rhombohedral distortion of 0.26° using the Ge511 monochromator (report see page 49).
NOT FOR PUBLICATION Inquiries about copyright and reproduction, etc. should be addressed to the authors. PAUL SCHERRER INSTITUT ISSN 1423-7326
March 2000
Scientific Report 1999
Volume III
Condensed Matter Research with Neutrons
ed. by: Jurg Schefer, Denise Castellazzi, Margit Shea-Braun
CH-5232 Villigen PSI Switzerland Phone: 056/310 21 11 Telefax: 056/310 21 99 www.psi.ch/fun
Electronic Version: http://lns00.psi.ch/fun99/reports99.htm Condensed Matter Research with Neutrons
Head: Dr. W.E. Fischer Secretary: R. Bercher Phone+41 (56)310.3402 FAX+41 (56)310.3131 WWW.PSI.CH/FUN
Laboratory for Condensed Matter Low Temperarture Neutron Scattering Theory Facilities ETHZ & PSI
Head: Head: Head: Prof. Dr. A. Furrer Dr. R. Morf Dr. S. Mango
Secretaries: D. Castellazzi M. Shea-Braun
Phone+41 (56)310.2087 Phone+41 (56)310.4459 Phone+41 (56)310.324! FAX+41 (56)310.2939 FAX+41 (56)310.2939 FAX+41 (56)310.3191 WWW.PSI.CH/LNS TABLE OF CONTENTS
EDITORIAL 1
HIGH TEMPERATURE SUPERCONDUCTORS 5
PSEUDOGAP IN A BACK-EXCHANGED H0Ba2Cu408 COMPOUND: CONFIRMATION OF A 7 LARGE OXYGEN ISOTOPE EFFECT D. Rubio Temprano, J. Mesot, S. Janssen, K. Conder, A. Furrer, H. Mutka and K. A. Muller
CHARGE ORDER IN Ho1.xCaxBa2Cu,Oy PROBED BY CEF INTERACTION 8 A. Podlesnyak, A. Mirmelstein, N. Golosova, B. E. Mitberg, F. Altorfer and A. Furrer
MAGNETIC ORDER OF THE RARE EARTH IONS IN Er2Ba4Cu70143 9 G. Bottger, V. Pomjakushin, D. Clemens, P. Fischer, B. van den Brandt and S. Mango
THE MAGNETIC RESONANCE IN UNDERDOPED Bi2212 AND ITS RELATION TO THE 10 ELECTRONIC SPECTRA M. Bohm, N. Metoki, K. Kadowaki, A. Hiess and J. Mesot
HIGHLY CORRELATED ELECTRON SYSTEMS 11
MAGNETIC ORDERING IN (La1.yPry)07Ca03MnO3 13 V. Pomjakushin, A. Balagurov, P. Fischer, L. Keller, D. Sheptyakov, O.Gorbenko and A. Kaul
CRYSTALLINE ELECTRIC FIELD AND MAGNETIC ORDER IN RNLB2C 14 U. Gasser, P. Allenspach and A. Furrer
THE SUCCESSIVE MAGNETIC PHASE TRANSITIONS OF Dy3Pd20Si6 15 T. Herrmannsdorfer, A. Donni, P. Fischer, L. Keller and H. Kitazawa
MAGNETIC ORDERING IN Tb2Pd2ln AND Ho2Pd2ln 16 T. Herrmannsdorfer, P. Fischer, G. Bottger, L. Keller, M. Giovannini and E. Bauer
NEUTRON DIFFRACTION INVESTIGATION OF MAGNETIC ORDERING IN Ce3Cu3Sb4 17 T. Herrmannsdorfer, P. Fischer, P. Wachter, G. Wetzel and K. Mattenberger
LOW DIMENSIONAL MAGNETISM 19
PHASE DIAGRAM OF THE QUANTUM SPIN SYSTEM KCuCI3 21 N. Cavadini, W. Henggeler, A. Furrer, K. Kramer, H.U. Gudel, H. Mutka and P. Vorderwisch
KCuCI3 SINGLE CRYSTAL SPECTROSCOPY ON FOCUS 22 N. Cavadini, J. Mesot and S. Janssen MAGNETIC EXCITATIONS IN THE QUANTUM SPIN SYSTEM TICuCI3 23 G.Heigold, N.Cavadini, W.Henggeler, A.Furrer, K.Kramer, H.U.Gudel and H.Mutka
SPIN-WAVE EXCITATIONS IN FINITE CHAIN SEGMENTS OF CsMn1.sMgxBr3 24 A. Furrer and H.U. Gudel
MAGNETIC PHASE DIAGRAM OF THE SI-DOPED SPIN-PEIERLS COMPOUND CuGe^SLOj 25 F. Semadeni, P. Boni, T. Masuda, K. Uchinokura and G. Shirane f-ELECTRON MAGNETISM 27
DYNAMICAL SCALING FUNCTIONS IN FERROMAGNETIC EuS BELOW Tc 29 P. Boni, D. Gorlitz, J. Kotzler, B. Roessli and F. Semadeni
CRYSTAL FIELD SPLITTING IN ErGa3 30 A. Murasik, A. Czopnik, E. Clementyev and J. Schefer
CRYSTALLINE-ELECTRIC-FIELD IN IONIC CONDUCTOR Ho01Sr09CoO3.5 31 A. Podlesnyak, A. Mirmelstein, N. Golosova , B.E. Mitberg and F. Altorfer
MAGNETIC EXCITATIONS IN Cs3Er2X9 (X=CI,Br,l) 32 D. Schaniel, P. Allenspach, A. Furrer, K. Kramer and H.U. Gudel
EXCHANGE SPLITTING IN Cs3Er2Br9 33 D. Schaniel, P. Allenspach, A. Furrer, K. Kramer and H.U. Gudel
METAMAGNETISM IN THE GREEN PHASE Er2BaCu05 34 D. Rubio Temprano, K. Conder, P. Allenspach, A. Furrer and V. Pomjakushin
THE BAROCALORIC EFFECT IN CeSb 35 Th. Strassle, K. Mattenberger and A. Furrer
THE BAROCALORIC EFFECT IN DILUTED Cex(La,Y)1.xSb 36 Th. Strassle, K. Mattenberger and A. Furrer
MAGNETIC EXCITATIONS IN SINGLET GROUND STATE FERROMAGNET PrNi 37
E. Clementyev, P. Allenspach, P. Alekseev and V. Lazukov d-ELECTRON MAGNETISM 39
MAGNETIC EXCITATIONS IN CsMn(SO4)2- (D2O)12 41 R. Basler, H. Andres, H. U. Gudel, C. Dobe, P. Tregenna-Piggott and S. Janssen
MAGNETIC ORDERING IN B2Cu04 42 J. Schefer, B. Roessli, U. Staub, A. Amato, G. Petrakovskii, P. Pattison and Ch. Baines SPIN-GLASS TRANSITION IN CuGa2O4 43 B. Roessli, A. Amato, C. Baines, F. Semadeni and G. Petrakovskii
MAGNETIC EXCITATIONS IN THE S=5/2 ANTIFERROMAGNET RbMnCL, 44 N. Cavadini, A. Trottmann, A. Furrer, K.Kramer and H.U. Gudel
CRITICAL FLUCTUATIONS IN THE WEAK ITINERANT Ni3AI 45 F. Semadeni, P. Vorderwisch, B. Roessli, T. Chatterji and P. Boni
ON THE ORIGIN OF THE BIQUADRATIC EXCHANGE INTERACTION IN CsMn014Mgo8GBr3 46 Th. Strassle and A. Furrer
STRUCTURE AND DYNAMICS 47
RHOMBOHEDRAL DISTORTION OF THE CUBIC CHEMICAL STRUCTURE OF HoB6 AT LOW 49 TEMPERATURES DUE TO QUADRUPOLAR ORDERING A. Donni, S. Kunii, L. Keller, P. Fischer, T. Herrmannsdorfer and V. Pomjakushin
PRESSURE DEPENDENCE OF THE PHONON DENSITY OF STATES IN Pr01La09NiO3 50 Th. Strassle and A. Furrer
DEUTERIUM ATOM DISTRIBUTION IN HEXAGONAL ZrCr2D4 51 P. Fischer and A. Skripov
CHEMICAL DISORDER AND MAGNETIC ORDERING IN Ce2Pd2ln INVESTIGATED BY 52 COMBINED NEUTRON AND RESONANT X-RAY SCATTERING T. Herrmannsdorfer, P. Fischer, D. Schaniel, L. Keller, M. Giovannini, R. Hock and E. Bauer
l1 STRUCTURE OF Y092Er008 BO3 53 M. Ren, J.H. Lin, Y. Dong, L.Q. Yang, M.Z. Su, LP. You and P. Allenspach
MULTILAYERS AND SURFACES 55
STRESS AND MAGNETIC ANISOTROPY IN Fe089Co011/Si MULTILAYERS 57 D. Clemens, M. Horisberger and S. Wehrli
ABSORPTION LAYERS FOR SUPERMIRROR POLARIZERS 58 S. Wehrli and D. Clemens
POLARIZATION EFFICIENCY OF MULTILAYER MIRRORS PRODUCED AT PSI 59 H. Grimmer, O. Zaharko, M. Horisberger, H.-Ch. Mertins and F. Schafers
SOFT X-RAY MAGNETIC CIRCULAR DICHROISM IN TRANSMISSION FOR POLARIMETRY AND 60 ELEMENTALLY RESOLVED MAGNETISM O. Zaharko, H. Grimmer, A.Cervellino, H.- Ch. Mertins and F. Schafers
SOFT X-RAY RESONANT MAGNETIC SCATTERING OF Fe/C MULTILAYERS 61 O. Zaharko, H. Grimmer, H.- Ch. Mertins and F. Schafers INSTRUMENTAL AND SUPPORT ACTIVITIES 63
POLARISATION ANALYSIS WITH TASP USING REMANENT BENDERS 65 P. Boni, B. Roessli and F. Semadeni
HRPT DEVELOPMENTS AND FIRST EXPERIMENTS AT SINQ 66 P.Fischer, G.Frey, M. Koch, M. Konnecke, V. Pomjakushin, J. Schefer, R. Schneider, R. Thut, N. Schlumpf, R. Burge, U. Greuter, S. Bondt and E. Berruyer
TriCS: FIRST YEAR OF OPERATION 67 J.Schefer, P. Keller, M. Konnecke, O. Zaharko, Th. Strassle and J. Felsche
DRUCHAL USER OPERATIONS IN 1999 AND INSTRUMENTAL UPGRADE 68 F. Altorfer, E. Clementyev, R. Thut and M. Koch
SINQ TIME-OF-FLIGHT SPECTROMETER FOCUS: FIRST YEAR OF OPERATION 69 S. Janssen, C. Beck, J. Mesot, D. Rubio-Temprano, F. Altorfer, A. Furrer, L. Holitzner and R. Hempelmannn
THERMAL NEUTRON THREE-AXIS SPECTROMETER TNT 70 B. Roessli and P. Boni
3rd GENERATION SINQ PROJECT: LAUE DIFFRACTOMETER 71 O. Zaharko, J. Schefer and E. Lehmann
CREATION AND DECAY OF NUCLEAR POLARIZATION DOMAINS IN HYDROGENEOUS 72 MATERIALS P. Hautle, B. van den Brandt, J.A. Konter, S. Mango, H.B. Stuhrmann and O. Zimmer
SAMPLE SYNTHESIS LABORATORY AT LNS 73 K. Conder, P. Allenspach and A. Furrer CONDENSED MATTER THEORY 75
ACOUSTIC PLASMONS AND SUPERCONDUCTIVITY IN LAYERED CONDUCTORS 77 A. Bill, H. Morawitz and V.Z. Kresin
Li2Cu02 IN THEORY 78 B. Delley, H.B. Braun, A. Amato and U. Staub
SODIUM-NITRO-PRUSSIDE LATTICE VIBRATIONS 79 B. Delley, J. Schefer and Th. Woike
SURFACE RELAXATION OF THE HEMATITE (0001) SURFACE 80 B. Delley, A. Chaka and M. Scheffler
RANDOM MAGNETIC FLUX PROBLEM IN A QUANTUM WIRE 81 C. Mudry, P.W. Brouwer and A. Furusaki
HIGHER ORDER FRACTIONAL QUANTUM HALL STATES 82 R.H. Morf
INTERFACE WAVE FUNCTIONS AND ENERGY GAPS OF QUANTUM HALL STATES 83 R.H. Morf, N. d'Ambrumenil and S. Das Sarma
CROSS-OVER FROM UNIFORM MAGNETIZATION REVERSAL TO DOMAIN NUCLEATION IN 84 MAGNETIC NANOSTRUCTURES H.B Braun
MAGNETIC CORRELATIONS IN NANOSTRUCTURED FERROMAGNETS 85 H.B. Braun, J.F. Loeffler and W. Wagner
DIPOLAR INTERACTION IN 2D HONEYCOMB MAGNETS 86 H. B. Braun, B. Roessli and K. Kramer LIST OF PUBLICATIONS 87
INTERNAL REPORTS 94
CONFERENCE, WORKSHOP AND SEMINAR CONTRIBUTIONS 95
FUN SEMINARS AT PSI 104
LECTURES AND COURSES 106
MEMBERS OF SCIENTIFIC COMMITTEES 107
HIGHER DEGREES AWARDED 110
AWARDS RECEIVED 110
GUESTS 111
SINQ USER STATISTICS AND SINQ SCIENTIFIC COMMITTEE 112
STAFF 113 EDITORIAL
This year was a period of research topics. The activities on "Multilayers and consolidation of the Surfaces", a basic research object by itself, is however operation at the spallation also to a large extent motivated by the development of source of PSI and its optical components for neutron- and X-ray scientific exploitation at an instrumentation. While most of the solid-state work has increasing number of been done with neutrons, some contributions deal with instruments. The source other probes, like muons and synchrotron light. was in operation from mid- Exploiting the unique possibilities at PSI, to take march until Christmas 99, advantage of the complementary nature of the and received a total different probes is encouraged, in particular for our in- charge of proton beam of house research. nearly 5000 mAh. Initially The theory group on "Condensed Matter Physics" the beam current was carries an important part of the responsibility for the about 0.83 mA; after shortening the meson target in scientific atmosphere at PSI and in particular in the front of the source in autumn the beam transmission department. This "care taking" is one of the declared increased from 56% to 70%. As a consequence the tasks for this group. Note the intimate link of their beam current onto the spallation target was raised contributions with the work of the experimentalists. beyond 1 mA. Towards the second half of the year we had eight diffractive instruments in operation - two at It is by now a tradition that I have the occasion to thermal beam tubes and six at the guide system for congratulate at this place to some of our youngsters. cold neutrons. Furthermore, neutrons have been Nordai Cavadini and Daniel Rubio received the Young delivered to a set of five facilities for non-diffractive Scientist Award at the 2nd European Conference on use. These instruments are in the responsibility of the Neutron Scattering in Budapest for their work, spallation-source division. Hence, their performance presented at the conference. Let me however express and the corresponding results obtained will be my appreciation also to all the collaborators for their described elsewhere (Appendix VI of the annual dedication to our common case. report). This rapid progress from the commissioning phase to a consolidated operation of the source is the result of the competent and devoted commitment of the crews from the accelerator- and spallation-source divisions. Let me express at this place my full gratitude for their great performance. Walter E. Fischer The beam time available for customers at the various spectrometers was generally highly over booked. The share of spectrometer time allocated to various groups from many countries all over the world is shown in "SINQ user statistics". Indeed PSI is eager to open its facilities to an international community, the only criteria being the quality of the proposals. In order to do better, a considerable increase in neutron flux can be expected for the following year by the insert of a more efficient spallation target for the next beam period. Moreover, two additional spectrometers will be taken into operation. The contributions to the chapter "Instrumental and Support Activities" reflect the effort in our department to extend and improve the research possibilities with neutrons at PSI to the benefit of the users (and our self, of course). The principal part of this annual report gives an overview of the research activities in the FUN- department. Highly correlated electron systems and the investigation of magnetism are our emphasized LABORATORY FOR NEUTRON SCATTERING
The Laboratory for Neutron new instruments (diffractometers HRPT and TriCS) are Scattering (LNS) is a joint described in detail in the present Annual Report. venture between the Eidgenossische Technische Most of the scientific activities of the LNS staff members Hochschule Zurich (ETHZ) in 1999 were based on the excellent possibilities offered and the Paul Scherrer Institut at SINQ. In addition, an increasing number of research (PSI). According to the projects made use of complementary synchrotron x-ray laboratory's designation and muon techniques. The research topics covered the neutron scattering constitutes areas of highly correlated electron systems and the main research area. Neutron scattering magnetic compounds as well as structural investigations measurements provide information on an atomic scale of novel materials. Some of the highlights include about the static and dynamical properties of condensed the first observation of a large oxygen isotope effect matter, which - in many cases - is obtainable in no on the pseudogap in the slightly underdoped high- other way. The majority of the work performed by the temperature superconductor HoBa2Cu408, LNS is therefore centered around neutron scattering the first observation of the magnetic resonance and the spallation neutron source SINQ at PSI. peak in the slightly underdoped high-temperature superconductor Bi2Sr2CaCu208> The tasks of the staff members of the LNS are twofold: - the magnetic phase diagrams of the CMR (i) Operation, further development and extension of the manganate (La,.yPry)07Ca03MnO3 and the spin- instrumentation set up at SINQ for neutron scattering Peierls compound CuGe]xSixO3, experiments. This requires a permanent effort to the magnetic excitations in the quantum spin improve the efficiency of all the instruments through system TICuCI3, further developments in neutron optics, neutron the barocaloric effect in Ce^La.Y^Sb which may detection, neutron polarisation and shielding against open the technique of pressure-induced cooling to unwanted background, (ii) Establishing a strong in- temperatures below 4 K, house research programme. This is considered to be the unusual softening of a magnetic excitation in the essential not only for the qualification of the scientists, singlet ground-state ferromagnet PrNi, but also for creating a fruitful scientific environment and the interplay of quadrupolar and magnetic ordering competence which makes SINQ an attractive place for in HoB6, excellent work in neutron scattering from which the correlation between stress and magnetic particularly the young generation of scientists as well as anisotropy in magnetic multilayer systems. the external user community will benefit. The laboratory's motif for the year 1999 was teamwork. The continuous operation of SINQ from March to When I chose this motif at the beginning of the year, I December 1999 imposed a major load on the scientific did not expect it to become a pure necessity because of and technical personnel of the LNS. During this period the unusually high fluctuation rate of the LNS personnel some 160 external users from all over the world had to in 1999: We had 6 departures (mostly doctorate and be looked after for preparing and performing neutron post-doctorate students) and 13 entries of new scattering experiments as well as for assisting in the collaborators. It is a pleasure for me to realise that this data analysis after the measurement cycles. At the fluctuation rate had no detrimental impact at all on the same time improvement programmes were carried out laboratory's daily life; in fact, all the new collaborators for some of the spectrometers, new devices for the have been integrated and accustomed to the sample environment were implemented, and the particularities of the laboratory rather quickly - thanks to commissioning work for new instruments was the teamwork practiced by all. Therefore I wish to continued. In particular, the two triple-axis express my gratitude to the LNS staff members for their spectrometers DriichaL and TASP have seen a engaged and competent co-operation, and I extend my significant improvement of their performance by a factor thanks to all those inside and outside PSI who of 4 through the installation of new analyser systems contributed to achieve the laboratory's goals in 1999. and cooled beryllium filters. In addition, the polarisation analysis has been installed at TASP by using the novel Albert Furrer technique of remanent supermirror benders. A particular Head of the LNS highlight was the commissioning of the high-resolution time-of-flight spectrometer FOCUS in May 1999, which is a joint project between the University of Saarbrucken (funded by a grant from the BMBF) and the PSI. All these activities as well as the preparatory work for the • 3
CONDENSED MATTER THEORY GROUP
The Condensed Matter Theory Group has been en- generalization of density functional methods for the cal- gaged in four main areas of condensed matter reser- culation of magnetic properties. ach: (i) strongly correlated electron systems, (ii) mag- netic systems in low dimensions, (iii) the calculation of A highlight of our theoretical results is the success- electronic structure based on density functional theory ful prediction of the probability of spontaneous mag- and (iv) disordered systems. These projects have been netization reversal in magnetic nanostructures by H.B. undertaken in collaboration with researchers from the Braun. He has anticipated the role of solitons in these Laboratory for Neutron Scattering and from the Muon systems for magnetization reversal. These ideas have Spin Rotation Group at PSI and also in various collab- been confirmed in recent Monte Carlo simulations by orations with people from universities. This is exempli- researchers at Duisburg University. fied by the reports in the present annual report. The condensed matter theory group wants to help es- Collaborations are also going on with the Laboratory tablish stronger links between the various groups active for Micro- and Nanostructures, in particular in the ther- in condensed matter research at PSI. It is planned to mophotovoltiac project. These will become more im- start a literature seminar series that will be organized portant in the near future for the development of low- by C. Mudry. The purpose will be to discuss important bandgap photovoltaic cells. recent work in condensed matter research, and also for those participating, to become more acquainted with In April 1999, our new postdoc, Andreas Bill, joined each other's research interests. Another effort to stim- our group and in September, our new staff member, ulate collaboration and exchange of ideas will be made Christopher Mudry, started his work at PSI. A new post- by a series of seminars by B. Delley who will discuss doc, Rene Windiks from the Humboldt University in various successful applications of density functional Berlin, has been selected for work with B. Delley in the theory in condensed matter theory. The link to univer- area of electronic structure calculations. He is sched- sities will be strengthened in the future by lectures on uled to start his postdoc position in May 2000. The goal condensed matter physics that will be given at Zurich of this work is to extend the present capabilities of den- University by C. Mudry starting in the fall of 2000. sity functional methods to the calculation of electronic structures in the vicinity of surfaces and interfaces as well as to chemical reactions. This topic will be of par- ticular interest in the future as the Swiss light source (SLS) will be a very useful instrument for this kind of Rudolf H. Morf investigations. Another area of potential interest is the Head of the Condensed Matter Theory Group LOW TEMPERATURE FACILITIES
The year 1999 has seen a number of developments of A continuous flow 4He cryostat developed for DOLLY, which a few will be sketched in what follows. the p.SR experimental station, has been put into Preparatory work in view of the application of Dynamic operation on the beam and after initial difficulties it is Nuclear Polarization and NMR techniques at low now performing as well as it did in laboratory tests. temperatures to the study of the structure of biological A dilution refrigerator suited for the study of powder macromolecules by polarized neutron scattering has probes on DMC has been built and used for a first produced in the laboratory very promising results, experiment: work is in progress to improve its which need now to be confirmed by an experiment performance. with neutrons. A test of principle will be conducted in First contacts with colleagues of the SLS are quickly summer 2000 at the ILL reactor by a collaboration developing to interesting projects, partly in including researchers from PSI, IBS, ILL, GKSS and collaboration with Swiss universities, for the realization newly CEA-Saclay. In case of a positive outcome a of low and eventually very low temperature series of measurements using the new technique instrumentation fulfilling the very demanding could be performed at PSI. The necessary requirements of research with synchrotron light. instrumentation is in large part available and could be It is therefore a pleasure for me to acknowledge that shared with other users interested to use it. our colleagues, who in return provide us with The particle physics experiments using the active interesting problems and challenges, are putting the polarized targets recently developed by the group are capabilities of the group at good use. delivering results, which confirm the potentialities of this new instrument. Several other experiments using it are being prepared at PSI and other laboratories. The collaboration with colleagues not only of the Department for Condensed Matter Research with Neutrons, but also of the department Particles and Salvatore Mango Matter and of the Synchrotron Light Source Project is Head of Low Temperature Facility Group taking form. High Temperature Superconductors
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i 30- NEXT PAGE(S) ifi left BLANK PSEUDOGAP IN A BACK-EXCHANGED HoBa2Cu4O8 COMPOUND: CONFIRMATION OF A LARGE OXYGEN ISOTOPE EFFECT
D. Rubio Temprano1, J. Mesot1, S. Janssen1, K. Conder1, A. Furrer1, H. Mutka2 and K. A. Muller3 'Laboratory for Neutron Scattering, ETHZ & PSI, CH-5232 Villigen PSI 2lnsitute Laue Langevin (ILL), F-38042 Grenoble Cedex, France 3 Physics Institute, University of Zurich, CH-8093 Zurich
We have recently found a huge isotope effect CBO -^18O) of 50 K on the pseudogap temperature T*, which may be an indication that electron-phonon induced effects have to be incorporated in any modei of superconductivity. A confirmation of this important finding has now been performed using the corresponding back-exchanged compound.
By means of inelastic neutron scattering 18 measurements on the slightly underdoped high-Tc HoBa2Cu, O, compound HoBa2Cu408, we have found that the pseudogap temperature T* is shifted from 170 K to 220 K upon replacing 16O by 18O [1]. This huge isotope effect, which is absent in NMR and NQR experiments, suggests that the mechanism leading to an isotope effect on the pseudogap has to involve a time scale in the range 10'8 s» x » 10"13 s.
Additional crystal-field (CF) measurements (see fig. 1) have been performed on the high- resolution time-of-flight spectrometer FOCUS at SINQ under the same experimental conditions.
0.16 - T = 20 K
50 100 150 200 250 300 350 0.04 - Temperature (K)
*m-**f*?T ... i Fig.2: Temperature dependence of the HWHM - 2 -1 .5 - 1 -0.6 0 0.5 1 1.5 2 corresponding to the rV'-^rV" CF ground Energy Transfer (meV) state transition in both 18O and 16O compounds. Lines represent the calculated normal state Fig.1: Energy spectrum of neutrons scattered from 18 linewidth. HoBa2Cu4 08 at 20 K. The back-exchanged data confirms the huge Fig. 2 shows IN5 (full circles) and FOCUS (open isotope effect on the pseudogap. This finding circles) results for the intrinsic linewidth corresponding a) (1) 18 strongly supports that electron-phonon induced to the r3 -^r4 CF transition. For the O compound effects play an important role in high-Tc some intermediate temperatures have been superconductivity. Indeed, a recent theoretical model measured on FOCUS in order confirm and define AT* 16 [2] is able to explain quantitatively the oxygen isotope more accurately. For the O compound, however, the effect and the extremely fast underlying time scale. FOCUS data corresponds to the back-exchanged sample. The nice consistency of this set of data when compared with the data from the original 16O [1] D. Rubio Temprano et al, Phys. Rev. Lett, (to be compound confirms the stability of the "1248" family of published). compounds against oxygen isotope substitution as [2] A. Bussmann-Holder et at, Phys. Rev. Lett. well as the reliability of the present data. (submitted for publication). CHARGE ORDER IN PROBED BY CEF INTERACTION
A. Podlesnyak1, A. Mirmelstein1, N. Golosova1, B. E. Mitberg1, F. Altorfer2 and A. Furrer2 'Institute for Metalphysics, Ekaterinburg, Russia laboratory for Neutron Scattering, ETHZ & PSI, CH-5232 Villigen
Inelastic neutron scattering (INS) has been used to study the transitions between the lowest lying levels of 3+ the J-multiplet of the Ho ions in Ho^.xCaxBazCu3Oy (0 Studies of weak effects in the CuO2 planes due to 2 (2, 3)) shows that the lines are the result of a doping can improve our understanding of the origin of superposition of at least two ground state CEF the normal as well as the superconducting states in transitions with slightly different energies. high-Tc copper oxides. In this report we present INS The results can be summarised as follows, (i) Calcium data on a series of Ho1.xCaxBa2Cu3Oy samples to doping of the underdoped tetragonal Ho-123 leads to support an idea of charge order in the CuO2 planes the lowering of the local symmetry, (ii) Local due to hole doping [1]. inhomogeneity in the CuO2 planes, which is a characteristic feature of underdoped cuprates [3], still 100 - exist in deeply overdoped regime (Tc=30 K for x=0.3, y=7). Such a behaviour of the CEF interaction under Ca doping is found to be consistent with the model of the CEF generated by charge ordered structure in the CuO2 planes [1]. 50 - D EF 4 6 8 1O12 14 Energy transfer (meV) Fig. 1: Energy spectra of neutrons scattered from Ho^CaxBagCusOes at T=1.5 K for Ea=7 meV and Q=1.8 A"1. 1. x=0; 2. x=0.1; 3. x=0.25. Measurements of INS were performed using the SINQ triple-axis spectrometer DruchaL Since the energies of the low-lying levels are most sensitive to the doping we focus on the low energy part of the experimental 0 2 4 6 8 10 12 14 spectra. The CEF level scheme for Ca-free Energy transfer (meV) HoBa2Cu3Oy was consistently determined previously [2]. We now consider the behavior of the transitions of Fig. 1 and Fig. 2 in detail. A Gaussian fit (solid lines in Fig. 2: Energy spectra of neutrons scattered from the Figures) of the spectra for underdoped tetragonal Ho^CaxBagCusCv at T=1.5 K for Ea=7 meV and Q=1.8 1 Ho^CaxBagCusOsa shows an appearance, due to Ca A" . 1. x = 0;2. x = 0.1; 3.x = 0.3. substitutions, of transition C forbidden for this symmetry [2]. This gives clear evidence for a formation [1] A.Mirmelstein, A.Podlesnyak, V.Bobrovskii, of orthorhombic clusters in the tetragonal lattice. Such I.Zhdakhin, J. Phys.: Condens. Matter., to be a lowering of the local symmetry reflects the charge published. order occurring in the CuO2 planes due to hole doping [2] U.Staub, J.Mesot, M.Guillaume, P.AIIenspach, [1]. In the Ho-123 samples of orthorhombic symmetry A.Furrer, H.Mutka, Z.Bowden and A.D.Taylor, all the CEF levels are singlets [2]. A detailed Phys. Rev. B 50, 4068 (1994). identification of the shapes and line widths of the [3] J.Mesot, A.Furrer, J. Supercond. 10, 623 observed peaks of overdoped Ho1.xCaxBa2Cu307 (Fig. (1997). MAGNETIC ORDER OF THE RARE EARTH IONS IN Er2Ba4Cu7014.3 G. Bottger1'2, V. Pomjakushin', D. Clemens', P. Fischer1, B. van den Brandt, S. Mango3 1 Laboratory for Neutron Scattering, ETHZand PSI, CH-5232 Viiiigen PSI 'Stallikerstr. 4, CH-8142 Uitikon 3Low Temperature Facilities, Paul Scherrer Institute, CH-5232 Viiiigen PSI Er2Ba4Cu70U3 undergoes a magnetic phase transition at 7=500 mK. The magnetic structure of this compound has been determined by means of neutron diffraction. The results indicate a long-range magnetic ordering of the Ef* ions in contrast to its sister compound ErBa2Cu30e3. The coexistence of high-temperature superconductivity and antiferromagnetic ordering of the rare earth ions in layered perovskite compounds of the R2Ba4Cu6+n014+n.8 type (n=0,1, 2; R=most rare-earth elements and Y) placed these compounds in the center of many scientific investigations. Er2Ba4Cu70,5.5 (0<5<1), which belongs to the R2Ba4Cu6+n014+n.5 family, is probably the least famous and least investigated compound of this family of high- temperature superconductors. 123 124 247 20 n N= 0 2 1 Space group Pmmm Ammm Ammm Fig. 1: Magnetic Bragg peaks (V2 V2 1) and (V2 V2 4) in Structure Er2Ba4Cu70143 measured at 7=0.2 K. • buckled CuO2 planes X X X • single Cu-0 chains X - X Er2Ba4Cu70143 is still in progess, it seems to be clear • double Cu-0 chains - X X that the described diffraction experiment does Tc 0...94 K =80 K 30...95 K not confirm that expectation. A comparison of the 3+ 8 0 THE MAGNETIC RESONANCE IN UNDERDOPED Bi2212 AND ITS RELATION TO THE ELECTRONIC SPECTRA M. Bohm 1-2, N. Metoki3, K. Kadowaki4, A. Hiess2, J. Mesot' 'Laboratory for Neutron Scattering, ETHZ & Paul Scherrer Institut, CH-5232 Villigen PSI 2 Institut Laue-Langevin, Av. Des Martyrs, F-38042 Grenoble Cedex 9, France 3Advanced Science Res. Center, Japan Atomic Energy Res. Inst, Tokai, Ibaraki 319-1195, Japan 4Institute of Materials Science, University of Tsukuba, Ibaraki 305, Japan We report inelastic neutron scattering (INS) measurements on underdoped Bi2Sr2CaCu2O8+5 (Bi2212). We observe the existence of a magnetic resonance, whose energy is in good agreement with the energy of the mode inferred from angle resolved photoemission spectroscopy (ARPES). The appearance below Tc of a strong resonance peak underdoped regime of Bi2212 (Tc=70K). The sample at Q=(jr/a,7t/a) (a is the Cu-Cu distance in the CuO2- was built of an alignement of five crystals with an 3 planes) in the spin susceptibility of YBa2Cu30x [1] overall volume of about 50 mm and mosaicity < 3°. (YBCO) is a real challenge for the theories of high- The measurements were performed on the triple-axes temperature superconductivity. The energy of this spectrometer IN8 at the Institut Laue-Langevin in excitation lies around 40 meV in optimally doped Grenoble, France. We used a vertically focused YBCO and decreases with underdoping. Cu(111)-monochromator and a vertically and In order to confirm that this peak is a general feature horizontally focused PG(002)-analyser with fixed final of the HTSC, it is of great interest to perform similar energy. measurements on Bi2212 samples. Figure 1 shows energy scans at two symmetry- Furthermore, in Bi2212, ARPES measurements have equivalent Q-points in reciprocal space. Whereas a shown that at (n,0) the electronic spectra change clear signal can be identified at the smaller Q-value, it drastically below Tc. A sharp coherent peak is strongly suppressed for the larger one, indicating its accompanied by a so-called 'hump' appears which magnetic origin. A fit to the difference of these two can be explained by the interaction of the charge data-sets locates the centre of the peak at (34±1)meV. carriers with a bosonic mode, whose energy also This value is in good agreement (see Fig. 2) with the decreases with underdoping [2]. energy of the mode inferred from ARPES spectra on crystals of the same origin [2]. This result indicates 3500 J. I i i i i l that the resonance/mode is intimately connected to • Q(7i/a,jt/a) the pairing mechanism. 3000 A Difference Underdoped Overdoped^ 2500 - AE T=10 K 2000 \ :s/20min ] 40 1 c \ 1500 [CO L CD 1000 - tensit y 500 30r - - CD 0 LLJ ARPES-ref.[2] I I i^ i i i INS-ref.[3,4] 20 25 30 35 40 45 50 Energy transfer [meV] INS-this work 20b Fig. 1: Energy scans (E, fixed) at two symmetry- 35 50 65 80 95 80 65 equivalent points in reciprocal space, as well as the TC[K] difference (triangles) of both data-sets. Fig. 2: Doping dependence of the resonance inferred from INS data (closed symbols) together with the Due to the weak chemical bonding along the c-axes, mode inferred from ARPES (open circles). samples of Bi2212 which are sufficiently large enough for INS were difficult to obtain, so that a direct [1] J. Rossad-Mignod etal., Physica C, 185 (1991) 86. comparison with ARPES was not possible. [2] J.C. Campuzano etal., Phys. Rev. Lett., 83 (1999) Recently, INS measurements were reported on 3709. optimally-doped Bi2212 single crystals [3,4]. We [3] H. A. Mook etal., cond-mat/9811100. present here the first INS measurements in the [4] H. F. Fong etal., Nature 398 (1999) 588. 11 Highly Correlated Electron Systems * 'I 5 7ooa 6000 5000 I? 4000 § 3000 % 200oL 0 -1000 •2000 NEXT PAGE(S) ?o* left BLANK 13 MAGNETIC ORDERING IN V. Pomjakushin 1, A. Balagurov2, P. Fischer1, D. Sheptyakov2, L. Keller1, O.Gorbenko 3, A. Kaul3 1 Laboratory for Neutron Scattering ETHZ & PSI, CH-5232, Villigen PSI 2 Frank Laboratory of Neutron Physics, JINR 141980, Dubna, Russia 3 Chemical Department. Moscow State University, 119899, Moscow, Russia The magnetic structure of the colossal magnetoresistance manganates (La^Pr^oyCaoaMnOs (0.5/sys0.75) has been studied by neutron powder diffraction on DMC/SINQ. The magnetic state changes from the pure FM state (y<0.6) to the canted AFM ordering (CAF). The AFM component corresponds to a Mn37Mn4+ charge ordered state of pseudo-CE type with two propagation vectors [0 0 1/2] and [1/2 0 1/2]. CAF can be treated either as a homogeneous state, or as a mixture of FM and AFM regions. The experiment in exter- nal magnetic fields up to 4T performed to resolve this question favours a phase separation scenario. The low temperature state of the perovskites A^ would account for a non-monotonous temperature de- xA'xMn03 (where A = La or a rare earth, A' = Ca.Sr) pendence of the AFM-moment [1]. However, we did corresponds to a ferromagnetic metal or antiferromag- not find direct evidences of the phase separated netic insulator with a tendency towards charge order- state. To clarify this question we performed an ex- ing of manganese ions. The particular state depends periment in external magnetic fields. To avoid possible on the doping level, i.e. on the proportion of Mn37Mn4+, reorientation of the FM-crystallites the sample was and on the relation between the Mn-O and A-0 bond made as ceramic pellet. The magnetic field was di- lengths which in turn depend on the average A-cation rected perpendicular to the scattering plane. If the radius 100 4 "Tc AFM (1/2 0 0) 2 10 f FMM : (1/2 0 1/2) I. 1.5 104 8 50 CAFMI 4 0 no 1.18 1.19 1.2 1.21 10 15 20 25 30 35 40 CRYSTALLINE ELECTRIC FIELD AND MAGNETIC ORDER IN RNi2B2C (R = Tb, Ho, and Tm) U. Gasser, P. Allenspach, and A. Furrer Laboratory for Neutron Scattering, ETH Zurich & PSI, CH-5232 Villigen PSI The instruments DruchaL and TASP were used in order to measure CEF-excitations for a determination of the CEF Hamiltonian and the magnetic properties of TbNi2B2C as well as TmxYi-xNi2B2C. On DMC the magnetic orders of TbNi2B2C and HoNi2B2C were measured and the temperature dependencies of the magnetic wave vectors as well as the size of the magnetic moments were investigated. n n The borocarbides ffNi2B2C do not only provide the TbNi2 B2C (Tm « 15 K) and HoNi2 B2C. The tempera- opportunity to study the coexistence of superconduc- ture dependence of the magnetic order was carefully in- tivity with magnetic order for R= Dy, Ho, Er, and Tm, vestigated, since in HoNi2B2C magnetic phase transi- but they also show a great variety of magnetic orders tions take place at Tm = 8 K and TN « 5 K and several and Fermi-surface nesting in a relatively simple chemi- magnetic phases coexist between Tm and TM- More- cal structure. The richness of magnetic structures that over, the observed magnetic phases in HoNi2B2C and have been observed in the borocarbides is one of the their temperature dependence are known to vary from major motivations to study this compound-series in de- sample to sample [3]. The wave vector k « (0.55,0,0) tail. The variety of magnetic structures is due to the (r.l.u.) of the magnetic order in TbNi2B2C is determined competition between the RKKY interaction that favors by strong Fermi-surface nesting that is also observed in incommensurate structures with long wave length and ErNi2B2C and GdNi2B2C. The moments are oriented the single-ion anisotropy that tends to align the spins parallel to k and the structure is, therefore, a longitudi- parallel to an easy axis. The anisotropy is determined nally polarized spin density wave. At low temperatures by the crystalline electric field (CEF) at the rare earth peaks belonging to the third order harmonic 3k are ob- site. Measurements of CEF-transitions are, therefore, served indicating that the spin density wave is squaring crucial for the determination of the magnetic ground up and no lock in transition is observed. state of the if!3+-ions and the understanding of the magnetic ordering in the borocarbides. n Samples with the compositions TbNi2 B2C and 11 Tma;yw1_;l:Ni2 B2C (M = Y or Lu) were measured on DruchaL and TASP in order to determine CEF- transitions in the energy range between 0 and 15 meV. The measurements were done in fixed final energy mode {Ef = 6.0, 4.7, or 3.0 meV; depending on the desired resolution). For Ef < 5.0 meV a Be-filter was used in order to suppress higher orders. After the monochromator a collimation of 80' or 40' was used de- pending on the resolution and the analyzer was used in focusing mode. The measured spectra were analyzed by fitting the CEF cross-section to the data (Fig. 1). The CEF-parameters that were derived for TbNi2B2C are in good agreement with calculated values that were extrapolated from CEF-parameters that have been de- termined in a study of the singel ion magnetism of 8 10 12 14 energy loss (meV) HoNi2B2C, ErNi2B2C, and TmNi2B2C (Table 1) [1,?]. n Fig.1: Neutron spectrum of TbNi2 B2C taken on Table 1: CEF parameters A™ for RNi2B2C (R= Tb, Ho, DruchaL. The full line results from a CEF Er, and Tm) in meV. The values for R = Ho and Er are profile-fit to the data. given for comparison. /tO A A\> A4 A% At i [1] U. Gasser, P. Allenspach, F. Fauth, W. Henggeler, Tb -17.7 3.0 -61.9 1.4 11.6 J. Mesot, A. Furrer, S. Rosenkranz, P. Vorder- Ho -15.0 2.3 -71.4 -0.4 11.7 wisch, and M. Buchgeister, Z. Phys. B 101, 345-352 Er -6.3 3.3 -73.9 -0.61 10.1 (1996). Tm -13.0 2.2 -63.1 -1.3 15.6 [2] U. Gasser, P. Allenspach, J. Mesot, and A. Furrer, Physica C 282-287, 1327-1328 (1997). [3] J.W. Lynn, S. Skanthakumar, Q. Huang, S.K. Sinha, Diffraction measurements were carried out on DMC Z. Hossain, L.C. Gupta, R. Nagarajan, C. Godart, in order to determine the magnetic structure of Phys. Rev. B, 55, 6584(1997). 15 THE SUCCESSIVE MAGNETIC PHASE TRANSITIONS OF Dy3Pd20Si6 T. Herrmannsdorfer', A. Donni2, P. Fischer1, L Keller \ H. Kitazawa3 'Laboratory for Neutron Scattering, ETHZ & PSI, CH-5232 Villigen PSI 2Department of Physics, Niigata University, Niigata 950-2181, Japan 3National Research Institute for Metals, Tsukuba 305-0047, Ibaraki, Japan The low-temperature magnetic structures of Dy3Pd20Si6 have been studied by neutron powder diffraction. Successive antiferromagnetic ordering at Tm -5.8 K and TN2~ 1.8 K was found for the two Dy sites 8c and 4a, with k, = [1,1,1] and k2 = [0,0,1], respectively. Unlike the situation found for Tb3Pd20Si6 and Nd3Pd20Ge6, the 4a magnetic Dy moments point along the high symmetry axis [1,1,1]. The members of the intermetallic compound series and 5.9(3) uB for the 8c and 4a sites, respectively. As R3Pd20X6 (R = rare-earth, X = Si, Ge) have attracted can be seen from Fig. 1, the 4a sites have not reached interest due to the observation of exceptional effects magnetic saturation at T = 1.5 K. like the heavy-fermion behaviour of Ce3Pd20Si6, the quadrupolar ordering of Ce in Ce3Pd20Ge6 [1], and due 50 k, = [1,1,1] to their unusual low-temperature magnetic properties k2 = [0,0,1] like the multiple magnetic sublattice ordering. 5 40 Following the complete magnetic structure de- termination of Tb3Pd20Si6 [2], we extended our studies i! to Dy3Pd20Sis. t I Neutron powder diffraction data were collected down 2 10 LaJUlLiJU to 1.5 K at the SINQ instrument DMC (X= 2.556 A). • t I II II III ^^^^ I Due to the heavy neutron absorption of Dy, a double- wall V cylinder with 14 mm and 15 mm inner and outer diameter, respectively, was used as a sample container. Fig. 2: DMC Neutron powder diffraction pattern of Dy3Pd20Si6 at T = 1.5 K. .... 1200 At the present stage, it is known that the three com- . 1000 • • (mi pounds Nd3Pd20Ge6, Tb3Pd20Si6 and Dy3Pd20Si6 all show o (110) successive magnetic ordering with k, = [1,1,1] and k2 = /a . • O [0,0,1]. For the 4a sites, the angle between the 1CO • * magnetic moments and k2 was determined to be 90°, 1 § — _o 60 °- o_ - itegrate d Inten s ~ 200 BS 0 . " °o 1.5 1.6 1.7 1.8 1.9 2,0 0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 Temperature / K Fig. 1: The temperature dependence of integrated magnetic neutron intensities of Dy3Pd2oSi6, Bragg reflections (111) and (110). This enabled us to collect good quality neutron data with typical counting times of 1 h per temperature for the temperature dependence of the zero field Fig. 3: The magnetic unit cell of Dy3Pd2oSi6. Note that magnetisation (Fig. 1) and 20 h per temperature for due to configurational symmetry, the orienta- the magnetic structure determination (Fig. 2). tion of the 8c moments cannot be determined from a zero field powder experiment. Dy3Pd20Sie shows two successive magnetic second order transitions at TN1 = 5.8 K and TN2 = 1.8 K (Fig. 1). The refinement of the diffraction data at T = 1.5 K is [1] J. Kitagawa, N. Takeda, M. Ishikawa, T. Yoshida, shown in Fig. 2. The magnetic structure of Dy Pd Si 3 20 6 A. Ishiguro, N. Kimura, T. Komatsubara, Phys. (Fig. 3) is similar to that of Tb Pd Si [2]. In contrast to 3 20 6 Rev. B 57, 7450(1998) Tb3Pd20Si6, the 4a magnetic moments of Dy3Pd20Si6 are not oriented along the c-axis, but close to the high [2] T. Herrmannsdorfer, A. Donni, P. Fischer, L symmetry direction [111]. At 1.5 K, the values of the Keller, G. Bottger, M. Gutmann, H. Kitazawa, J. ordered magnetic moments were refined to 8.5(2) uB Tang, J. Phys.: Condens. Matter 11, 2929 (1999) 16 MAGNETIC ORDERING IN Tb2Pd2ln AND Ho2Pd2ln T. Herrmannsdorfer1, P.Fischer', G.Bottger1, L.Keller1, M. Giovannini2, E.Bauer3 'Laboratory for Neutron Scattering, ETHZ & PSI, CH-5232 Villigen PSI sDipartimento di Chimica e Chimica Industriale, Universita' di Genova, 1-16132 Genova, Italy 3 Institut fur Experimentalphysik, T.U. Wien, A-1040 Wien, Austria The rare-earth intermetaiiic compounds Tb2PdJn and Ho2PdJn order magnetically at TN~33K and 8.5 K, respectively, with a propagation vector k= ±(1A, 1A, ¥2). A symmetry analysis of low-temperature neutron powder diffraction data shows that the magnetic structures of the compounds can be described by cos- modulated magnetic moments that are aligned along the c-axis. In the system of the tetragonal intermetaiiic dependence of the zero-field magnetisation was compounds R2Pd2ln (space group P4/mbm, R = rare- measured at the SINQ-instrument DMC (A, = 2.556 A) earth), exceptional magnetic effects like the (Fig.1). stoichiometry-induced transition from ferromagnetism Tb Pd ln and Ho Pd ln undergo a second-order to antiferromagnetism in solid solutions of the Kondo- 2 2 2 2 magnetic phase transition at T ~ 33 K and 8.5 K, system Ce Pd ln were observed [1]. To explore the N 2 2 respectively. At 1.5 K, the ordered magnetic moments remarkable variety of magnetic ordering phenomena in have reached full saturation. The magnetic reflections the rare-earth series, we started a systematic neutron of both samples were indexed with a propagation diffraction investigation. Here we report on the vector k = ± (%, VA, VZ). A group theoretical analysis of compounds with R = Tb and Ho. the possible magnetic modes was performed using the 1.0 program MODY [2], leading to four complex irreducible representations x2 for the rare-earth sites (4h) which 0.8 r decompose into three orbits. Best fits were obtained on the basis of the representation x2, with cos- 0.6 modulated magnetic moments aligned along the c-axis • Tb2Pd2ln (Tab.1). The corresponding refinement of the D1A S. 0.4 DMC2.56A diffraction data for Tb2Pd2ln is shown in Fig.1. Ref 1.5/4 1/4 1/2 3E 0.2 ~^T • —p (4h) coordinates spin arrangement 0.0 " , i , i , i , i , i T . 10 15 20 25 30 35 40 1 x, 1/2+x, 1/2 Temperature / K 2 1/2+x, -x, 1/2 3 1/2-x,x, 1/2 4 -x, 1/2-x, 1/2 , ) Tab. 1: Rare-earth site (4h) and corresponding arrangement of the magnetic moments. The residual peak asymmetries visible in the difference plot of Fig. 1 for some of the high-intensity magnetic peaks at low scattering angles is most probably caused by either a small deviation from commensurability or by a lattice distortion associated with the magnetic phase transition. 0.05 0.10 0.15 0.20 0.25 0.30 0.35 1 sine IX /A' Fig. 1: The temperature dependence of the reduced [1] M. Giovannini, H. Michor, E. Bauer, G. Hilscher, ordered magnetic moment (upper part) and P. Rogl, T. Bonelli, F. Fauth, P. Fischer, T. observed, calculated and difference neutron Herrmannsdorfer, L. Keller, W. Sikora, A. powder diffraction pattern at T= 1.5 K (lower Saccone, R. Ferro, Phys. Rev. B 61 (at press) part)ofTb2Pd2ln. [2] W. Sikora, in: T. Lulek et al. (Eds.), "Symmetry For the magnetic structure determination, low- and Structural Properties of Condensed Matter", temperature neutron data were collected at the Zajaczkowo, World Scientific, Singapore, p. 484 instrument D1A, ILL (X = 2.483 A). The temperature (1994) 17 NEUTRON DIFFRACTION INVESTIGATION OF MAGNETIC ORDERING IN Ce3Cu3Sb4 T. Herrmannsdorfer', P. Fischer', P. Wachter2, G. Wetzel2, K. Mattenberger2 'Laboratory for Neutron Scattering, ETHZ & PSI, CH-5232 Viiligen PSI 2Laboratory for Solid State Physics, ETH Honggerberg, CH-8093 Zurich By means of neutron powder diffraction, we have established canted antiferromagnetic Ce ordering corresponding to k = 0 in Ce3Cu3Sb4. At a temperature of 2 K, the value of the ordered magnetic Ce 3 moment is found to be 2.1(2) fjB, close to the free ion value of Ce *. Recent bulk magnetic, X-ray diffraction, transport and Rwp = 11.6 %, RIM = 9.1 % [3]) was obtained for a optical measurements on the intermetallic compound three-dimensional canted antiferromagnetic structure Ce3Cu3Sb4 (space group 143d) have shown that this system is not a ferromagnetic semiconductor, but a semimetal with magnetic polaron effects on the electrical resistivity [1], Due to the presence of Ce ions it orders magnetically below Tc = 11.3 K, exhibiting a spontaneous magnetic moment in zero external magnetic field of approximately 1/3 of the theoretical saturation moment. In order to establish the magnetic ground state of Ce3Cu3Sb4, neutron diffraction data were collected on a polycrystalline sample with cold neutrons (X = 2.556 A) at the SINQ instrument DMC. Measurements with good counting statistics (about one day per temperature) were made in the paramagnetic and in the magnetically ordered state. In the difference magnetic neutron diffraction pattern l(2 K) -1(18 K) (Fig. 1), additional antiferromagnetic reflections as well as ferromagnetic Bragg peaks, both with considerably lower intensity than the nuclear reflections, could be Fig. 2: The magnetic unit cell of Ce3Cu3Sb4. clearly identified. 8000 as illustrated in Fig. 2. The ordered magnetic Ce moment was refined to 2.1(2) uB, which agrees within 3+ 6000 - error limits with the free ion value of Ce , i.e. there o obs. o -cal. seems to be no crystal field or Kondo reduction of the dif. ordered magnetic moment. For a magnetic single 4000 - hkl CO o domain crystal of Ce3Cu3Sb4, the present model of o LLJ CM magnetic ordering yields 1.6(2) uB as the resultant 2000 - ferromagnetic moment component per Ce3* along the o direction [1,1,1], in fair agreement with > 1.1 uB that is 0 - to be expected from magnetisation measurements at LJJ H<100kOeinRef. [1]. -2000 0 10 20 30 40 50 60 70 80 26 [°] [1] P. Wachter, L. Degiori, G. Wetzel, H. J. Schwer, Fig. 1: Magnetic neutron powder diffraction pattern of Phys. Rev. B 60, 9518 (1999) Ce3Cu3Sb4 in the ordered state at T « 2 K. [2] W. Sikora, in: T. Lulek et al. (Eds.), "Symmetry and Structural Properties of Condensed Matter", All magnetic Bragg peaks could be indexed within the Zajaczkowo, World Scientific, Singapore, p. 484 chemical unit cell, i.e. with k = 0. Also the I centering (1994) was found to hold for the magnetic reflections. With the help of the program MODY [2], we performed a [3] T. Herrmannsdorfer, P. Fischer, P. Wachter, G. group theoretical analysis of jhe possible magnetic Wetzel, K. Mattenberger, Solid State Commun. modes for the space group 143d and the Ce sites 112, 135(1999) (12a). The best agreement between the observed and the calculated magnetic neutron intensities (%2 = 1.25, NEXT PAGE(S) left BLANK 19 Low Dimensional Magnetism (tro'2)" (0.5 0 2) 'momentum [r. I. u.] NEXT PAGE(S) left BLANK 21 PHASE DIAGRAM OF THE QUANTUM SPIN SYSTEM KC11CI3 N. Cavadini1, W. Henggeler1, A. Furrer1, K.Kramer2, H.U. GudeP, H. Mutka3, P. Vorderwisch4 'Laboratory for Neutron Scattering, ETHZ & PSI, CH-5232 Viliigen PSI 2Department for Chemistry and Biochemistry, Uni Bern, CH-3000 Bern 9 3lnstitut Laue-Langevin, B.P. 156, F-38042 Grenoble Cedex 9 4Hahn-Meitner-lnstitut, BENSC, D-14109 Berlin Wannsee Quantum disordered antiferromagnets show unconventional properties which cannot be explained by standard spin wave theory. In the dimer limit, low lying magnetic excitations are of singlet-triplet nature and involve pairs of strongly correlated spins. Detailed studies of the field-temperature phase diagram in the three-dimensional S=1/2 dimer system KCUCI3 have been performed. Monoclinic KC11CI3 belongs to the family of S=1/2 stressing the robustness of the model approach. The quantum disordered antiferromagnets which feature a decrease of the gap A for B || b* occurs at a rate nonmagnetic singlet ground state and a finite spin guBB ~ -0.12 meV/T, in agreement with the critical excitation gap. Above the gap A-2.7 meV well defined field BC~22.4T determined from high-field singlet-triplet modes are observed which are magnetization investigations [2]. Regarding the dispersive in all directions of the reciprocal space. temperature investigations, the dispersion relation These properties can be explained in the framework was measured in the range between kBT«A and of an antiferromagnetic (AF) Heisenberg dimer model. kBT~A. A progressive flattening of the bandwidth with In particular, the gap A arises from the quantum increasing T was observed, following the tendency to fluctuactions induced by the dominant AF intradimer reach the nondispersive intradimer excitation limit exchange counpling [1] and is not due to crystalline which corresponds for KCUCI3 to 4.3 meV (Figure 2). anisotropy. A change of external parameters like the This behaviour is generally expected in the case of a magnetic field or the temperature characteristically thermally driven reduction of the spin-spin correlations influences the dynamical properties of such gapped in the system, and can be successfully reproduced by singlet-triplet systems. In the following, investigations a RPA dimer approach. At the same time, the lifetime of the phase diagram in KCUCI3 are discussed. of the excitations was observed to severely renormalize, indicating the collective origin of the above described effect. 3 6 9 12 15 field strength [T] 0 10 20 30 40 temperature [K] Fig.1: Observed field dependence of the magnetic excitations in KCUCI3 at T~2K. The threefold linear Fig.2: Observed temperature dependence of the splitting is expected for a Heisenberg singlet-triplet extrema of the dispersion relation in KCUCI3. The system (continuous line). continuous line refers to a model explained in the text. Upon application of an external magnetic field, the singlet-triplet excitation modes have been observed to [1] N. Cavadini, W. Henggeler, A. Furrer, H.U. undergo a linear Zeeman splitting up to 14T (Figure Gudel, K. Kramer, and H. Mutka, Eur. Phys. J. B 1). At this field strength, the imposed splitting is 7,519-522(1999) comparable with the bandwidth of the modes (Figure [2] W. Shiramura, K. Takatsu, H. Tanaka, K. 2 at T=2K) - nevertheless, no deviations from the Kamishima, M. Takahashi, H. Mitamura, and T. simple dimer model expectations are observed, Goto, J. Phys. Soc. Jpn. 66, 1900-1903 (1997) 22 KCuCI3 SINGLE CRYSTAL SPECTROSCOPY ON FOCUS N. Cavadini1, J. Mesot1, S. Janssen1 1 Laboratory for Neutron Scattering, ETHZ & PSI, CH-5232 Villigen PSI First investigations featuring single crystal time-of-fiight spectroscopy at SINQ are reported. An attempt to measure the dispersive singlet-triplet magnetic excitations in KCuClj on the whole scattering plane was performed. Despite the low intensity of the S= 1/2 Cu2+ spins, magnetic signal could be resolved in a broad range of momentum transfer. The insulator KCUCI3 shows a nonmagnetic singlet background conditions encountered, which prevented ground state separated by a spin gap from well detailed investigations in case of faint and broad defined triplet excitations modes. These excitations distributed signal. The planned intensity gain and solid are dispersive in all directions of the reciprocal space angle increase, together with a better background and can be modeled in the framework of an control are expected to allow successful studies of antiferromagnetic dimer model [1]. Effective S=1/2 magnetic dynamics on single crystals. exchange couplings between the dimers account for the dispersion relation of the modes above the gap. Relying on the clear characterization of the magnetic system, we performed single crystal test expectation measurements on FOCUS, with the following results. energy 1.0 10° PG002 E[=10.53meV / 0.5 k|=2.25 A-1 / 40° sample 2 r 20c KCuCI3 (T) 0.0 measurement -log(l0/1) ••••/ 130° 1.0 Fig.1: Scheme of the adopted FOCUS configuration. In the inset, the oriented sample is shown scaled 1:1 0.5 with respect to k,. A large KCUCI3 sample was mounted in the a*c* plane with the a* axis at 13° with respect to the incoming beam (Figure 1). This orientation satisfied the 202 0.0 Bragg condition in the detector bank, which we used 0.0 1.0 2.0 as a reference during the runs. Inelastic measurements in the energy loss configuration were performed over the whole dynamical range at T-1.4K. Fig.2: Dispersion relation from KCUCI3 on the For each detector, magnetic intensity is generally portion of the a*c* plane covered by the first 75 expected where the dispersive singlet-triplet detectors (top). The scale reproduces the excitations match the respective (K,CO) loci. Those corresponding (H 0 L) momentum transfer after Fig. 1. points are denoted by the thick continuous curve in Measured magnetic intensity normalized against a Figure 2 (top). The observed intensity followed the void run (bottom). above expectations, albeit restricted to the lower half of the detector bank and to momenta corresponding to [1] N. Cavadini, W. Henggeler, A. Furrer, H.U. rather flat excitations around E~5 meV (Figure 2, Gudel, K. Kramer, and H. Mutka, Eur. Phys. J. B bottom). Both limitations arise from the difficult 7, 519-522 (1999) 23 MAGNETIC EXCITATIONS IN THE QUANTUM SPIN SYSTEM TICuCI3 G.Heigold1, N.Cavadini1, W.Henggeler1, A.Furrer1, K.Kramer2, H.U.GudeP, H.Mutka3 'Laboratory for Neutron Scattering, ETHZ& PSI, CH-5232 Villigen PSI zDepartment for Chemistry and Biochemistry, Uni Bern, CH-3000 Bern 9 3lnstitut Laue-Langevin, B.P. 156, F-38042 Grenoble Cedex 9 Inelastic neutron scattering investigations have been performed on the insulator TICuCI3 to study the magnetic interactions between the S=1/2 Cu2* ions. This compound shows a nonmagnetic singlet groundstate which is separated by an energy gap A-0.9 meV from well-defined dispersive singlet-triplet excitations. Like for the isostructural KCuCI3, the observed dispersion and intensities can be described by an antiferromagnetic (AF) dimer model. From high field magnetization measurements, it is known that TICuCI3 undergoes a quantum phase transition at Bc~6 T [1], which is believed to correspond to the closing of the gap. This fact promotes TICuCI3 a potential candidate for studies near a quantum critical point. Quantum spin systems featuring a nonmagnetic predictions (see Figure 1). From the dispersion singlet groundstate and a magnetic energy gap have relation, the relevant exchange couplings were attracted much interest in the last years, both determined. In Figure 2, selected directions in theoretically and experimentally. Although neutron reciprocal space are shown. Using a model based on scattering can provide clear and conclusive insights in weakly coupled AF dimerized Cu2+ ions, the measured such compounds, only a few complete investigations energies and intensities can be quantitatively have been presented so far, this mainly due to the explained. Compared with KCuCI3 [2], the dispersion absence of large enough single crystals. Motivated by branches are steeper and the energy gap is smaller the clear results obtained from high quality single indicating that the unbalance between intradimer and crystals of the dimer compound KCuCI3, we started the interdimer couplings is less pronounced. TICuCI3 is dynamical study of the isostructural TICuCI3, for which thus probably best described by an isotropic first inelastic measurements are presented below. Heisenberg hamiltonian with a dominant exchange between nearest neighbor Cu2+ ions supplemented by important three dimensional correlations. To check 120 - the expected linear Zeeman splitting of the triplet | • modes, neutron scattering measurements in high 'iE' (h k I) = ft (-0.5 0 2.0) 1 fields are planned. o 90 1 T=1.5K • J • •• c 60 I j - KCuCI •'•' '•, o 3 o TICuCI 30 3 \ n CD . I 4 ***** JL CD n > • V E? (-0.5 0 2 0) :•**••.. CO CD 2 - c 50 T=15K , r [-x 0 2x] • ? ? 0 ******* * **' (-0.5 0 2) (0 0 1) [oox(0] 0 2) 12 3 4 momentu'Um [r.l.u.. ] energy transfer [meV] Fig. 2: Low-energy cuts from the observed magnetic Fig. 1: Measured excitations from TICuCI at T=1.5K 3 excitations in TICuCI . The lines compare the (above) and T=15K (below). The renormalization at 3 for TICuCI, and higher T is indicative of their magnetic nature. The dispersion relations determined asymmetry of the peaks is caused by the large KCuCI3. curvature of the dispersion. [1 ] H.Tanaka, K.Takatsu, W.Shiramura, T.Ono, T.Kambe, K.Kamishima, H.Mitamura , T.Goto, TICuCI crystallizes in the monoclinic space group 3 Physica B 237-238,120 (1997) P2,/c with two non-identical dimers per magnetic unit [2] N.Cavadini, W.Henggeler, A.Furrer, K.Kramer, cell. Generally two excitation modes are expected. In H.U.Gudel, H.Mutka, Eur. Phys. J. B 7, 519 the a*c*-plane however, for symmetry reasons only one branch is detected agreeing with the theoretical (1999) 24 SPIN-WAVE EXCITATIONS IN FINITE CHAIN SEGMENTS OF A. Furrer1 and H.U. Qiidel2 'Laboratory for Neutron Scattering, ETH Zurich and PSI, CH-5232 Villigen PSI 2Department of Chemistry and Biochemistry, University of Bern, CH-3000 Bern 9 Inelastic neutron scattering was employed to study magnetic excitations in the diluted one-dimensional Heisenberg antiferromagnet CsMn^Mg^ (x=0, 0.05, 0.10, 0.25, 0.50). The spectral response is interpreted in terms of spin-wave excitations in finite chain segments of Mrf+ ions, which are found to exist as long as the chain length exceeds twice the wavelength of the spin excitation. This limit determines the crossover into the mesoscopic regime. Understanding the spectral features of correlated low- spin wave is ns=A/R+1=3. The energies, linewidths and dimensional magnetic systems in the presence of line shapes of the spectral response are found to be nonmagnetic impurities has become a central issue in best described by taking the lower limit in the current studies of high-temperature superconductors, summation procedure to be n,=5 or 6. Smaller and spin-ladder systems, spin-Peierls compounds, etc. In larger values of n, fail to reproduce the observed data. one-dimensional systems nonmagnetic dilution This means that spin-wave excitations at the zone separates the infinite chain into segments of length i. boundary exist in segments whose length l involves at Whenever l is large compared to the wavelength X of least five or six Mn2+ ions, which is roughly twice the the collective spin excitation, the spin-wave approach is wavelength X of the collective excitations. We found the adequate. On the other hand, when l becomes same condition, n,=2ns, for the spectra taken at other comparable to X, the system is in a mesoscopic regime wavevectors. Thus an empirical criterion emerged from and the spin-wave approach breaks down. The our analysis that spin-wave-like excitations exist in applicability of the spin-wave formalism is therefore chain segments provided that their length l exceeds dependent on both the impurity concentration and the twice the wavelength X of the spin excitation. wavelength of the spin excitation. But where does the crossover to the mesoscopic regime occur? To our Reference knowledge this question has never been answered by a [1] A.R. McGurn & M.F. Thorpe, J. Phys. C: Solid clear-cut experiment. In the present work we have State Phys. 16, 1255(1983). addressed this question by a neutron spectroscopic study of the magnetic response in the diluted one- dimensional antiferromagnet CsMni_xMgxBr3 (x=0, - x=0 0.05,0.10,0.25,0.50). In Fig. 1 we show the dependence of the energy spectra upon variation of the Mg concentration x for spin excitations at the zone boundary. For the pure compound (x=0) the spectral response is perfectly i i i i i symmetric and can be described by a Gaussian 5 - x=0.05 /^v centered at an energy of 8.9 meV. Its linewidth I / % corresponds to the instrumental energy resolution. For • 4 - -As *? low impurity contents (x=0.05 and 0.10) the spectral ^. 3 response is gradually shifted to lower energies, the ~ 2 peak width increases, and the peak shape develops an S 1 t»x^^* asymmetry on the low energy side. Similar features as 1 1 1 1 shown in Fig. 1 were observed for higher impurity 5 - x=0 10 ^% contents (x=0.25, 0.50) as well as for other wavevectors 4 — in the Brillouin zone. 3 A X We analyzed the observed energy spectra 2 - according to an exact model by McGum and Thorpe [1] 1 in which the spin-wave energies are expressed as a i function of the chain length. The spectral response is 6 7 8 9 10 then given by a weighted sum over all the chain Energy transfer [meV] segments with a lower and upper limit of the chain length n, and nu»1, respectively. The results are shown Fig. 1: Energy spectra of neutrons scattered from as lines in Fig. 1. At the zone boundary the spin waves CsMn^xMg^Brj at T=10 K and Q=(0,0,1.5). For x=0 have the wavelength ?»=2R where R corresponds to the the line denotes a Gaussian fit. For x*0: n,=4 (• • • •), separation of the Mn2+ ions along the chain, hence the n,=5 ( ), n,=6 ( ), n,=8 ( ). number of Mn2+ ions involved in a single period of the 25 MAGNETIC PHASE DIAGRAM OF THE Si-DOPED SPIN-PEIERLS COMPOUND F. Semadenil, P. Boni1, T. Masuda 2, K. Uchinokura2 and G. Shirane 3 1 Laboratory for Neutron Scattering, ETH Zurich & PSI, 5232 Villigen PSI, Switzerland 2 Department of Advanced Materials Science, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan 3 Department of Physics, Brookhaven National Laboratory, Upton, New York 11973-5000, U.S.A. The magnetic phase diagram of the CuGei-xSiiVO3 spin-Peierls (SP) compound has been established by means of elastic neutron scattering over an extensive range of doping concentration (0.56%-2.7%). The magnitude of the magnetic moment \it^ versus Si-doping exhibits a maximum at a critical concentration xc=1.7%. This indicates the existence of a boundary between a dimerised AF phase in the low doping regime, and a uniform Neel state above xc. The discovery of an inorganic spin-Peierls (SP) com- CuGei.xSixO3 pound by Hase et al. [1] has been the starting point of a great amount of investigation in order to understand the mechanism underlying the SP transition. Since CuGeO3 is particularly sensitive to the introduction of impurities, this compound is believed to be the ideal system to study doping effects on the SP ground state. For site-doped (Mg2+ ions) compounds, the existence of a critical doping concentration xe has been empha- sised, where the SP and AF order parameters show a sudden change [2,3]. Recent susceptibility measure- ments [4] and neutron diffraction experiments [5] on Si- doped compounds indicate that such a phase boundary is presumed to also occur in bond-doped SP systems. We report here a comprehensive study of the magnetic phase diagram of Si-doped CuGeO3. Fig. 1 shows a superlattice antiferromagnetic peak for the Si=0.82%- doped compound, namely under the presumed thresh- old concentration. The background at 5 K indicates that the peak is not contaminated by higher order neutrons. CuC'e0.9918S'0.0082O3 800 ' i ' I ' • (0 1 1.2) I 0.00 0.01 0.02 0.03 x(Si) 600 - - o co 1.5 K Fig.2: Magnetic moment as a function of Si doping at I T=1.5 K and Q=0 (a). T exhibits a similar be- 400 _ N haviour upon doping variation as shown in the i o lower plot (b). The solid lines are a guide to the O 200 _ eyes. • 5.0 K • op The current results give indications for the existence n Q ° 1 i 1 i of a compositional phase boundary. It will be how- -131.5 -131.0 -130.5 ever necessary to perform experiments closer to xc, Q say 1.5% and 2.2%, in order to confirm this hypothesis. Nevertheless, the observation of a SP superlattice peak Fig.1: Rocking curve of the (0 1 1/2) superlattice below xc, that could not be seen above this boundary, peak at 1.5 K and 5 K for CuGei_.rSi.TO3, with strongly supports such a transition. x=0.82%. The AF ordering temperature T,v as well as the mag- netic moment fx£ff are reported in Fig. 2, as a func- [1] M. Hase et al., Phys. Rev. Lett. 70, 1993 (3651). tion of Si-doping concentration. The order parameter [2] T. Masuda et al., Phys. Rev. Lett. 80, 1998 (4566). Hejf exhibits a maximum at the critical concentration [3] H. Nakao et al., J. Phys. Soc. Jpn. 68, 1999 (3662). xc = 1.7%. This increase is also visible for the ordering [4] T. Masuda et al., to be published temperature up to the same concentration. However, [5] S. Katano et al., Phys. Rev. B 57, 1998 (10280). for higher concentrations JN exhibits a plateau-like be- haviour. NEXT PAGE(S) left BLANK 27 f-Electron Magnetism NEXT PAGE(S) left BLANK 29 DYNAMICAL SCALING FUNCTIONS IN FERROMAGNETIC EuS BELOW Tc P. Bon?, D. Gorlitz2, J. Kotzlef, B. Roessli', and F. Semadenf 1 Laboratory for Neutron Scattering ETHZ & PSI, CH-5232 Viligen PSI, Switzerland 2 Institut fur angewandte Physik, Universitat Hamburg, D-20355 Hamburg, Germany Inelastic scattering of neutrons from EuS is used to determine the dynamical scaling function of the mag- netic excitations below Tc. In contrast to an isotropic ferromagnet the spin waves are significantly damped even reasonably far away from Tc, i.e. at 0.93 Tc. The strong damping is caused by the dipolar interactions in EuS. The dynamical scaling functions for the spin wave energy and damping are in good agreement with the predictions of recent mode-mode coupling theory. Recently, Schinz and Schwabl [1] have used mode- dependence is similar as for the critical fluctuations at 25 mode coupling theory to calculate for the first time the 7C, where A = 2.1 meVA . dynamical scaling function for isotropic ferromagnets In order to compare the results directly with mode- with large dipolar interactions. They have been taken mode coupling theory we have calculated the dynami- fully into account and not only been treated as a per- cal scaling functions for the spin wave energy Eq and turbation as it was done in previous work [2]. In order the linewidth rq. They are given by the ratios g{R) = to test the results we have measured the spin waves Eq(T)/rq(Tc) and f(R) = rq(T)/rq(Tc) and are plotted in with transverse polarization (Goldstone modes) with Fig. 2 versus the radial scaling variable R by open and respect to the reduced momentum transfer q (8S± q). closed symbols, respectively. R is a function of q, the Polarization analysis has been used to separate the inverse correlation length KZ and the dipolar wave spin wave excitations from the intermediate mode and number qd. The angle 0.02 P 2 - • ^-" 0.00 >^ i *. . i • . 03 • . • • i °- R=((K /q)2+(q /q)2)1/2 I 002 z d Fig. 2: Dynamical scaling functions of EuS for the spin 0.01 wave energy (open symbols) and linewidth (closed O.i 0.00 0.04 0.08 0.12 0.16 0.20 symbols). The solid lines and the broken lines indicate the predictions of mode-mode coupling theory for the dipolar and the isotropic case, respectively. Fig. 1: Spin wave energies (top) and linewidths (bot- tom) of EuS at 15 K and 15.9 K. The results clearly show that mode-mode coupling theory agrees well with the experimental results and The following features are important: First, the spin explains nicely the large deviations of the spin dynam- wave dispersion becomes linear at small q in contrast ics in EuS from an isotropic Heisenberg model. Far to an isotropic ferromagnet that has a quadratic dis- away from 7C (7= 10 K), f[R) approaches the iso- persion relation. This effect is caused by the dipolar tropic result. interactions [3,4] and is related to the reduction of the number of Goldstone modes from two to one due to [1] H. Schinz and F. Schwabl, Phys. Rev. B 57, 8430; the dipolar interactions. Second, the spin waves are 8438(1998). significantly damped even at 7= 15 K. This effect is [2] B. P. Toperverg and A. G. Yashenkin, Phys. Rev. B again due to the dipolar interactions that do not con- 48,16505(1993). serve the order parameter M and lead to a finite life- [3] T. Holstein and H. Primakoff, Phys. Rev. 58, 1098 time of the spin waves. In fact, the damping of the spin (1940). waves is essentially given by r = Aq25, i.e. the in- [4] P. Bonietal., Phys. Rev. B52, 10142(1995). 30 CRYSTAL FIELD SPLITTING IN ErGa3 A. Murasik1 ,A. Czopnik2 , E. Clementyev3 and J. Schefer3 11nstitute of Atomic Energy 05-400 Otwock-Swierk, Poland 2lnstitute for Low Temperature and Structure Research of Academy of Science 50-950 Wroclaw 3 Laboratory for Neutron Scattering ETHZ&PSI, CH-5232 Villigen, PSI, Switzerland The splitting of the J = 15/2 multiplet of Ei3+ in a cubic crystal field has been determined by inelastic scattering from a polycrystalline sample of ErGa$ Least-squares fits applied to the observed spectra taken at various temperatures gave crystal field parameters: B4 = (-11.68 ± 0.01)x10'2 and BQ = (9.69 ± 0.01)x10'2 meVyielding the /> doublet as a ground level with the overall splitting of 10.92 meV. The results are used to calculate the temperature-depended zero-field magnetisation. process was observed although in few cases also the The neutron diffraction studies of ErGa3 performed de-excitation of low lying levels was allowed for, to recently, revealed that in the range of (2.7 - 2.8) K there confirm the magnetic nature of the observed scattering. occurs an abrupt reorientation of the Er3+ spins from The scattered neutrons were analysed with an analyser nearly [110] direction towards the [100] axis. Since the energy kept constant either at Ef = 4.97 or 8.05 meV. knowledge of the crystal field splitting seems to have On the basis of observed intensities and their considerable significance for our under-standing of the temperature variation, we have been able to determine temperature dependence of magnetic anisotropy in two CEF parameters required for cubic symmetry. ErGa3, we decided to investigate the crystal field Least-squares fits applied to the spectra taken at splitting in this compound by using the direct various temperatures gave crystal field parameters: B4 spectroscopic method of neutron inelastic scat-tering. 2 2 = (-11.68 ± 0.01 )x10" and B6 = (9.69 ± 0.01 )x10" For Er3+ in a cubic environment, the ground multiplet 4 meV yielding the r7 doublet as a ground level with the l15/2 splits into 5 levels Fn, two doublets F7, rs, and 1 2 3 overall splitting of 10.92 meV. The obtained results three quartets r8( ), TV ), F8( ). The initial have been used to calculate the zero-field experiments have been performed on the triple-axis magnetisation, which shows good agreement with the spectrometer DRUCHAL installed at the SINQ experimental data obtained earlier. spallation source of the Paul Scherrer Institute (Villigen, Switzerland). The 20 g polycrystalline sample of ErGa3 (used earlier in the neutron diffraction study) was encapsuled in a cylindrical aluminium container of 10 mm diameter and 50 mm length, filled with the helium [111], [110] gas and inserted into the close-cycle helium [100] refrigerator. The neutron inelastic experiments have been carried out in the energy range allowing to detect o ErGa, • experiment [100] el.+quasiel. [110] ErGa T = 40K — [111] 1 k, =1.97A' E^S.OSmeV 3 two-level model i ; yj LL T C! r r -> (r . r) 8 ."'"» ; LU i N • 0.0 0.2 0.4 0.6 0.8 1.0 T/T., •ill f It I -4-3-2-10 1 2 3 4, 5 6 7 8,.,., 9 10. 11 Energy transfer [meV] Fig. 2: Zero-field magnetisation calculated from the determined CEF parameter. By a comparison with two Fig. 1: Typical energy spectrum at 40 K. Arrows indi- level curve it is seen that the CEF anisotropy is weak cate positions of particular CEF-transitions. and is solely due to excited levels. At -2.7 K it can be easily perturbed even by other weak interactions yielding the rotation of moments, already observed. the energy transfers from -3.5 meV to about 10 meV. During experiments mainly the neutron energy loss 31 CRYSTALLINE-ELECTRIC-FIELD IN IONIC CONDUCTOR Ho01Sr09CoO3.8 A. Podlesnyak1, A. Mirmelstein1, N. Golosova1, B. E. Mitberg1,and F. Altorfer2 'Institute for Metal -Physics, Ekaterinburg, Russia 2Laboratory for Neutron Scattering, ETHZ & PSI, CH-5232 Villigen The crystal field interaction at the rare earth site in Ho01Sr09CoO3.s has been studied using inelastic neutron scattering. The experimental data enable us to estimate CEF Hamiltonian. It is well known that the perovskite-like compounds fact provides therefore an opportunity to determine Ln1.xSrxCo03.5, Ln = rare earth, under the certain x unambiguously the CEF Hamiltonian parameters. We and 5 exhibit the highest oxygen diffusion (and, recently reported on results from measurements of hence, ionic conductivity) among the materials with CEF interaction in Ho01Sr09CoO3.6 at TOF high enough electrical conductivity. The changes of spectrometer KDSOG (Dubna, Russia) [5]. There we rare-earth content x as well as oxygen deficiency <5 have been able to observe three CEF transitions out result in oxygen vacancies redistribution, which in turn of ground state above 10 meV. The 17-fold affect the crystal structure and electronic properties of degeneracy of the ground state J-multiplet % of the the samples, thus leading to essential changes in the Ho3+ ions is split by cubic CEF into one singlet, two electronic and ionic conductivity [1]. The effects of dublets, and four triplets [6]. In order to determine the oxygen nonstoichiometry have been widely studied remaining, missing levels we focus here on the low by different chemical methods, as thermogravimetry energy part of the spectra. [2], iodometric titration [3], etc. The cubic to Triple-axis spectrometer DruchaL was used to rhombohedral structural transition and magnetic determinate the low-energy CEF transitions of the phenomena as a function of rare-earth content x have ground-state J-multiplet % of the Ho3+ ions. The been studied by a number of research groups [1,4]. analysis of the data involves standard parameterization However, there are still unresolved questions in technique that has been worked out by Lea, Leask particular regarding the local defect structure and and Wolf [6]. The observed energy spectra at defect-dependent properties, as well as in theoretical T=1.5 K exhibit well defined inelastic lines at 1.2 and approaches of unique oxygen diffusion behavior of 3.4 meV (Fig. 1). This experimental situation most these oxides. probably can be realized for the parameters x=-0.77 and W<0 (see Fig. 2 in ref. [6]), so that we are able to estimate CEF parameters as ts Eobs[meV] Ecaic [meV] 1.2 1.19 3.4 3.49 12.6 12.29 17.8 18.36 19.4 19.38 - 20.81 Table 1. Observed and calculated energies of Ho3+ CEF excitations in Ho0.1Sr09CoO3.5,. [1] A.N.Petrov, O.F.Kononchuk, A.V.Andreev, et al., Sol. St. Ionics 80,189 (1995). [2]J.Mizusaki, Y.Mita, S.Yamaguchi, K.Fueki, J. Sol. St. Chem. 80, 102 (1989). [3] M.Seppanen, M.Kuto, P.Taskinen, Scand. J. Of 0 2 4 6 8 Metallurgy, 9, 3 (1980). Energy transfer (meV) [4] G.Thorton, B.S.Tofield, A.W.Hewat, J. Sol. St. Chem. 61, 301 (1986). Fig. 1: Energy spectra of neutrons scattered from [5] A.Podlesnyak, A.Mirmelstein, G.Chimid, 1 HoaiSra9Co03.8 at T=1.5 K for Ea=7 meV, Q=1.8 A . Experimental report of FLNP-JINR, KDSOG-M instrument, date of experiment: 20.01.99-4.02.99. For certain x- and 5-values cobalt perovskites [6]K.R.Lea, M.J.M.Leask and W.P.Wolf, J. Phys. Ln1.xSrxCo03.5 do crystallize in the cubic structure. This Chem. Solids 23, 1381 (1962). 32 MAGNETIC EXCITATIONS IN Cs3Er2Xg (X = CI,Br,J) D. Schaniel1, P. Allenspach ', A. Furrer1, K. Kramer2, H.U. Gudel2 1 Laboratory for Neutron Scattering, ETHZandPSI, CH-5232 Villigen PSI 2Department for Chemistry and Biochemistry, University of Bern, CH-3000 Bern 9 Neutron spectroscopy was used to investigate the crystalline electric field (CEF) in the rare earth compounds Cs3Er2X9 (X=CI,Br,l) and Cs2ErCI5. The CEF parameters of C$>ErCI5 and the dimer compounds Cs3ErsX9 (X=CI,Br) were determined. The lack of any exchange splitting associated with the CEF transitions in the dimer compounds can be explained by the large single ion anisotropy for Er. 3 Cs3R2X9 (R: rare earth; X=CI,Br,l) is a compound of C3v instead of C3 for the Er ""), this compound potential use in upconversion lasers. Structurally it exhibits a similar energy splitting pattern as forms rare earth dimers. Dimer splittings of up to Cs3Er2X9 (X = CI.Br) (see Table 1, Fig. 2). some hundred ueV due to the exchange interaction Determination of the CEF parameters is in have been observed for different rare earths in this progress. structure [1,2,3,4]. For a long time the absence of any observable dimer splitting in the Er compound was a mystery. Earlier inelastic neutron scattering 100 (INS) measurements in Cs3Er2CI9gave evidence for a rather huge splitting (> 1 meV) [5], but these signals could be attributed to the crystalline electric field (CEF) splitting of a secondary phase, namely Cs2ErCI5, present in the parent compound at concentrations ranging up to 25%. A pure sample of Cs2ErCI5 has also been investigated by INS and the CEF parameters have been determined [6]. Table 1: Observed and calculated energy splitting in Cs3Er2X9 (X = CI,Br,l). Energies are given in meV. 3+ *: optical measurements on Er doped 0.01 Cs3Lu2Xc,:1%. E(ij) X=CI(obs) X = CI(calc) X = Br(obs) X = Br(cato) X = I (obs) E(1,2) 0+20 0 0+20 0 0+20 Fig.1: Magnetic susceptibility for Cs3Er2CI9 E(3,4) 5.86+0.05 5.92 5.10+0.05 5.05 5.05+0.05 calculated from CEF-Parameters. E(5,6) 11.56+0.10 11.26 8.70+0.20 8.92 8.50+0.20 E(7.8) 12.30+0.50 13.46 9.80+0.20 10.00 8.95+0.20 E(9,10) 13.46+0.10 13.61 11.10+0.20 10.89 9.60+0.20 E(11,12) '34.8+1.0 34.2 27.52 DruchaL E(13,14) *35.8+1.0 35.4 29.54 E = 5.6 meV, Q = 3.2 A, T = 10 K E(15,16) '37.1+1.0 37.5 30.15 f The CEF parameters of Cs3Er2X9 (X = Cl, Br), deter- mined from the INS measurements, reproduce the observed energy splitting very well (see Table 1) [6]. The magnetic susceptibility calculated from these parameters shows an extreme single ion anisotropy (in excess of 102 for Xc/lab (see Fig. 1)), which is responsible for the lack of any dimer signal. This 6 8 10 12 almost pure Ising system (compared to almost energy transfer (meV) perfect Heisenberg systems for the other rare earths) splits the lowest CEF state in the presence of the Fig.2: Crystal field excitations in Cs3Er2l9. ferromagnetic dimer interaction into a magnetic ground-state doublet (|tt),|ii>) and a non- [1] A. Donni et al., J. de Phys. 49, C8-1513 (1988) magnetic excited doublet (I ti), I it)) at 0.20 meV [2] A. Furrer et al., Phys. Rev. Lett. 62, 210 (1989) and 0.28 meV for X = Br and Cl, respectively, [3] A. Furrer et al., Phys. Rev. Lett. 64, 68 (1990) without any allowed transitions between these two dimer states. [4] A. Furrer et al., Inorg. Chem. 29, 4081 (1990) [5] P. Allenspach et al., Physica B 234-236, 744 Further INS measurements on Cs3Er2l9 were per- (1997) formed. Despite its different symmetry (point group [6] D. Schaniel, Diploma Thesis, LNS-199 (1999) 33 EXCHANGE SPLITTING IN Cs3Er2Br9 D. Schaniel \ P. Allenspach 1, A. Furrer1, K. Kramer2, H.U. Gudel2 1 Laboratory for Neutron Scattering, ETHZ and PSI, CH-5232 Villigen PSI department for Chemistry and Biochemistry, University of Bern, CH-3000 Bern 9 Measurements of specific heat, dynamic susceptibility and magnetization were performed with use of the PPMS System (Quantum Design) on the rare earth dimer compound Cs3Er2Brg in order to understand the exchange coupling in the Er3* - Er3* dimers. While the intradimer coupling is antiferromagnetic in most rare earth dimers, we found a ferromagnetic coupling for the Er3* dimer. Cs3Er2Br9 crystallizes in the space group R-3c. The point group of the Er3+ ion as well as of the dimer 16 H = 8000 Oe- 3 complex [Er2Br9] " is C3. Energy splittings of the H = 6000 Oe 4 ground state multiplet l15/2 due to the crystalline H = 4000 Oe electric field (CEF) were observed by neutron 12 H = 3000 Oe spectroscopy and the CEF-parameters were determined [1]. H = 2000 Oe Superimposing an exchange Hamiltonian of the form H = 1000 Oe Hex = -2JS1'S2 on the magnetic Hamiltonian formed by H= 100 Oe the CEF parameters, one gets the energy splitting of the dimer levels, depending on the value of the exchange constant J. J was determined by fitting the exchange Hamiltonian to the specific heat in zero magnetic field, yielding J = 0.0011 meV. We measured the specific heat of Cs3Er2Br9 in different external magnetic fields (Fig. 1). The data Fig. 2: Magnetization in different external magnetic were interpreted by using the formula fields (parallel to the crystallographic c-axis). The dynamic susceptibility shows a clear frequency Cp = + Cp (1) dependence at temperatures below 10 K. Frequency Since we deal with an insulating compound y is equal scans at several different magnetic fields were to zero. (3 = 2.11-10"2 J/mole -K4 resulted from the fit to performed in this temperature regime (see Fig. 3). ma9 the data. Cp is the magnetic contribution to the The real and imaginary part of the susceptibility specific heat. The strong upturn at the lowest (m',m") were fitted using the following formulas: measured temperatures in the zero-field curve originates from long-range magnetic ordering which rrv*Aco) = ^,^2 :«*» = 2 2 (2) occurs at a temperature of about 1 K. l+4> 7", where %0 is the static susceptibility and T, a characteristic relaxation time. %0 and T, were used as 3 =2.11'1Cr2 (J/mol-K4) __„,-/'' / free parameters. By lowering the temperature, the relaxation time increases and the formulas (2) are not / adequate anymore, because of the onset of long- 0 D dataH = 0.0 T H 'X 7 range ordering. 3 / Vp mag P +0-T 2o 6 O dataH = 0.2T r—r—i 1 1 1 1 1 1—i—r- Opmag T = 3.3 K D / •/ dataH = 0.75T " • x X o m1: H - 250 Oe mag 3 j \^ Cp +p'T m'-Fit * dataH = 1.5 T - . m": H = 250 Oe — - Cpmag +|3-T3 °x m"-Fit o 4 6 - T(K) Fig. 1: Specific heat of a Cs3Er2Br9 crystal in different magnetic fields. The magnetic field is parallel to the crystallographic c-axis. . i . . 9 a . it . 1000 Fig. 2 shows magnetization measurements F(Hz) compared to calculations using J = 0.0011 meV. The Fig. 3: Frequency dependence of the dynamic qualitative agreement is good and confirms the susceptibility at H = 250 Oe and T = 3.3 K. ferromagnetic exchange coupling (positive sign of J). [1] D. Schaniel, Diploma Thesis LNS-199 (1999) 34 METAMAGNETISM IN THE GREEN PHASE Er2BaCu05 D. Rubio Temprano, K. Conder, P. Allenspach, A. Furrer, V. Pomjakushin Neutron diffraction experiments and magnetization in static and oscillating fields were performed in the metamagnetic Er2BaCu05 green phase in order to determine the magnetic order and its evolution as a function of the external field. The non-superconducting green coloured oxides R2BaCu05 (R=rare earth) were first discovered in 1982 by Michel and Raveau [1] who determined their crystallographic structure. They have attracted much attention in the past years since they have very ~ 20 often been found as impurities in the synthesis of high-temperature superconducting RBa2Cu3Ox and RBa2Cu408 oxides. These three families of compounds have perovskite-like structures which include common elements such as CuO5 pyramids and Cu-0 chains. For the case of Er2BaCu05 (Pnma, Z=4), magnetic diffraction measurements revealed the 10 15 20 existence of long-range magnetic ordering in the rare Temperature (K) earth sublattice below a critical temperature TN=15 K, as found by Golosovski et al [2]. According to this Fig 1: Temperature dependence of the DC work, there exists antiferromagnetic (AF) ordering magnetization at H=0.5, 1, 1.5 and 2 T for defined by a propagation vector of k=[0 1/2 0] and Er BaCu0 . The inset shows a hysteresis loop with the spin projections alternating in sign as given by 2 5 measured at T=4.2 K and up to 9 T. FxFyG2 in Bertaut's notation. The measured total magnetic moments (at T=1.3 K) of the two Er sites amount to 8.5(1) and 7.1(1) /jB, respectively, which are T = 4.2 K significantly lower than the free-ion value of 9 fJB. However, possible changes of the magnetic structure induced by the presence of an external field have not yet been studied. Such a study would be of interest taking into account that the R2BaCuOs compounds present a metamagnetic behaviour [3]. The Er2BaCu05 sample was prepared by conventional solid state reaction from high purity Er2O3, BaCO3 and CuO powders in air at 950 °C during 50 h. The single phase character was checked by x-ray and neutron diffraction. AC magnetic susceptibility (H=10 Oe, co=1000 Hz) and DC magnetization (2 < T < 30 K, 0 < H < 2 T) were measured using a commercial Fig.2: Neutron diffraction patterns (nuclear and Quantum Design PPMS device. Neutron diffraction magnetic) of Er2BaCu05 taken at T=4.2 K and measurements were performed down to 50 K for H=0,1 and 2 T. (nuclear) and 4.2 K (magnetic) at the SINQ instrument DMC (X=3.8 A) equipped with a superconducting A detailed analysis of the magnetic structure cryomagnet (0 < H < 2 T). as a function of the magnetic field are currently being performed. Fig. 1 shows the DC magnetization of Er2BaCu06 for H=0.5,1,1.5 and 2 T. We can observe how, in addition to the AF component, a new magnetic contribution appears somewhere between 0.5 and 1 [1] C. Michel and B. Raveau, J. Solid State Chem. T. This critical field can be determined by using the 43, 73 (1982). derivative of the hysteresis loop at T=4.2 K (see inset), [2] I. V. Golosovski et al, Sov. Phys. Solid State 3 4 resulting to be H=0.93 T. Furthermore, significant (5), May 1992. changes of the magnetic structure with the magnetic [3] R. Z. Levitin et al, J. Magn. Magn. Mat. 90-91 field can also be inferred from the neutron diffraction (1990) 536-540. measurements at T=4.2 K (see fig. 2). 35 THE BAROCALORIC EFFECT IN CeSb Th. Strasslea, K. Mattenberger6 and A. Furrera a Laboratory for Neutron Scattering, ETH Zurich & PSI, CH-5232 Villigen PSI 6 Laboratory for Solid State Physics, ETH Zurich, CH-8093 Zurich Recently a new adiabatic cooling technique based on a pressure-induced structural phase transition was described for Pri-sLa^NiC^. Here we report on the pressure-induced magnetic phase transition of CeSb used for cooling. In the barocaloric effect [BCE] a change of the isother- mal total entropy of the system is used to establish adiabatic cooling in a similar manner as in the well- known magneto-caloric effect [1]. However in the BCE we rather use external pressure in order to change the magnetic entropy than huge external magnetic fields which are still needed for magneto-caloric cooling. The magnetic entropy Sm of a rare-earth compound can be written as o.o 16.0 18.0 20.0 22.0 Sm = — with pi = (1) T[K] Fig.1: Measured BCE -AT versus initial temperature upon the release of p = 0.26 GPa. with Ei the crystal-field energy levels of the rare-earth ion at temperature T and R the molar gas constant. The cooling effect can be calculated, if the entropies at Any change of the Bi's immediately affects Sm and adi- ambient and elevated pressure are known. Based on abatic cooling from state 1 to state 2 gets possible, if a molecular field calculation we expect bigger cooling rates -AT than actually observed (Fig. 2). This largely results due to the inertia of the thermocouple as well S(T, 1) < S{T, 2), with S = Se- (2) as insufficient insulation of the sample. Thus we may not have realized fully adiabatic transitions. So far we could observe a maximal cooling of -2 K at a pressure the total entropy of the system and Si and S - its e release of 0.53 GPa. lattice and electronic contributions, respectively. The change in the energy levels may arise by a pressure- 5.0 induced structural phase transition as in the case of Pri_xLaxNiO3 or due to Zeeman splitting in a pressure- - induced magnetic transition as in the case of CeSb [2]. P=° -J- CeSb crystallizes in the NaCI structure. Below TN « 16 K antiferromagnetic ordering occurs in various phases. From neutron diffraction TN is known to in- —* . crease linearly upon uniaxial pressure as dTN/dp « 8K/ GPa [3]. Thus the application of pressure at tem- — 0.0 peratures just above 16 K results in a magnetic phase transition from the paramagnetic into the ordered state 1 and vice versa upon the removal of pressure. Since in T[K] the ordered state the isothermal entropy is smaller than Fig.2: BCE based on a molecular field calculation in the paramagnetic state, cooling can be realized by (left: experimental entropy forp = 0 (from cp) adiabatic release of pressure (see Eq. 2). [gray] and calculated entropies for p = 0 For the direct measurement of the barocaloric effect, a [dashed black] and p > 0 [solid black], right: cubic single crystal of 3 mm side length was mounted expected cooling -AT). on an uniaxial pressure device allowing the in situ change of p at cryogenic temperatures. The tempera- ture of the crystal was tracked by a thermocouple glued on one side of the crystal. The socket and the anvil of [1] K. A. Muller, M. Koch, S. Fischer, F. Fauth, A. Furrer, the pressure device were made of ZrO ensuring good Appl. Phys. Lett. 73, 1056 (1998) insulation and thus almost adiabatic conditions for small [2] Th. Strassle, A. Furrer, K. A. Muller, J. Alloys and time scales. Fig. 1 shows the cooling effect -AT in Compounds, in press function of the initial temperature T prior to the release [3] B. Halg, A. Furrer and O. Vogt, Phys. Rev. Lett. 57, of pressure. 2745(1986) 36 THE BAROCALORIC EFFECT IN DILUTED Th. Strassle0, K. Mattenberger6 and A. Furrera a Laboratory for Neutron Scattering, ETH Zurich & PSI, CH-5232 Villigen PSI b Laboratory for Solid State Physics, ETH Zurich, CH-8093 Zurich It has been shown that the rare-earth compound CeSb shows a barocaloric effect of about -2 K at T = 20 K upon removal of p = 0.53 GPa. In order to lower the working temperature to the technicaiiy interesting range below 4 K, we have partially substituted the Ce ions by non-magnetic La and Y ions. These diluted systems show an anomalous barocaloric effect at low temperatures, that may be explained by the occurrence of a cluster-glass state. The monopnictide CeSb shows a pressure-induced magnetic phase transition close above TN(p = 0) = 16.1 K. This property may well be used for cooling by the barocaloric effect [BCE] as reported previously [1]. For Cea:(La,Y)1_a;Sb the partial dilution of Ce ions by non-magnetic Y and La ions decreases TN by dTN/dx « -50 K and enables the use of these com- pounds for the BCE at lower temperatures. We have investigated the BCE on a single crystal of Ceo.85(La0.9oYo.o5)o.i5Sb which orders at TN(p = 0) = 7.9 K. Neutron diffraction revealed that TN increases with dTN/dp « 12.4 K / GPa by the application of uni- axial pressure. The cooling effect has been measured T[K] in the very same way as for CeSb [2]. Fig. 1 shows the BCE in function of the initial temperature prior to the Fig.1: Barocaloric effect in release of pressure. With increasing pressure the max- imum of the cooling shifts to higher temperatures with the same rate as TN increases. A maximum cooling of -AT s» 0.42 K could be observed at around 10 K upon /("•; ambient 2000 - 0.13 GPa " the release of 0.24 GPa. The shape of -ATN shows 7 V~\r • qualitative difference compared to the case of CeSb. 1 / \ * 0.26 GPa For long-range ordered systems the BCE is expected to vanish for temperatures below TN(p = 0) [1]. However, 1500 _ in Ceo.85(Lao.95Yo.o5)o.i5Sb cooling below TN{p = 0) can still be observed. Further the BCE seems to sat- 1 S yr S,/jS- urate above 0.24 GPa as the application of 0.36 GPa does not increase -AT. mnn 0.3 0.4 0.5 The first feature may be understood by the effect q0 [reduced units] of dilution. Both neutron diffraction and macro- scopic measurements did not show long-range order Fig.2: Pressure dependence of the magnetic bragg in CeajO-a.Y^-sSb (x > 0.15) at ambient pressure. peak (llg0) in Ceo.85(Lan.95Y0.o5)o.i5Sb (uniax- However Fig. 2 indicates that the magnetic Bragg peak ial pressure along c-axis). (llqo) becomes more intense and sharper as pressure is increased. This means that the magnetic correla- tion length increases by pressure application which can well explain cooling below TN, as the magnetic entropy decreases with increasing correlation length. Macro- o scopic measurements on Ce3;(La,Y)i_xSb, x = 0.7 and x = 0.85 at ambient pressure have further shown char- acteristic features of the occurrence of a cluster-glass state at temperatures below TN(p —> 0) namely, qualita- 3K, 200G o 50G tive difference between zero-field and field-cooled mag- 2K.200G A 50G netization, frequency dependent sharp cusps in the AC- susceptibility, as well as a time-dependence of the mag- netization upon the change of an external magnetic field at constant temperature (Fig. 3, x = 0.7). Fig.3: Ce0 7(La0 76Yo 24)0 3Sb: The time-dependence [1] see previous report Of x = M/H(T = 2,3 K;H = 50,200,400 Oe) [2] Th. Strassle, A. Furrer, K. A. Muller, "Cooling by Adi- after zero-field cooling follows a power law. abatic Application of Pressure - the Barocaloric Ef- fect", Physica B, in press 37 MAGNETIC EXCITATIONS IN SINGLET GROUND STATE FERROMAGNET PrNi E.CIementyev', P.AIienspach1, P.AIeksee\f, V.Lazukov2 'Laboratory for Neutron Scattering, ETHZ & PSI, CH-5232 Villigen 2RRC Kurchatov Institute, 123182 Moscow, Russia A dramatic softening of one of the magnetic modes has been found at the magnetic Bragg point in singlet ground state ferromagnet PrNi. The softening occurs just above the Curie temperature. This fact is indicative of the unconventional ,,soft-mode driven" mechanism of magnetic ordering in PrNi. In rare-earth compounds based on non-Kramers ions triple axis spectrometer Druechal (SINQ-proposal with a singlet ground state magnetic ordering occurs II/99S-30) two inelastic transitions were found in a-c only if the exchange interaction exceeds a critical scattering plane (see Fig.1). A dramatic softening of value related to the crystal field (CF) splitting [1]. The one of the modes has been observed at the zone so-calied "soft mode driven" phase transition is center at temperatures just above Tc (Fig.2). Namely, considered as a plausible mechanism of magnetic the magnetic mode along [100] drops down to almost ordering (see [2] and references therein). This zero at the magnetic Bragg point (2,0,0). Further scenario fundamentally differs from the conventional studies are necessary to obtain the whole CF splitting one, namely the alignment of magnetic moments due scheme for PrNi and to estimate the strength of the to any kind of exchange interaction. In spite of anisotropic magnetic coupling in this system. extensive experimental studies the suggested softening of the low-energy magnetic mode at In view of the new findings PrNi provides a possibility temperatures close to Tc was never observed in a of a deep insight into the mechanism of magnetic ferromagnetic singlet ground system. ordering in a compound with a singlet ground state. PrNi is a singlet ground state ferromagnet with Tc=20K and the easy magnetisation direction along the "c" crystallographic axis. This binary compound has an orthorhombic crystal structure (space group Cmcm). The local point symmetry at Pr site gives rise to the 3 splitting of the Pr3+ ground multiplet H4 into 9 singlets. 160 - 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Energy (meV) Fig. 2: The temperature dependence of the magnetic mode along [100] in PrNi. Q=(2,0,0) corresponds to the zone center. Lines are guided to the eye. [1] Y.L.Wang and B.R.Cooper, Phys. Rev. 185 696 Fig. 1: Magnetic modes in PrNi along [100] and [001] (1969). for Q=(0.58,0,0) - black circles and Q=(0,0,0.7) - [2] R.J.Birgeneau et. al., Phys. Rev. B 6 2724 open circles measured at T=25K. (1972). In recent measurements of the dispersion of the low- energy magnetic modes in single crystal PrNi on the NEXT PAGE(S) left BLANK 39 d-Electron Magnetism NEXT PAGE(S) left BLANK 41 MAGNETIC EXCITATIONS IN CsMn(SO4)2(D2O)12 Ft. Basler1, H. Andres', H. U. Gudei1, C. Dobe1, P. Tregenna-Piggotf, S. Jansserf department fur Chemie und Biochemie, Universitat Bern, Freiestrasse 3, CH-3000 Bern 9 2Labor fur Neutronenstreuung, ETHZ & PSI, CH-5232 Villigen In CsMn(SO4)2-(D2O)12a co-operative Jahn-Teller effect is responsible for a cubic to orthorhombic phase transition occurring at ca. 160 K. We resolved five inelastic transitions on FOCUS with X = 5.32 A within the 5 Ag ground state of the orthorhombic phase. Both energies and intensities of these magnetic transitions were modelled successfully by an effective Spin Hamiltonian accounting for an axial and rhombic zero-field splitting. The title compound CsMn(SO4)2-(D2O)12belongs to the An increase of the temperature to 15 and 30 K is well known family of alums. It undergoes a cubic to concomitant with the appearance of three hot orthorhombic phase transition at ca. 160 K due to a transitions $and y at 0.199(3), 0.4646(7) and + co-operative Jahn-Teller effect. The [Mn(OD2)6f 0.6717(8) meV, respectively. The corresponding coordination distorts from S6 to C, point symmetry. transitions I', IT, a', f5' and y on the neutron energy Thus the ground state of Mn(lll) in the low temperature 5 gain side (negative energy transfer) are observed at phase is a zero-field split Ag term. The following effective Spin Hamiltonian, acting in the basis of five elevated temperatures. The measured transition S=2 spin functions, describes this splitting: energies are nicely reproduced by eq. (1) as seen in table 1. The best fit parameters are D = -0.5608(7) meV and E = 0.0339(3) meV. Additionally, table 1 shows the comparison between the measured relative intensities at 30K and the ones calculated according to the dipole approximation for The parameters D and E are the axial and rhombic non-interacting ions. The agreement between zero-field splitting parameters, respectively. experimental and calculated intensities is excellent From high-field high-frequency EPR data it has been and confirms of the mode!. impossible to unambiguously determine these zero- field splitting parameters. Therefore an inelastic neutron scattering (INS) experiment on a polycrystalline sample of the title compound was label energy normalized performed on the SINQ time-of-flight spectrometer [meV] intensities FOCUS. In Figure 1 the spectra obtained at T = 1.4, energy loss 15 and 30 K (X = 5.32 A ) are shown. 30 K exp calc exp calc I 1.5864(7) 1.5867 0.56(2) 0.58 II 1.790(1) 1.790 0.46(4) 0.48 a 0.199(3) 0.204 0.22(1) 0.14 P 0.4646(7) 0.4651 0.39(1) 0.42 Y 0.6717(8) 0.6687 0.38(1) 0.40 Table 1: Experimentally determined energies and relative intensities with estimated errors of the various INS transitions for neutron energy loss at 30 K. The values are normalised to transition I at 1.4 K. [1] R. Caciuffo, G. Amoretti, A. Murani, R. Sessoli, ir r Y P' «' a P A. Caneschi, D. Gatteschi, Phys. Rev. Lett., 81 i i_ i I I i i I i I t i i i (21), 4744(1998). -10 12 neutron energy loss [meV] Fig 1: INS spectra of CsMn(SO4)2-(D2O)12 at T = 1.4, 15 and 30 K measured on FOCUS {X = 5.32 A). The transitions are labeled on the bottom of the Figure. 42 MAGNETIC ORDERING IN B2Cu04 J. Schefer', B. Roessli1, U. Staub2, A. Amato3, G. Petrakovskif, P. Pattison5 Ch. Baines3 laboratory for Neutron Scattering, ETHZ and PSI, CH-5232 Villigen PSI, Switzerland 2Swiss Light Source (SLS), CH-5232 Villigen PSI, Switzerland 3juSFt-group, PSI, CH-5232 Villigen PSI, Switzerland 4Institute of Physics, SB RAS, Krasnoyarsk, Russia 5Swiss-Norwegian Beam Line, ESRF, F-38042 Grenoble, France We investigated the magnetic behavior of B2Cu04 copper-oxide by means of magnetic susceptibility, heat capacity, juSR and neutron diffraction, all facilities located at PSI. The magnetic structure has been determined at 12 K using the TriCS single crystal diffractometer. Incomensurate magnetic ordering has been observed at 4 K on the TASP spectrometer. Copper-oxides compounds with half-integer (S=1/2) ), where 5, and 82 may be equal. Full data Cu2+ spins and strong antiferromagnetic interactions sets from this incomensurate structure will be collected like Sr2CuC>4, Ca2CuC>3 exhibit new phenomena due at TriCS (4.2K) and at D10 (2K). to quantum fluctuations. One of the most striking feature is that in presence of important magnetoelastic —i—•—i—i—T—•—i—i—i—i—i—>—r coupling the lattice might become instable and 1.2 -c^222 : X X 1.0 V Q —o— 310 - undergoes a phase transition, similar to the \ B2CUO4 ; 0.8 —A— 130 ' dimerisation observed in the quasi-one-dimensional -^3-10" 0.6 Spin-Peierls compounds CuGeO3. -o— 1 -30: N J3 0.4 -3—1-10- 0.2 ^Sl --o— 110 ^ 0.0 —~—5 0.2 - We investigated the magnetic behavior of CuB2O4 0.4 8 copper-oxide by means of magnetic susceptibility, - 12 14 16 18 20 22 24 26 28 heat capacity, |iSR (Fig. 1) and neutron diffraction (Fig.2). Temperature [K] Fig 2: Neutron diffraction intensities of selected 100 reflections measured on TriCS. N I cc c o o 0.01 0.1 1 10 100 Temperature [K] Fig. 1: Depolarization rate measure with muon spin rotation at PSI. Both, the susceptibility and the specific heat measurement show sharp transitions at 20 K and 10 K. No nuclear phase transition has been observed down to 4.2K (powder diffraction on SNBL/ESRF). Therefore all phase transitions observed below 20K Fig. 3: Two possible magnetic models allocated to the are of pure magnetic origin. Using the single crystal chains (dark) and the layer (white) copper atoms diffractometer TriCS/SINQ, we reduced the possible respectively, which may be combined. The tetragonal magnetic structures of CuB2O4 at 12.5K two c-axis is vertical. Oxygen and Bor atoms are not commensurate possibilities shown in Fig.3, which will shown. be evaluated finally on D10. Preliminary results are published in [1]. Moreover, we found that the magnetic [1] G. Petrakovskii et al., JMMM 205,105-109 (1999) structure below 10 K is incommensurate with 43 SPIN-GLASS TRANSITION IN CuGa O B. Roessli1, A. Amato2, C. Baines2, F. Semadeni1, G. Petrakovskii3 laboratory for Neutron Scattering, ETH Zurich & Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland 2Laboratory for Muon-Spin Spectroscopy, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland 3Institute of Physics, Academy of Sciences, Siberian Branch, 64000 Krasnoyarsk, Russia Positive muon spin relaxation (^SR) have been used to characterise the magnetic ground-state of the spinel compound CuGa2O4. We observe a paramagnetic to spin-glass transition below T/~2.5K. At low temper- ature, the muon signal ressembles the dynamical Kubo-Toyabe expression reflecting that the spin freezing process in CuGa2O4 results in a Gaussian distribution of magnetic moments. De Seze pointed out that a spin-glass phase transi- the Gaussian internal field distribution (second moment tion can occur in a geometrically frustrated system with A2/T2) fluctuates at a rate v. This function, which, Ising spins and antiferromagnetic interactions only [1]. with the exception of some limiting cases, cannot be Following [1], Villain [2] proposed that spin glasses expressed analytically, directly depends on the param- can be obtained in materials with geometric frustra- eters v and A. Figure 2 shows the temperature de- tion and Heisenberg-type exchange interactions like cu- pendence of the parameter A which exhibits a clear in- bic spinels. Spinel compounds have the chemical for- crease below ~ 3K. The fluctuation rate v was found mula AB2O4. When both sublattices are occupied by to be constant below T/-2.5K {y ~ 3.7 MHz). In- magnetic ions the ground state is a ferrimagnet. The terestingly, the DKT function describes the data more B sublattice builds connected tetrahedra and antifer- satisfactorily than the model of "coexisting static and romagnetic interactions induce topological frustrations dynamical fields" developed by Uemura et al. [6] in- [3] which can lead to a spin-glass state when non- dicating that in CuGa2O4, by decreasing the temper- magnetic impurities are introduced [2]. The /iSR ex- ature, an increasing part of each Cu2+ moment be- periments were performed on the LTF spectrometer at comes quasi-static (characterized by a slow fluctuation the Paul-Scherrer Institut, Switzerland. The data were rate v), whereas the remaining part does not affect the recorded using the zero-field method which is very sen- muon polarization due to fast fluctuations not accessi- sitive to determine both static and dynamic effects in ble within the /iSR time-window. spin-glasses [4]. Fig.2: Temperature dependence of the A-parameter of the DKT function. This parameter mirrors the width of the quasi-static field distribution below Tf and therefore the value of the quasi- 0.5 1.0 1.5 2+ Time (us) static Cu moment. Fig.1 Zero-field /xSR signal measured in CuGa2O4 [1] L. de Seze, J. Phys. C: Solid State Phys. 10, L353 at T = 650mK (top) and T = 4.5K (bottom). (1977). The negative values at t = 0 reflects the phase [2] J. Villain, Z. Physik B 33, 31 (1979). between the direction of the positron detectors [3] P.W. Anderson, Phys. Rev. 102, 1008 (1956). and the initial muon polarization. Note the dif- [4] K. Emmerich, E. Lippelt, R. Neuhaus, H. Pinkvos, ferent y-scales. Ch. Schwink, F.N. Gygax, A. Hintermann, A. Schenck, W. Studer and A.J. van der Wai, Phys. For temperatures below T=3.5K, the muon depolariza- Rev. B 31, 7226 (1985). tion increases [see Fig. 1 (top)] and assumes a Gaus- [5] R. Kubo, Hyperfine Interact. 8, 731 (1981). sian character at short times. The best description [6] Y.J. Uemura, T. Yamazaki, D.R. Harshman, M. of the data is obtained using the so-called dynami- Senba, and E.J. Ansaldo, Phys. Rev.B 31, 546 cal Kubo-Toyabe (DKT) function [5], which reflects that (1985). 44 MAGNETIC EXCITATIONS IN THE S=5/2 ANTIFERROMAGNET RbMnCI3 N. Cavadini1, A. Trottmann1, A. Furrer1, K.Kramer2, H.U. GudeP 1 Laboratory for Neutron Scattering, ETHZ & PSI, CH-5232 Villigen PSI 2Department for Chemistry and Biochemistry, Uni Bern, CH-3000 Bern 9 Inelastic neutron scattering investigations have been performed to study the influence of magnetic anisotropy on the classical S=5/2 spinwave spectrum in RbMnCI3. Evidence of a spin gapped excitation branch is reported, whose origin is proposed to depend on the collective internal dipolar field at T«TN. RbMnCI3 is a classical three-dimensional magnetic from the magnetic order at T -2.0 -1.5 -1.0 -0.5 0.0 energy transfer [meV] c/2 e Fig. 1: Schematic view of the spin-spin correlations in RbMnCI3, as described in the text. 1.00 1.02 1.04 Energy scans performed at T«TN in the vicinity of the minima of the dispersion relation indicate the momentum transfer [r.l.u.] presence of a spin-gapped excitation branch at small q, accompanied by a barely resolved gapless branch in Fig. 2: Observed profiles near q~0 on the energy the tails of the elastic line (Fig. 2, top). Both excitations gain side (top). Data at T«TN and T»TN are compared. have been observed to disappear well above TN=94K, Observed dispersion near q~0 (bottom). which is a clear confirmation of their magnetic origin. To account for the experimental observations, a spin exchange model including anisotropic internal fields [1] M.Melamud etal., Phys.Rev. B 3, 821 (1971) has been developed. The origin of these fields is [2] H.A.AIperin et al., J. Appl. Phys. 52, 2225 ascribed to the induced dipolar contributions arising (1981) 45 CRITICAL FLUCTUATIONS IN THE WEAK ITINERANT Ni3AI. F. Semadeni1, P. Vorderwisch2, B. Roessli1, T. Chatterji3 and P. Boni1 1 Laboratory for Neutron Scattering, ETH Zurich & PSI, 5232 Villigen PSI, Switzerland 2 Hahn-Meitner Institut, 14109 Berlin, Germany 3lnstitutLaue Langevin, BP 156, 38042 Grenoble Cedex 9, France The magnetic excitations in the weak itinerant ferromagnet Ni3AI have been investigated in the ordered phase and at Tc by means of inelastic neutron scattering. The linewidth of the scattering at Tc shows that the dynamical critical exponent z « 2.47 ± 0.16 is close to the value expected for an isotropic Heisenberg model. Moreover, the temperature dependence of the spin waves compares well with the predictions ofMMT, whereas above Te the scaling function is compatible with the itinerant character of the compound. A comprehensive investigation of the spin dynamics of temperature Tc is reported in Fig. 1. The fit yields the weak itinerant ferromagnet Ni3AI has been under- a dynamical critical exponent in good agreement with 25 taken in order to search for critical fluctuations. The the Heisenberg model (r(Tc) = Ag' ), whereas the inspection of the dynamical scaling behaviour for the itinerant model would enforce a q3 dependence of the spin-wave and paramagnetic fluctuations is thought to linewidth. provide a determinant criterion for this purpose. The scaling function for the spin waves is presented in Fig. 2, versus the scaling variable x — n~(T)/q, where K~{T) is the inverse correlation length. The experimen- tal scaling of the linewidth below Tc can be well de- scribed by MMT, assuming K^ as the only fit parameter. The results yield K^ = 0.10 ± 0.04 A~\ that compares well with the value from a polycrystalline sample above To [5]. In the paramagnetic phase, on the other hand, the scal- ing behaviour shows better agreement with the predic- tions of the itinerant model [3] (see Fig. 3). 0.04 0.06 q (A1) Fig.1: Quasielastic linewidth measured at Te. The z solid line is a fit to the Aq model. For the 1.2 - dashed line the exponent has been fixed at 3, I i as expected from itinerant theory. r 0.8 - The scaling functions were determined by calculating Resibois-Piette the ratio T(T)/T(TC) for both types of excitations, and „_____ i + x2 have been compared with mode-mode coupling the- 0.0 0.2 0.4 0.6 ory (MMT) including critical fluctuations [1,2] and self- x consistent renormalisation theory (SCR-RPA) where critical scattering is not taken into account [3,4]. Fig.3: Dynamical scaling function for the paramag- netic fluctuations. The dashed line represents the Resibois-Piette scaling function, whereas the solid line (1+.r2) is provided by SCR. The present results indicate that there is a need to intro- duce critical fluctuations in SCR, in order to explain the dynamical scaling behaviours observed in Ni3AI below Tc, within the framework of an itinerant model. [1] P. Resibois, C. Piette, Phys. Rev. Lett. 24, 514 (1970). x= tc/q [2] H. Schinz, F. Schwabl, Phys. Rev. B57,8438 Fig.2: Dynamical scaling function for the linewidth of (1998);P/7ys. Rev. B 57, 8456 (1998). the spin waves as a function of the scaling pa- [3] T. Moriya, Spin Fluctuations in Itinerant Electron rameter x = K~/q. The solid line is given Magnetism, Springer, Berlin, 1985: and references by mode-mode-coupling theory for an isotropic therein. ferromagnet [4] G.G. Lonzarich, L. Taillefer, J. Phys. C 18, 4339 (1985). The linewidth of the magnetic fluctuations at the Curie [5] N.R. Bernhoeft et al. Phys. Rev. B 18, 422 (1983). 46 ON THE ORIGIN OF THE BIQUADRATIC EXCHANGE INTERACTION IN CsMn'0.14nM'Mg10.86poc*Br, Th. Strassle and A. Furrer Laboratory for Neutron Scattering, ETH Zurich and PSI, CH-5232 Villigen PSI The origin of the biquadratic exchange term in a Heisenberg Hamiitonian may be based on Neel's idea of magnetostriction [1], which was discussed in detail by Kittel [2]. The dimer system CsMn0UMg086Br3 allows to prove the Neel-Kittel idea experimentally by determination of the bilinear exchange parameter J in function of the distance R between the dimer ions. By means of external pressure both J and R can be changed thus supplying the needed relation J(R). We succeeded in measuring R(p), however in order to measure J(p) further improvements in the experimental setup are necessary. MgxBra crystallizes in the hexagonal space by the pressure cell and the small sample volume. The group P63/mmc. For low Mn concentration (x=0.86) observed shifts in the NaCI Bragg peaks gave an dimers of Mn2+ (S=5/2) ions form. The excitation estimation of the applied pressure of 0.3 GPa at spectra of these dinners can only be described by 4.2 K. Table 1 shows the lattice parameters for p=0 inclusion of a biquadratic coupling term in the and p=0.3 GPa. By comparing the lattice parameters Hamiitonian [3,4] at ambient pressure both at 4.2 K and at room temperature, one finds a strong thermal expansion, 2 which may explain the loss of pressure from initially = -2JS1S2-4K(S1S2) (1) 0.8 GPa at 293 K down to 0.3 GPa at 4.2 K. We observed a striking behavior of the lattice parameters Applying Neel-Kittel's idea to a magnetic dimer in of CsMn Mg Br : c tends to increase while a hexagonal symmetry yields 014 08S 3 decreases when pressure is applied. The overall decrease in volume yields a compressibility similar to K = [R (aj/3R)| J2/(16aV) 0 R (2) NaCI. Comparing the effect of both a decrease of 2 temperature and an increase of pressure suggests a = (a/2c) (c11+c12)-(a/c)c13+(1/2)c (3) that the c-axis behaves mechanically stronger than the a-axis. Further experimental and theoretical work on V denotes the volume of the unit cell, R is the 2+ o the exchange mechanism between the Mn ions and equilibrium distance between the Mn2+ ions in the the bonds to neighboring atoms must be done in dimer, a and care the lattice parameters, and c are the lk order to explain this effect in detail. elastic constants, which are known from phonon dispersion measurements [5]. A measurement of the interdimer distance R=c/2 and the bilinear exchange Table 1: Lattice parameters of CsMn014Mg086Br3 parameter J in function of pressure can supply the (* values from ICSD database) relation J(R) essential to check Eq. 2 experimentally. Small variations of the lattice parameters were TfK] / pfGPal a [A] c[Al c/a expected, therefore both the elastic and the inelastic 4.2/0 7.4823(47) 6.4575(64) 0.863 measurements were performed on the triple axis 4.2 / 0.3 7.4147(44) 6.5008(83) 0.877 spectrometer TASP. The powder sample was 293/0 7.609 * 6.520 * 0.857 prepared in a zero-matrix clamp cell made of Zr-Ti alloy allowing pressures up to 1 GPa. NaCI was added to For the inelastic measurements the poor signal to the sample as a pressure gauge. All measurements noise ratio did not allow the measurement of even the have been done at 7=4.2 K. For the elastic strongest |0>H>|1> excitation of the dimer at 1.8 meV. measurements neutron energies of 4.0 meV and 13.7 meV with aflat analyser were chosen. The beam This experiment has revealed one of the two important was collimated 20' before the monochromator, 40' relations to proof Neel-Kittel's idea namely the R(p) before and 40' after the sample. To avoid higher order dependence of the dimer ions. Improvements in the Bragg peaks a Be and a graphite filter were used, inelastic part of the measurement will enable us to respectively. All inelastic measurements were done in measure J(p) in the near future which would allow the the energy loss mode with a fixed monochromator calculation of the biquadratic exchange parameter K. energy of 5.0 meV and focusing analyser. Higher order contamination was avoided by a cooled Be filter placed in front of the analyser. The weak signal did not [ 1 ] L. Neel, Bull. Soc. Frang. Min. Crist. 77, 257 (1954) allow the use of any collimators. [2] C. Kittel, Phys. Rev. 120, 335 (1960) [3] U. Falk et al., Phys. Rev. Lett. 52, 1336 (1984) The Bragg peaks turned out to be very weak in [4] U. Falk et al., Phys. Rev. B 35, 4893 (1987) intensity due to the absorption of the incident beam 47 Structure and Dynamics ijf; NEXT PAGE(S) left BLANK 49 RHOMBOHEDRAL DISTORTION OF THE CUBIC CHEMICAL STRUCTURE OF HoB6 AT LOW TEMPERATURES DUE TO QUADRUPOLAR ORDERING A. Donni1, S. Kunii2, L Keller3, P. Fischer3, T. Herrmannsdorfer3 and VPomjakushin 3 1 Dep. of Physics, Niigata University, Niigata 950-2181, Japan 2 Dep. of Physics, Tohoku University, Sendai 980-8578, Japan 3 Laboratory for Neutron Scattering, ETHZ and PSI, CH-5232 Villigen PSI Below TQ =6.1 K and TN = 5.6 K HoB6 is found by high-resolution neutron diffraction to be rhombohedrally distorted according to space group R-3m, with the angle a increasing from 90 to 90.26 at 2.1 K. At 10 K and 293 K the HoB6 sample appears to crystallize with space group Pm-3m, apart from a minor second HoB12 phase. At room temperature HoB6 crystallizes in a cubic At low temperatures high-resolution neutron diffraction CaB6-type chemical structure according to the space mesurements were performed on HRPT with HoB6, group Pm-3m and undergoes a ferro-quadrupolar using X = 1.886 A, primary collimation 12' and phase transition at TQ = 6.1 K [1]. From bulk secondary collimation 24' (sample diameter 8 mm). As magnetic measurements HoB orders antiferro- 6 illustrated in Fig. 2, there is clear evidence for a magnetically at temperatures below TN = 5.6 K [2]. rhombohedral distortion of HoB6 at 5.9 K and 2.1 K, In order to investigate the relations of chemical similar to DyB6 [3]. Note that the Bragg peaks of HoB12 structures, quadrupolar and magnetic ordering, a do not indicate significant positional shifts in the same 11 polycrystalline sample of Ho B6 was investigated on temperature range. Profile refinements based on the SINQ powder diffractometers HRPT and DMC space group R-3m yield for HoB an increase of the (SINQ project II/99L-15) in the temperature range 6 rhombohedral angle a from 90° at 10 K to 90.185(1)° from 2.1 K to 293 K. In contrast to X-ray diffraction, neutron scattering is particularly sensitive to the light at 5.9 K and 90.264(1)° at 2.1 K. Thus quadrupolar B atoms and to magnetic ordering phenomena. The ordering induces the rhombohedral distortion which sample preparation of incongruently melting hexabo- increases in the magnetically ordered state. rides is described in ref. [2]. Low-temperature The analysis of the antiferromagnetic ordering investi- neutron measurements were performed in a ILL type helium cryostat. gated on DMC is in progress. Fig. 1 illustrates the HRPT neutron diffraction pattern 5000 of the HoB6 sample at room temperature for a neutron wavelength X = 1.197 A in the high intensity mode of instrument operation. The profile refinement confirms the CaB6 type chemical structure of HoB6 at room temperature with a lattice parameter a = 4.097 A and the positional parameter xB = 0.1987(1). Moreover it revealed the presence of a 11 volume % fraction of HoB12 in the sample. i 160 zCO LJJ Fig. 2: Temperature dependence of the high scattering angle part of the HRPT neutron •z. 5000 - diffraction pattern of HoB6 + HoB12 {X = 1.886 o A, high resolution mode). DC Z> LU References 0 2 0 40 60 80 100 120 140 160 [1]T. Goto et al., to be publ. in proceedings of SCES'99. Fig. 1: Observed, calculated and difference HRPT [2] K. Takahashi and S. Kunii, J. Solid State Chem. neutron diffraction patterns of HoB + HoB 6 12 133, 198 (1997). (higher and lower hkl bars, respectively, A, = [3] K. Takahashi et al., Physica B 241-243, 696 1.197 A) at room temperature. (1998). 50 PRESSURE DEPENDENCE OF THE PHONON DENSITY OF STATES IN Pr0 iLa0 9Ni03 Th. Strassle and A. Furrer Laboratory for Neutron Scattering, ETH Zurich & PSI, CH-5232 Villigen PSI The generalized phonon density of states in Pr0.iLa0.9NiO3 has been measured at ambient pressure and at p«2 GPa. The knowledge of the phonon density of states is crucial in order to estimate the usage of the Pri-^La^NiOa nickelates for cooling by the barocaloric effect. Our results indicate that the influence of the lattice can well be described by the empirical Gruneisen theory. The barocaloric effect [BCE] reflects a cooling tech- nique similar to the well-known magneto-caloric effect [1]. An adiabatic change of the system's total entropy S(T) can lead to cooling or heating, respectively. In the BCE the application of relatively weak external pres- sure results in a change of the system's magnetic en- tropy, whereas for the magneto-caloric effect magnetic fields of a few Tesla are necessary. The total cooling of the system -AT is given by S(T,p = 0) = S(7' + AT,P > 0) (1) where S(T,p) = Sm(T,p) + St(T,p) + Se-(T,p) de- notes the total entropy of the system with its magnetic, phononic and electronic terms, respectively. Thus in or- der to model the cooling effect knowledge of the pres- sure dependence of 5; gets inevitable. From an empiri- cal point of view the pressure dependence of Si can be modelled by the GrOneisen theory relating the shift of the Debye temperature QD to thermodynamic proper- ties of the system [2]: = • Ap (2) V E [meV] with an overall Gruneisen parameter r, the volume Fig.1: Measured neutron spectra and GPDOS of V, the thermal expansion coefficient a, the specific Pr0.iLa0.9NiO3 at 293 K and p = 0. heat cv and the applied pressure change Ap. The lattice has a detrimental effect on the BCE: First AT The peaks at 9.5,15.5 and 24.0 meV are in accordance is decreased due to the monotonically increasing 5; to the observed phonon dispersion and Raman spec- and second, when pressure is applied elastic heating tra of the isostructural LaGaO3 [3]. For the measure- occurs due to the hardening of the phonons yielding ment of the GPDOS under pressure the sample was St{T,v > 0) < Si{T..p = 0). The latter effect com- placed in a pressure cell made of steel allowing pres- petes the BCE and turns out to be almost linear in T sures up to 2 GPa at room temperature. Within the for T < QD- However, this model is correct only, if the given resolution the spectrum at p = 1.8 GPa did not system's phonon spectra can be described adequately show any shift or change in the observed peak po- by a Debye-Einstein model. sitions and intensities, respectively. From the simple In order to study the influence of pressure upon the Gruneisen model one expects the peaks to shift by application of pressure we measured the generalized about Aw/u w 0.7 % / GPa, which is certainly at the phonon density of states [GPDOS] of a powder sam- limit of the instrumental resolution for the given con- ple at room temperature by means of inelastic neutron figuration. Therefore we conclude that for pressures scattering on the triple axes spectrometer DruchaL. p < 1.8 GPa no significant change in the phonon spec- The neutron energy gain configuration was chosen with trum occurs and that the use of the Gruneisen model a fixed incoming neutron energy of 13.7 meV in con- is justified in order to get a first approximation of the 1 stant Q-mode with Q = 4.75 A" . This configuration pressure dependence of the phononic entropy. allowed to measure the GPDOS up to around 80 meV using cold incoming neutrons. The background was determined by measuring the spectrum at 15 K where no phononic contributions are expected in energy gain [1] K. A. MOIIer, M. Koch, S. Fischer, F. Fauth, A. Furrer, mode. Appl. Phys. Lett. 73, 1056, (1998) In Fig. 1 we show the measured phonon spectra of [2] Th. Strassle, Diploma Thesis (ETH Zurich, 1999) Pro.iLa0.9Ni03 together with the extracted GPDOS. [3] W. Marti, Ph.D. Thesis (ETH Zurich, 1995) 51 DEUTERIUM ATOM DISTRIBUTION IN HEXAGONAL ZrCr2D4 P. Fischer 1 and A. Skripov 2 1 Laboratory for Neutron Scattering, ETHZ and PSI, CH-5232 Villigen PSI 2 Institute of Metal Physics, Urals Branch of the Academy of Sciences, Ekaterinburg 620219, Russia The hexagonal Laves phase C14-ZrCr2D4 was investigated by means of neutron diffraction at room temperature. It shows a statistical deuterium distribution over 4 D sites comparable to ZrMn2D3. Short- range D correlations visible as diffuse peak suggest the possibility of order-disorder phase transitions at lower temperatures. Systematic NMR studies of C15- and C14-type Fig. 1 shows the corresponding observed and Laves compounds ZrCr2Dx indicate for both sytems calculated diffraction pattern. The broad diffuse structural phase transitions around 250 K associated hump in the background around scattering angles of with deuterium ordering [1]. Recently Kohlmann et 55° is caused by short-range order of deuterium, al. investigated carefully such a transition by means similar to the cubic phase [2]. As often for hydride of neutron diffraction in the C15 compound ZrCr2D38 samples, one observes considerable line broade- [2]. With respect to deuterium mobility/diffusion it is ning. It is best described by function 5 of FullProf [3]. important to know in detail the structural differences The refinement shows that the sample contains a between these cubic and hexagonal systems. volume fraction of about 26 % of the cubic phase 2 and yielded the temperature factors B?r = 0.23(4) A , As a test experiment we therefore investigated at 2 2 BCr = 0.72(6) A and BD = 0.9(1) A . The lattice room temperature on HRPT with X = 1.886 A in the parameters of the dominant^ hexagonal phase a = high-intensity mode a small sample (diameter 5 mm, 5.437 A and c = 8.901 A are in approximate height 28 mm, measurement time of the order of one agreement with the mentioned X-ray values. The day) of C14-ZrCr D with X-ray lattice parameters a 2 4 neutron intensities yield for space group P63/mmc a = 5.448 A and c = 8.840 A [1]. For the profile deuterium concentration x = 3.5(2) and indicate that refinement with FullProf [3] we started from the the sites (24I), (12 k) and two sites (6h) are occupied structural parameters of ZrMn2D3 [4] and of C15- to approximately 30 %, 26 %, 36 % and 26 % by ZrCr2D4 of ref. [2]. deuterium atoms, respectively. The refined positional parameters are close to those of ZrMn2D3. However, these results should be regarded as preliminary due to an nonnegligible influence of the cubic phase. References [1]A. V. Skripov, Yu. G. Cherepanov, B. A. Aleksashin, S. V. Rychkova and A. P. Stepanov J. Alloys Comp. 227, 28 (1995). [2] H. Kohlmann, F. Fauth and K. Yvon, J. Alloys Comp. 285, 204 (1999). •1 104 2 0 140 160 [3] J. Rodriguez-Carvajal, Physica B 192, 55 (1993). [4]J. J. Didiheim, K. Yvon, D. Shaltiel, and P. Fig. 1: Observed, calculated and difference neutron Fischer, Solid State Commun. 31, 47 (1979). diffraction pattern of ZrCr2D4 (X = 1.886 A, 620 = 0.05°) at room temperature. The vertical bars indicate Bragg peak positions for the dominant C14 phase, for the minor C15 phase and for V from the sample container, respectively (from above to the bottom). 52 CHEMICAL DISORDER AND MAGNETIC ORDERING IN Ce2Pd2ln INVESTIGATED BY COMBINED NEUTRON AND RESONANT X-RAY SCATTERING T. Herrmannsdörfer1, P. Fischer ', D. Schaniel '', L.Keller1, M. Giovannini2, R. Hock3, E. Bauer" 1 Laboratory for Neutron Scattering, ETHZ& PSI, CH-5232 Villigen PSI 2 Dipartimento di Chimica e Chimica Industriale, Università' di Genova, 1-16132 Genova 3 Lehrstuhl für Kristallographie und Strukturphysik, Universität Erlangen-Nürnberg, D-91054 Erlangen 4 Institut für Experimentalphysik, T.U. Wien, A-1040 Wien Resonant X-ray diffraction experiments on solid solutions of the Kondo-system Ce2PdJn are used in combination with magnetic neutron data to obtain information about site disorder and magnetic ordering. Solid solutions based on the Kondo-system Ce2Pd2ln same Bragg reflection between the different (space group P4/mbm) exhibit a critical influence of wavelengths could be observed. An example of a data the chemical composition upon their magnetic set is given in Fig. 1. One should note the relatively properties. It was discovered that the nominal 2:2:1 wide accessible sin(0) / X-range and the excellent stoichiometry is just on the border to a region resolution given by the instrument which separates the extending towards compositions with an In deficiency. Bragg reflections even at this short wavelength. Furthermore, a stoichiometry-induced transition from ferromagnetism to antiferromagnetism with increasing Pd content was reported [1]. I 20 Ce2Pd2ln BM16 0.4453 Ä Several possible models concerning site-occupancies T = 7K between Ce and Pd on the four crystallographic sites " 15 4h, 4g, 4e and 2a may be employed to account for the magnetic ordering effects already found by neutron diffraction. Since the chemical disorder is closely correlated to the magnetic ordering, an accurate il i • knowledge of the site occupancies is essential. MU uni n in in ninniiiiiliiiiii iniiuiiiii in lliliiiniliiiiluiiiiii ii 0.3 0.4 No. of electrons /A'1 Element scattering length /fm (neutral atoms) Fig. 1 : X-ray diffraction pattern of a Ce2Pd2ln sample Pd 46 5.91 with nominal 2:2:1 composition, taken at the K In 49 4.065 absorption edge of In. Ce 58 4.84 Neutron data sets for these compounds taken at the ILL diffractometers D1A and D1B as well as at the Tab. 1 : Scattering contrast of the atoms in Ce2Pd2ln. SINQ instrument DMC are already availabe [1]. A detailed analysis of the experiments is in progress. From a diffraction experiment, site occupancies can With the combination of the complementary resonant only be determined accurately if there is sufficient X-ray and magnetic neutron data in joint refinements, it scattering contrast between the elements that can is to be expected that accurate information about the occupy the same crystallographic site. This can either structural interplay between site disorder and magnetic be achieved via a significant difference in the atomic ordering in these compounds will be obtained. form factors (X-ray case) or scattering lengths (neutron case). In our sample, the situation concerning the scattering contrast is rather delicate, since in both [1] M. Giovannini, H. Michor, E. Bauer, G. Hilscher, cases the relative differences are quite small (Tab. 1). P. Rogl, T. Bonelli, F. Fauth, P. Fischer, T. In order to increase the scattering contrast, resonant Herrmannsdörfer, L. Keller, W. Sikora, A. X-ray scattering data sets were collected for samples Saccone, R. Ferro, Phys. Rev. B 61 (at press). of various stoichiometries at the high resolution powder diffraction beamline BM16 at ESRF Grenoble. The incoming X-ray energies were chosen to be just below the K-absorption edges of Pd and In. A He Kyostat was used to achieve temperatures down to 5 K. Additional contributions to the real part f of the atomic form factors due to resonant scattering of approx. -5 electrons were obtained. Therefore, considerable changes of intensities belonging to the 53 11 STRUCTURE OF Y092Er008 BO, M. Ren\ J. H. Un\ Y. Dong\ L Q. Yang", M. Z. Su\ L P. Yotl, and P. Allenspach3 1 State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, Beijing 100871, P. R. China 2Laboratory of Microscopy, Peking University, Beijing 100871, P. R. China 3Laboratory for Neutron Scattering, ETHZ and PSI, CH-5232 Villigen PSI u YOS2Er008 BO3 was measured on DMC at different temperatures ranging from room temperature up to 850 degrees Centigrade. The structure was refined using simultaneously X-ray and neutron diffraction data. For a satisfactory agreement with the data a triclinic cell has to be assumed. Lanthanide orthoborates LnBO3 (Ln : rare earth and DMC at different temperatures between room yttrium) show high VUV (vacuum ultraviolet) temperature and 850 degrees Centigrade did not transparency and exceptional optical damage 3+ 3+ yield any variation in the structure. This is in contrast thresholds and, moreover, the Eu - and Tb -doped to GdBO where a phase transition was observed at LnBO exhibits extraordinarily high luminescent 3 3 around 800 degrees [1, and Fig. 1]. Unfortunately, at efficiency under VUV excitation and, thus are considered to be attractive candidates as VUV that time no higher temperatures could be achieved luminescent materials used in the gas discharge with the furnace in order to define the temperature of 11 panels. In the last couple of years considerable effort the phase transition for Y092Er008 BO3. has been devoted to improve the performance of these materials. A problem of these materials in application is the color purity. For the Eu3+ -doped X-Ray GdBO3, for example, the emission spectrum is 5 composed of almost equal contributions from the D0- 7 5 7 F, (580 nm) and D0- F2 (613 nm) transitions, which give rise to an orange-red color instead of deep red. In terms of application, concentration of the main £•• 5 7 emission in the D0- F2 transition is required. It is 5 7 known that the D0- F2 transition is hypersensitive to . ii h the symmetry of the crystal field (CEF). Therefore, improving the performance, as well as any sensible interpretation of the luminescent properties, requires a well-defined crystal structure of these compounds. 10 20 30 40 50 60 70 90 100 110 LGdBO3 -25 Neutrons 2.GdBO3 -800 3.GdBO3 -1000 4.OdBO3-70D 5.G(iBOj-500 ; s (a.u. ) -A. 11 5 n 43 ill A A 1 ] J Two Theta / degree 20 30 40 JO 11 Fig. 2: X-ray and neutron data for Y092Er008 BO3. Two Theta/degree Fig. 1: Temperaure dependent X-ray diffraction From the X-ray data taken in Beijing and the Neutron patterns for GdBO . data from SINQ in a simultaneous fit (Fig. 2) a new 3 structure model (triclinic instead of rhomohedral) could be obtained which is able to explain the In order to define the position of boron and oxygen previously not understood spectroscopy data for more accurately we employed neutron diffraction on 3+ YBO3:Eu . Further neutron diffraction and a supposedly isostructural compound to GdBO3, spectroscopy investigations are planned. 11 namely Y092Er008 BO3. The addition of Er will enable us in a future accepted experiment to measure the Reference CEF splitting in this material. Test measurements on [1] M. Ren et al. Chem. Mater. 11(6), (1999) 1577. NEXT PAGE(S) left BLANK 55 Multilayers and Surfaces NEXT PAGE(S) left BLANK 57 STRESS AND MAGNETIC ANISOTROPY IN Fe089Co011/Si MULTILAYERS D. Clemens', M. Horisberger1, S. Wehrli2 'Laboratory for Neutron Scattering ETHZ&PSI, CH-5232 Viliigen PSI sEcole Polytechnique Federate Lausanne, CH-1015 Lausanne In a saturated state FegS9Co011/Si is a material combination that has no contrast in scattering length density for spin down neutrons. If Si substrates are used as a support no absorption subcoatings are needed. In order to expand the q-range over which Fe0S9Co01/Si supermirrors reflect spin up neutrons the stress within the multilayer stack has to be minimized. As we found in-plane magnetic anisotropy in another Fe-Co alloy this property is investigated in magnetic hysteresis measurements. Fe089Co011/Si and Fe/Si are among the most problem of magnetically dead boundary layers which appropriate systems that can be applied to multilayer has been encountered in other samples is not present transmission polarizers. They have a high reflectivity in our Fe089Co0l1/Si multilayers. flupfor spin up neutrons because of the large contrast in The production of supermirrors up to m=2 (multiples of their scattering length densities pso between Si and the the critical angle of Ni) has been achieved. Higher m saturated magnetic layers. Contrary to this the spin supermirrors revealed the problem of stress. With a, s 2 9 2 down reflectivity f?down is that of a Si substrate. between 2 • 10 dyn/cm and 6 • 10 dyn/cm the We produced Fe089Co0l1 single films and Fe089Co011/Si (tensile) stress is considerably higher than in multilayers with constant bilayer period Dbi = 14 nm in Fe050Co048Vo02/Ti:N polarizing multilayers. Thus the a sputtering process under different Ar gas pressure m=3 do not last on the substrate. We are in the course pAr. In one series (pAr = 0.25 Pa) we also varied Dbi of optimizing the adherence and parallely the magnetic beween 10 and 30 nm in order to cover thickness behaviour of this system. Fe089Co011/Si as produced values that are typical for a supermirror structure. The during this series shows an in plane easy axis deposition has been performed in a automated perpendicular to the direction where it has been found Leybold Z600 DC magnetron sputtering plant. Float in Fe050Co048V002. The remanence is not as pronounced glass and polished Si wafers served as substrates. but still sufficient for zero field applications. The surface curvature of so-called Si stress wafers - the substrates that were used to determine the -ICO -50 0 „,, 50 ICO "50 200 residual stress of the multilayer - has been characterized in preliminary profilometry 0.002 \ measurements. These substrates were profiled in a second run after having deposited the multilayer 0.001 sequence. -O.0D1 \ J -0.032 -2O0 -150 -100 -50 0 100 150 :O3 H[Oe] Fig. 2: In plane magnetic hysteresis as measured with a vibrating sample magnetometer for the sample in Fig. 1 [1] D. Clemens, Th. Krist, P. Schubert-Bischoff, Fig. 1: ft (filled symbols) and f? (open triangles) J. Hoffmann, F. Mezei, Physica Scripta 50, 195 up down (1994) for a 10x14nm Fe089Co01,/Si multilayer (TOPSI/SINQ, ,1=0.4735 nm) The neutron data in Fig. 1 has no obvious peak in f?down at the angle where the peak in R appears. The 58 ABSORPTION LAYERS FOR SUPERMIRROR POLARIZERS S. Wehrli1, D. Clemens2 1Ecole Polytechnique Federate Lausanne, CH-1015 Lausanne 2Laboratory for Neutron Scattering ETHZ&PSI, CH-5232 Villigen PSI Absorption layers for supermirror polarizers were investigated. Different layer systems of TiGd and TiNx were probed by neutron reflectometry. It was possible to optimize the absorption properties without affecting the reflectivity of the polarizer. Applications to supermirror polarizers showed a significant improvement of the polarization at small q (< 0.2 nm1). Appropriate magnetic materials allow for the production of supermirror polarizers for neutrons [1-4]. Frequently used materials are Ti/Co, Si/Fe, and TiNx/Feo.5Co0.48V. The latter material combination is transparent for spin down neutrons. Rup/dOwn denotes Polarizer m = 1.8 with TiGd(450A) the reflectivity for spin up and spin down neutrons, —A—Spin up \ respectively. The polarization P={RUp- •—v—Spin down « —o— Polarization A ftdown)/(ftup+ftdown), of supermirror polarizers is limited at small q, because of remaining fldown from the 0.3 0.4 substrate (here float glass). Glass has a positive q (nm'1) scattering length density psc and totally reflects 1 neutrons up to g=0.15 nm" . This can be avoided with Fig. 2: R and P of an m=1.8 supermirror polarizer on additional absorption layers introduced between the 45 nm TiGd (TOPSI, ^=0.4735 nm). TiNx/FeCoV layers and the substrate. A TiGd alloy was used as absorber material with good sputtering favorable (Fig. 1). Further improvement was achieved 1 properties. Neutrons penetrate easily into the alloy, with system 3 giving /?down< 10% for q > 0.06 nm" . whereas pure Gd reflects at small q. The absorbing <7rms = o.7±O.1 nm on top of the absorption layers, a layer should not be too thick in order to avoid value that is similar to the surface of the glass, as additional interface roughness which leads to diffuse determined by comparing reflectivity measurement scattering and lowers Rup and therefore P. Bilayer with simulations. 45 nm TiGd and system 3, both with systems consisting of TiN and TiGd were probed. The an m=1.8 polarizer on top are shown in Fig. 2 and 3 x 1 combination of TiGd with TiNx should lead to a smaller and shows a lower Raown below q=0.2 nm" and module of the psc and therefore to a better penetration therefore a better polarization as flup is not affected. of the neutrons into the absorption layers. Three different absorbing systems, two of them coated with (20 x [TiNx(5 nm) + FeCoV(5 nm)]) were produced and their absorption properties and roughness o was determined. System 2 has clearly superior absorption properties than System 1, although both 100 nm TiGd in total, signifying that the TiGd/TiNx multilayer is Magnetic multilayer 20x[TiN(50A)+F8CoV{50A)] 0.3 with absorpiion layers -O- System 1: TiGd(1000A) —•—System 2: TiGd(5O0A)+10xfriN(S0A)+TiGd(5OA)| Fig. 3: R and P of an m=1.8 supermirror on a multi- £ 0.2 — System 3 TiGd(1O00A)+10xrjiN(75A)+TiGd(25A)| layer absorber system (TOPSI, ^=0.4735 nm). 0.1 [1] F. Mezei, Comm. Phys. 1, 81 (1976); F. Mezei and P. Dagleish, Comm. Phys. 2, 41 (1977) 0.0 [2] O. Elsenhans etal. Thin Solid Films 246,110 (1994) 0.00 0.05 0.10 0.15 0.20 0.25 [3] P. Boni et al., Physica B 241 -243, 1060 (1998) q (nm'1) [4] O. Scharpf, Physica B 156-157, 639 (1989); 174,514(1991) Fig. 1: Rdown curves (TOPSI/SINQ, /l=0.4735 nm) 59 POLARIZATION EFFICIENCY OF MULTILAYER MIRRORS PRODUCED AT PSI H. Grimmer', O. Zaharko \ M. Horisberger', H.-Ch. Merlins2 and F. Schafers2 1 Laboratory for Neutron Scattering, ETHZand PSI, CH-5232 Villigen PSI 2 BESSY, Albert-Einstein-Strasse 15, D-12489 Berlin High quality multilayer structures were produced at PSI. Their polarization efficiency was determined by measuring the reflectance in s- and p-geometry with the new BESSY soft x-ray polarimeter. The multilayers have been produced with TIPS! by Fig. 2 shows that the polarization efficiency is maximal sputter deposition on silicon wafers and consist of close to 0=45°, as expected. This maximum appears alternating layers of Ti/Ni, V/Ni or Co/C. They were at an energy E below the L3 edge of Ti for the designed for high reflectivity at the Brewster angle 0B multilayer of Figs. 1,2. Absorption edges influence the for wavelengths close to the absorption edges of Ti polarization efficiency, as shown in Fig. 3 for the L3 (454 eV), V (512 eV) or C (284 eV). Linearly p-polar- edge of V, below which both R(180°) and R(90°) have ized radiation is not reflected at the Brewster angle; 0B maxima. = 45°. The reflectance of these mirrors for soft x-rays was measured with the BESSY soft x-ray polarimeter at beamline PM-4 of BESSY I, with predominantly -•— R«3=180°)/R(P=90°) linearly polarized radiation. Fig. 1 gives the o R(p=180°) in% —•— R fls =90°) in % reflectance of a Ti/Ni multilayer as a function of the rotation angle (3 of the sample around the optical axis. The linearly polarized light is s-polarized for p=0° or 180° and p-polarized for (3=90° or 270°. 44.1° 2.5 Photon energy 1 150(V/Ni), period = 1.73 nm A E = 434.5 eV 500 505 510 515 o 1.5 Photon energy E [eV] • Fig. 3: Bragg peak reflectances R(180°) and R(90°) in | 55 75(Ti/N')+AI \ / % and their ratio as functions of the photon energy E; 0.5 2.03 nm 0 is the angle where the Bragg peak appears. 90 180 270 360 ]B [degrees] .o'o.o.o-o-°'-0- 16 : o.0.0.0ooc \ 100(Co/C)+AI i Fig. 1: Reflectance for 0=45°, where 0 denotes the 14 v,.O-00 \ period 3.065 nm : angle between sample surface and incident beam. c 12 : —o— R(p=180 ) \ i : - •— R(p=90°) 1^46.0= j 5.75 - ' ' ' " 10 r —•— R(P=18OC) / R((5=9O°) 8 r e = 47.7° i "% 44 8C '• :e = 51 .o° 6 :•-•-•••"'" \ p \ •••-•••-•-•••-•••: 4 - 6' \ - 0) = 44.0 :© © *®-©-®-s© © ® » c 2 g \ °»t>OOO.ff '• , I 5.65 75(Ti/Ni)+AI 265 270 275 280 285 292905 Photon energy E [eV] period 2.03 nm Fig. 4: See caption of Fig. 3. 5.60 420 425 430 435 440 445 Fig. 4 gives analogous results as Fig. 3 for a Co/C Photon energy E [eV] multilayer. The polarization efficiency does not show Fig. 2: The polarization efficiency, i.e. the ratio the simple behaviour of Fig. 2 due to the K edge of R(180°)/R(90°) of the reflectances R(P) at the first carbon. Taking the partial polarisation of the beam at Bragg peak as function of the photon energy E. The PM-4 into account, the maximum polarization angle 0, where the Bragg peak appears, varies with E. efficiency, obtainable with a fully linearly polarized The parabola gives a fit to the experimental data. beam of the same energy resolution is estimated to be 23 for the multilayer of Fig. 4. 60 SOFT X-RAY MAGNETIC CIRCULAR DICHROISM IN TRANSMISSION FOR POLARIMETRY AND ELEMENTALLY RESOLVED MAGNETISM O. Zaharko1, H. Grimmer1, A.Cervellino2, H.- Ch. Mertins?, F. Schafers3 1 Laboratory for Neutron Scattering, ETHZ and PSI, CH-5232 Villigen PSI 2Laboratory for Crystallography, ETHZ, CH-8092 Zurich 3BESSY, Albert-Einstein-Strasse 15, D-12489 Berlin A new quantitative approach is developed to determine (i) the absolute degree of circular polarization for soft X-rays passing through a magnetic film and (ii) the spin and orbital magnetic moments of the absorbing atoms of the film taking into account experimental complications. This approach exploits the angular dependence of the X-ray absorption spectra (XAS) in transmission and is demonstrated on an example of a Fe050Co0A8V002/Ti multilayer. When elliptically polarized X-rays pass through a magnetic sample, the difference between the 1.3 absorption cross-sections of left- and right- circularly Co-edge polarized X-rays near the absorption threshold results in magnetic circular dichroism (MCD). The intensity 1.1 observed transmittance T contains information about the degree CO calculated of circular polarization of the incoming radiation and magnetic properties of the sample [1]: 0.9 T± = X+(1-X)[PL exp(U) + PR exp(R±)] (1) 0.7 where ± denotes parallel or antiparallel coupling of the directions of the film magnetization and of the beam 0.1 propagation, X is radiation leakage, PL and PR are the intensity fractions of the left- and right- circularly .0.0 polarized X-rays; L and R stand for: -0.1 + A^r/tanG) -0.2 In the case of a multilayer the summation extends over the individual layers, \\.u Aji, and r, are the absorption coefficient, dichroic contribution and 770 790 810 830 thickness of the i-th layer; 9 is the angle of incidence. Energy [eV] well with the existing knowledge of the electronic Using eq. 1 to fit of the angular dependence of the structure of bulk bcc Fe-Co alloys [5, 6]. transmittance, the normalized Stokes parameter S3 (S3=(PL-PR)/(PL+PR)) and the absorption cross Fig. 1: Fit of XAS and MCD at the Co L>, -edge. sections can be determined. The spin and orbital 3 magnetic moments m and m of the absorbing s L As result of the fit a significant radiation leakage was atoms can be derived from the sum rules [2, 3] using detected. An atomic force microscopy study has integration of the I^-r,) (XAS) and I^A^n) (MCD). A shown that the reason of the leakage is the fit of the XAS and MCD spectra with Gaussian- morphology of the Si N substrate [7]. convoluted Fano profiles allows an analytic 3 4 integration. [1] O. Zaharko, H. Grimmer, A.Cervellino, H.- Ch. Mertins, F. Schafers in preparation. The method has been applied to a Fe050Co048V00g/Ti [2] B. T. Thole, P. Carra, F. Sette, G. van der Laan multilayer, ex situ grown on a Si3N4 membrane. The Phys. Rev. Lett. 68, 1943 (1992). transmittance has been measured at the L2)3-edges [3] P. Carra, B. T. Thole, M. Altarelli, X.-D. Wang of Fe and Co at the bending magnet beamline PM3 Phys. Rev. Lett. 70, 694 (1993). at BESSY I using the BESSY soft X-ray polarimeter [4] F. Schafers et al. Appl. Opt. 38, 4074 (1999). [4]. Figure 1 presents the fit of XAS and MCD at the [5] R. Richter, H. Eschrig, J. Phys. F: Met. Phys. 18, Co L23-edges with Gaussian-convoluted Fano 1813(1988). profiles. The determined component resolved [6] H. Ebert, M. Battocletti, Solid State Commun. 98, magnetic moments of the Fe and Co atoms agree 785(1996). [7] The AFM study has been performed by D. Alliata. 61 SOFT X-RAY RESONANT MAGNETIC SCATTERING OF Fe/C MULTILAYERS O. Zaharko1, H. Grimmer1, H.- Ch. Mertins2, F. Schafers2 1 Laboratory for Neutron Scattering, ETHZ and PSI, CH-5232 Villigen PSI 2BESSY, Albert-Einstein-Strasse 15, D-12489 Berlin Resonant X-ray magnetic scattering is studied for three Fe/C multilayers near the Fe L23 edges. The resonant magnetic X-ray scattering (XRMS) near an absorption threshold [1] has become a novel experimental technique for the investigation of 20 '- magnetic materials. From XRMS the real and /**-. "= imaginary parts of the resonant magnetic amplitude • . •• i can be extracted and, applying sum rules [2], the spin : 2. ^ * • and orbital contributions to the magnetic moment of a 1 0 • • z resonant atom can be determined [3]. Therefore, • * XRMS can provide the same information as magnetic — ••* circular dichroism (MCD) in X-ray absorption • c spectroscopy (XAS) does [4]. n We aim to compare the two techniques and to - hv determine the component resolved magnetic moment of Fe from dichroic signals in XRMS and XAS, 20 - ' .1 independently. Here we report on the XRMS measurements in three Fe/C multilayers. 100 Fe/C bilayers were deposited on Si-wafers by 10 magnetron sputtering and characterized by grazing X- ray reflectometry (Siemens D500 diffractometer, Cu * • • +• Ko) and magnetometry (PPMS). The first and second * 3 multilayers have almost the same thickness of the Fe- 0 layers; the second and third multilayers - the same thickness of the C-layers, as was designed (see Table). All samples show in-plane magnetization with "i i i i 1 • ' saturation magnetic field below 200 Oe and coercivity 10 20 below 20 Oe in the film surface. 26 [deg] The resonant X-ray magnetic scattering was Figure: Reflectance asymmetry as a function of the measured at the BESSY I beamlines PM4, PM3 using angle of incidence 6 for three Fe/C multilayers the BESSY soft X-ray polarimeter [5]. The reflectance measured at the Fe L3 edge. of elliptically polarized light near the Fe L23 edges was measured with the magnetic field M applied parallel to The pronounced minima in curves 1 (26=25 deg) and the surface and the diffraction plane. The magnitude 2 (29=20 deg), correspond to the Bragg peak. A fit of of the effect is described by an asymmetry ratio A=(l+- the resonant X-ray reflectance and determination of Ly(l++L). where l+ and l_ are the reflectances at two the magnetic moment of the Fe atoms is under way. opposite directions of M. The highest asymmetry is observed for the first multilayer (Figure) with the [1] J. P. Hannon, G. T. Trammel!, M. Blume, D. Gibbs largest Fe-thickness. Phys. Rev. Lett. 61, 1245 (1988). [2] B. T. Thole, P. Carra, F. Sette, G. van der Laan Table: Parameters for the Fe/C multilayers refined Phys. Rev. Lett. 68, 1943 (1992). P. Carra, B. T. from the nonresonant X-ray reflectance (Cu Ka, IMD Thole, M. Altarelli, X. - D. Wang Phys. Rev. Lett. program [6], A - multilayer period, d - thickness of the 70,694(1993). individual layer, a- roughness of the interface) or [3] J. M. Tonnerre, L. Seve, D. Raoux, G. Soullie, B. obtained from XRMS (Fe L3 edge, A - asymmetry). Rodmacq, P. Wolfers Phys. Rev. Lett. 75, 740 (1995). [4] C. T. Chen, Y. U. Idzerda, H. - J. Lin, N. V. Smith, G. Meigs, E. Chaban, G. H. Ho, E. Pellegrin, F. A[nm] d 3[nm] dFe [nm] ] A% Sette Phys. Rev. Lett. 75, 152 (1995). 1 2.075 0.381 1.695 0.494 0.343 25 [5] F. Schafers et al. Appl. Opt. 38, 4074 (1999). 2 2.463 0 936 1.527 0.228 0.215 20 [6] D. L. Windt Computers in Physics, to be pub- 3 2.138 1 133 1.004 0.260 0.275 4 lished. NEXT PAGE(S) left BLANK 63 Instrumental and Support Activities »•' NSF *'• Sf: :: *:*: <> ®:9mm- !:ir!!i5u.-:^ 2.0 •' 2.5.. &. NEXT PAGE(S) left BLANK 65 POLARISATION ANALYSIS WITH TASP USING REMANENT BENDERS P. Boni, B. Roessli, and F. Semadeni Laboratory for Neutron Scattering ETH & PSI, CH-5232 Viligen PSI, Switzerland Polarized beam experiments have been conducted with the three-axis spectrometer TASP using polarizing benders with supermirrors m = 3. The polarization of the beam can be selected by magnetizing the mirrors either parallel or anti-parallel to the guide field by the application of a short field pulse of B = 200 G. There- fore, spin flippers are superfluous making the experiments very user-friendly. This year the triple-axis spectrometer TASP was equipped with polarizing supermirror benders before the sample and the detector (Fig. 1). The critical angle of reflection of the coatings is m = 3 times that of natu- n ral Ni leading to a critical wavelength X = 2.77 A [1]. Bender Sample Bender Detector The outer dimensions and the beam size of the bend- ers are identical with those of the collimators making the change between polarized und unpolarized beam a simple task. The surface of the mirrors at the exit of the bender a after the monochromator are parallel to the optical axis Bender Sample Bender of the instrument so that the beam is not displaced Detector with respect to the unpolarized beam setup. The beam Fig. 2: Principle of polarized neutron scattering using deviation by the bender is corrected for i) either by remanent benders. By switching the magnetizations of adjusting the Bragg angle and the position (translation) the supermirrors it is possible to measure the four of the monochromator or ii) by adjusting the position of cross sections (++), (-+), (-), and (+-). the monochromator and the zero of the scattering an- gle of the monochromator. The latter option is pre- ferred since the incident energy of the neutrons is not Fig. 3 shows an example of a polarized beam experi- changed. ment on TASP using remanent supermirror benders. The soft phonon in Heusler is completely suppressed in the spin-flip neutron scattering cross section [2]. guide k =1.97A'1 op-B29A-Ni MnGa-40-B29B E 120 2 bender E • non-spin-flip § 100 • spin-flip | 80 O f 60 bender 1 40 detectoi 2 20 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Fig. 1: Layout of the polarized beam spectrometer Energy Transfer (meV) TASP using remanent benders. Fig. 3: Inelastic polarized beam experiment in the par- The beam plug is equipped with permanent magnets amagnetic phase of Heusler alloy MnSn [1]. The that supply a guide field Hg = 10 G and electromag- measurements show that the magnetic cross section is netic coils that allow the application of a field of H = much smaller than the phonon cross section. 200 G for magnetizing the supermirrors. The polariza- tion of the neutron beam is defined by the direction of Polarization analysis using remanent benders has the magnetization M with respect to Hg that is always many advantages when compared with traditional set- kept fixed (Fig. 2). The polarization of the incident ups using spin flippers: beam can be changed by magnetizing the supermir- 1.) no need for spin flippers and their adjustment rors in the opposite direction. 2.) cross-talk between spin flipper and field at The analysis of the polarization of the scattered neu- sample position is minimized trons is provided by a bender, which is placed before 3.) no stabilized power supplies necessary the detector. Again, the surface of the mirrors is paral- 4.) ideal for white beam applications lel to the optical axis of the neutrons. Therefore, no adjustment at the analyzer is necessary. In practice, [1] P. Boni, et al., Physica B 267-268, 320 (1999). the deviation of the beam by the bender is only a few [2] P. Vorderwisch, U. Stuhr, and P. Boni, unpublished. mm and no displacement of the detector is necessary. 66 HRPT DEVELOPMENTS AND FIRST EXPERIMENTS AT SINQ P. Fischer ', G. Frey', M. Koch \ M. Konnecke \ V. Pomjakushin \ J. Schefer', R. Schneider', R. Thut \ N. Schlumpf2, R. Burge2, U. Greuter2, S. Bondt2, E. Berruyer3 1 Laboratory for Neutron Scattering, ETHZ and PSI, CH-5232 Villigen PSI 2 Paul Scherrer Institute, CH-5232 Villigen PSI 3 Cerca, F-26100 Romans, France Developments and status of the high resolution powder diffractometer HRPT for thermal neutrons at the SINQ target station are summarized. First experiments prove the expected excellent resolution and good intensity of thermal neutrons with wavelengths from 1.2 to 2.5 A. The high resolution powder diffractometer for thermal neutrons HRPT (Fig. 1) with 1600 detectors (PSD) started test operation including first measurements for users in 1999 at the SINQ target station. The instrument design and properties were presented at ECNS'99 and are described in ref. [1]. 0 2 0 4 0 6 0 8 0 100 120 140 160 Fig. 2: Observed (k = 1.886 A, high intensity mode, scattering angle step 526=0.05°), calculated and difference HRPT neutron diffraction patterns of Cs3MgD5 (+ traces of CsD). Insertion: Corresponding tetragonal structure of Cs3MgD5. A major activity 1999 was the development of a well functioning cooling system (1 kW heat to be removed) for the detector electronics. Moreover the latter had to be completed, and the user interface and histogram memory had been installed. This implied extensive tests needing a lot of manpower of PSI and LNS. For reliable user operation at SINQ further work is necessary with respect to the data readout system, which presently is in progress at Cerca. 1 1 1 i For 26M = 120° and a sample diameter of 10 mm : m 120 already resolutions down to 5d/d < 0.001 (d = lattice 0.008 - spacing) were reached, cf. Fig. 1. As an example for i'2'24' 6',-f2' applications Fig. 2 shows a precise determination of 0.006 - deuterium positions for the new metal deuteride T3 Cs3MgD5 [2]. Another illustrative example is the to 0.004 - detection of rather small structural distortions in case of HoB6 at low temperatures [3]. 0.002 _- References 0.000 1 . 1 1 i 1 I.I.I [1] P. Fischer et al., accepted for publ. in proceed- 20 40 60 80 100 120 140 160 ings of ECNS'99: Physica B 29 0 [2] B. Bertheville, P. Fischer and K. Yvon, accepted Fig. 1: HRPT at SINQ target station and observed for publ. in J. Alloys Comp. resolution functions for TriCS: FIRST YEAR OF OPERATION J. Schefer \ P. Keller', M. Konnecke ', O. Zaharko ', Th. Strassle ' and J. Felsche 2 1 Laboratory for Neutron Scattering, ETHZ and PSI, CH-5232 Villigen PSI, Switzerland 2Chemistry Department, University of Constance, D-78434 Constance, Germany The single crystal diffractometer TriCS is in operation since March 99 using a single detector. First user experiments have been successfully finished. Operation with 2-dimensinal detectors is scheduled for the year 2000. The single crystal neutron diffractometer TriCS (Fig.1) ErGa3 [1]. Details of these examples are given in a has been designed for nuclear and magnetic structure different report. determinations of samples with small to medium size unit cells (lattice parameter < 20 A depending on As a second example we show in Fig, 2 q-scans symmetry of the system studied). The layout of the along [11 q] in Ce085(La095Y005)016Sb, where increasing instrument will be published in [1]. uni-axial pressure along [00I] yields a higher TN. The We successfully reduced the focus of the size of the crystal for this measurement was monochromaotor using the TOPSI spectrometer for a approximately 3mm3. final and perfect alignment of the individual slabs, both for TriCS and powder diffractometer HRPT. The Scans along [1 1 q0] O ambient results of these tests will be published in [2]. A second T 0.13 GPa C002 monochromator with fixed focusing and a limited • 0.26 GPa height of 50 mm has been installed for measurements of magnetic Bragg peaks with higher flux but lower resolution. 0.4 q0 [reduced units] Fig 2: Pressure dependence of q-scans along [11q] in Ce085(La095YO05)015SbatT=2K. Results on a third system, B2Cu04, are shown in separated reports within this volume. The next steps are the implementation of the three 2- dimensional EMBL detectors [3] and the final shielding of the neutron flight path and the instrumental area. TriCS has been realized in collaboration with the University of Constance, who supported the construction of TriCS through a grant of BMBF, Germany. Fig. 1: TriCS at the thermal beam port R42 using the tilting option. The auxiliary equipment for temperature control has been completed with a four-circle cryostat (1.5-300K), which is available with a three months prior notice in order to complete all the tests and to order the helium [1] J. Schefer et al., Physica B (2000), accepted for dewar. Standard equipment covers 12 to 450 K in the publication (ICNS'99, Budapest) 4-circle mode. Any other equipment is available in out- [2] J. Schefer et al., Physica B (2000), accepted for of-plane measurements (tilting mode), of course with a publication (NOP99 Workshop, Nov. 1999, PSI) certain loss in resolution. [3] A.Gabriel and M.H.J. Koch, We successfully measured the magnetization curve for Nucl. Instr. Methods A313, 549 (1992) the cubic (a=4.2A) rare earth metallic compound 68 DRUCHAL USER OPERATIONS IN 1999 AND INSTRUMENTAL UPGRADE F. Altorfer, E. Clementyev, R. Thut and M. Koch Laboratory for Neutron Scattering, ETHZ & PSI, CH-5232 Villigen SINQ cold triple axis DruchaL on guide RNR13 underwent a major upgrade during 1999, when a new, optimised analyser was installed. The analyser covers approximately three times more area than its predecessor, thus increasing the neutron count rate for large samples by a factor up to three. Additional measures such as additional detector arm shielding and a new Be-filter also bettered the performance of the instrument DruchaL in its second year of routine user operations cylindrical Vanadium sample of 50 mm height and 10 attracted 21 principal users from 8 different institutes mm diameter before and after the new analyzer was from 5 countries. The instrument worked nearly mounted. One has to keep in mind, however, that the flawlessly throughout the year and great care was obtained values in gain factors depend strongly on the taken to improve its performance by adding three main dimensions of the sample, they will be significantly components that comprised the upgrade in 1999: lower for samples smaller than 10 mm in height. a) A new pyrolithic graphite analyzer Additional shielding on attached on the detector b) 1.50 arm c) A new Be-filter with a larger Be-block that covers more solid angle DruchaL started regular operations in 1998 with a pyrolithic graphite analyzer taken from a triple axis instrument installed at the PSI-SAPHIR reactor. This device was not optimized to the geometry of DruchaL. In August 1999 the new, three times larger device was delivered, oriented and installed. Figs. 1 and 2 show that DruchaL's performance was significantly increased without a considerable loss in resolution. _ £ 20000 - -•- New '5 0.00 In October 1999 additional detector arm shielding was added that improved the signal to noise ratio and 0< -2.0 -1.0 0.0 1.0 2.0 suppressed the background level well below the 1 count / minute threshold. Energy transfer [meV] During experiments in 1998 a Be-filter stemming from SAPHIR times was in operation. After its replacement Fig 1: Flux comparison between old and new DruchaL by the new device, counting rates were up, depending analyzer. A cylindrical Vanadium sample of 50 mm on sample size by roughly 40%. height and 10 mm diameter was used and an incident In order to offer DruchaL users a greater variety of neutron energy of 15 meV. sample environments an Euler cradle was adapted to the spectrometer and successfully used during an In order to document the count rate improvement we experiment in September 1999. did a series of measurements with a standard, 69 SINQ TIME-OF-FLIGHT SPECTROMETER FOCUS: FIRST YEAR OF OPERATION S. Janssen1, C. Beck2, J. Mesot1, D. Rubio-Temprano1, F. Altorfer1, A. Furrer1, L. Holitznet2, R. Hempelmannn2 1 Labor für Neutronenstreuung, ETHZ & PSI, CH-5232 Villigen 2 Physikalische Chemie, Universität des Saarlandes, D-66123 Saarbrücken After an intense test phase at the beginning of the 99 accelerator cycle the SINQ time-of-flight spectrometer FOCUS became routinely operational at the end of April. Until the end of the cycle already 31 officially scheduled user experiments have succesfully been performed. It could be shown that the performance of FOCUS already now is very competitive with existing spectrometers and unique in the sense of offering a high flexibility for the users. 0.1 After the basic adjustment and an intense test phase ^^-"---~=i--!—7,^=; jog nieV A Br Ba,Cu,C t \ the cold neutron time-of-flight spectrometer FOCUS •' L •='==7;=™.™ 10.2 meV T=25K i \ became operational at the end of April 99. Since that 0.08 =•-,;---..™- 8.7 nieV i i 31 scheduled user experiments have already been Î ; performed using a large variety of sample environment -: 0.06 A î \ equipment. !: t J \ I \ 5 0.04 • \ Ì \ 1 •V 1 / f: \ 0.02 > \ ! „-/ ..SJ...... \ 7 8 9 10 II 12 Neutron Energy Loss [meV| Fig. 2: Crystal electric field spectroscopy on the high- Tc superconducting compound ErBa2Cu408. The result was obtained by inelastic time focussing [2]. In particular the use of higher order reflections of the crystal monochromator in combination with inelastic time focussing provides a high energy resolution at large energy transfers. The crystal electric field spectrum [2] shown in figure 2 has been obtained with 2.0 10' an incoming energy of 15 meV providing an inelastic energy resolution below 0.4 meV at a neutron energy loss between 8 and 12 meV. By the use of the monochromatic focussing setup of the instrument the inelastic energy resolution in that range could even be reduced down to 0.3 meV. During the next shutdown the instrument will be upgraded by the installation of two additional banks of detectors. Together with the foreseen enhancement of SINQ's primary flux the machine will then provide FOCUS: measured neutron flux at sample 0.0 10° intensities comparable with the cold neutron TOF instrument IN6 at the ILL. FOCUS is operated in a 2 3 4 5 o 6 Neutron Wavelength X. [Â] close cooperation between the PSI and the 'University of Saarbrücken'. The generous financial support by Fig. 1 : Upper: total view of FOCUS as installed in the the german 'Bundesministerium für Bildung und SINQ guide hall. Lower: measured neutron flux at the Forschung' is gratefully acknowledged. sample position as a function of incident wavelength [1]. [1] S. Janssen, F. Altorfer, L. Holitzner, R. Hempelmann, Physica B, (2000), in press. During this first cycle of user operation FOCUS has [2] D. Rubio-Temprano, J. Mesot, S. Janssen, A. been used in a lot of different settings. Due to its high Furrer, K. Conder, H. Mutka, K.A. Müller, Phys. flexibility it is possible to optimise the instrument Rev. Lett., submitted. setting individually for the experimental requirements of the users [1]. 70 Thermal Neutron Three-Axis Spectrometer TNT B. Roessli and P. Boni (PSI) We discuss the possibility of building a three-axis spectrometer at the S8 thermal port. We show that with the combined use of supermirrors and focusing monochromators, it is expected that such an instrument will have a flux comparable to existing instruments already installed at medium and high-flux reactors. Currently the three-axis spectrometers Druchal and for the doubly focusing monochromator that is located TASP installed at the cold neutron guides at SINQ al- 1.5 m away. The distance between monochromator and low energy transfers up to hu> ss 8 - 10 meV. This en- sample has to be also 1.5 m in order to obtain a proper ergy transfer range is too restricted for a comprehen- monochromatic focusing. The scattered neutrons from sive study of collective excitations in many materials. the sample are reflected by means of a horizontally fo- We discuss the possibility of building a thermal three- cusing analyser on an area detector. The construction axis spectrometer at the S8 thermal port which would of the whole spectrometer is similar to the existing cold be complementary to the existing cold-neutron spec- three-axis spectrometers, i.e. sample table, analyser trometers for inelastic scattering. The schematic lay- and detector move on air cushions on a granite floor. out of the proposed thermal three-axis spectrometer is The useful range of thermal neutron energies at TNT shown in Fig. 1. With help of Monte-Carlo simulations extends from Et = 14 meV to about 100 meV allowing [1] we found that simultaneous use of supermirrors and transfers in E as large as 60 meV (« 15 THz). This im- of a doubly focusing monochromator leads to an aver- proves the presently possible ^-transfers at SINQ by a aged gain of ~ 5 at the sample position when compared factor of 6-8. It is well known that a comparison of the with a regular beam tube and flat monochromator. available flux at the sample position of three-axis spec- trometers at various neutron scattering centers is rather difficult because the resolution conditions and the ge- doably focusing ometries involved are usually different. Still, we try to horizontally focusing monochromator beam tube estimate the possible performance of TNT when com- pared with IN8 at the ILL and 2T at LLB. TABLE I. Comparison of the flux at the sample position of thermal three-axis spectrometers at the ILL and LLB. We as- sume that SINQ will provide in the year 2000 a source flux of « 0.8 • 10" n/cm2/s due to the installation of a lead target and due to an increase of the proton current by ~ 20%. Note, that the IN8 monochromator is only vertically focusing. item abr. TNT 2T IN8 2 source flux (n/cm /sec) Fo 0.8-10" 3 10" 12 • 10" FIG. 1. Layout of the proposed three-axis spectrome- vertical focusing G, 2.5 2.5 2.5 ter TNT at the beam port S8 at SINQ. The neu- horizontal focusing Gh 2 ~ 2 1 tron beam is extracted by means of a horizon- supermirror Gsm > 1.5 1 1 2 tally focusing beam-tube and focused on the area of source (cm ) A 120 — 78.5 78.5 sample with a doubly focusing monochroma- distance (cm) U 450 ~ 400 550 tor. solid angle S = 6•10~4 5 W~4 3•10~4 Li 10 figure of merit P 35 • 10 7^[ • 10IU 78 • 10IU The basic dimensions for the primary instruments are given by the geometrical cross section and the length of the beam-tube. In order to maximize the flux before the monochromator we suggest to extract the neutrons The figure of merit, P, has been calculated as a product by means of a horizontally converging guide coated of gains as listed in the table.We see, that all three in- with supermiror m = 4, thus reducing the width w of struments provide a similar flux at the sample position. the beam from 8 cm at the entrance to 1.5 cm at the [1] The McStas software package has been developed by K. exit.The exit of the beam-tube acts as the virtual source Nielsen and K. Lefmann, Rise National Laboratory. 71 3rd GENERATION SINQ PROJECT: LAUE DIFFRACTOMETER O. Zaharko', J. Schefer1 and E. Lehmann2 1 Laboratory for Neutron Scattering, ETHZ and PSI, CH-5232 Villigen PS I 2ASQ, PSI, CH-5232 Villigen PSI As a new instrument of the third generation, a Laue diffractometer is proposed to be installed at SINQ. It will allow fast and high quality data acquisition due to a combination of a large cylindrical image plate detector and a wide wavelength band pass. Structural biology, magnetism and materials science are the main research fields, which will profit from the LAUE instrument at SINQ. The first applications of the Laue diffraction technique The first feasibility study at SINQ performed with an at neutron sources already demonstrated its potential imaging plate at the neutron radiography facility in a series of successful experiments in magnetism NEUTRA is shown in Fig. 1. A white thermal beam and structural biology [1-4], Therefore, we propose to with a flux of 3-10s neutrons/cm2/sec/mA and a install such instrument at SINQ at a cold guide and/or diameter of 6 mm was centered on a strontium barium a thermal beam tube. niobat (SBN) single crystal (volume 125 mm3). The beam collimation was performed by the 2 cm and 6 The great advantage and potential of the Laue mm holes in two Li-doped plates at distance of 6 m. method is the speed with which data can be collected. The background caused by y-rays and ambient The combination of a broad wavelength band pass neutrons was reduced by a Pb-wall in front of the and a novel cylindrical neutron sensitive image plate sample and a shielding of 50% borated araldit around detector [5] provides an order of magnitude gain in the image plate. efficiency compared with conventional diffractometers. The required minimal size of the crystal is given by the size of the unit cell. The maximal possible unit cell can be increased by lowering the beam divergence. We propose to position the LAUE on a cold neutron guide as it will make feasible studies of small and medium size proteins and macromolecules. The neutron scattering lengths of hydrogen (bH=-3.7 fm) and deuterium (bD=6.6 fm) differ significantly. Therefore, neutrons are useful to locate individual hydrogen atoms of special interest, surrounding water or other small solvent molecules replaced/marked Fig. 1: Laue image from a= SBN single crystal with deuterium. This information becomes more and {tetragonal, a=12.43 A, c=3.91 A) taken with an image more essential for the biological research, as it often plate at NEUTRA. The crystal-to-detector distance is defines the functionality, group accessibility, mobility, 20 cm, [100] is parallel to the beam direction, [010] is structure dynamics, catalytic mechanisms. perpendicular to the image piate. As the instrument is very compact, the same LAUE The proposed instrument can be produced by an can be positioned on a thermal beam tube. It will external company under the license of EMBL and complement the traditional single crystal delivered ready for use. The required neutron optics diffractometer TriCS due to fast and extensive in front of the sample and the shielding around the reciprocal space data collection. This feature is instrument has to be designed by PSI. valuable to detect incommensurate structural and magnetic modulations and to follow structural and [1] N. Nimura; Y. Minezaki, T. Nonaka et all. Nat. magnetic phase transitions. The instrument can be Struct. Biol.4, 909 (1997). used for small samples and fast initial characterization [2] J. Habash, J. Raftery, S. Wiegerber et all. J. of topical new materials. Chem. Soc, Faraday Trans. 93 4313 (1997). [3] J. B. Forsyth, P. J. Brown, M. S. Lehmann et all. A prototype of such an instrument is in operation at JMMM, 177-181,1395(1998). ILL (Grenoble) since January 1998. The second, [4] P. Schobinger-Papamantelios, C. Wilkinson, D. improved version of the instrument, is under Myles et all. Abstracts of ECM-18, Praha, Czech construction: It has a vertical axis for the sample Republic (15-20 August 1998). mounting and is therefore compatible with [5] F. Cipriani, J.-C. Castagna, C. Wilkinson et all. J. conventional cryostats, furnaces and pressure cells. Neutron Res. 4, 79(1996). The detector employes neutron-sensitive image plates based on storage-phosphor with addition of Gd2O3. 72 CREATION AND DECAY OF NUCLEAR POLARIZATION DOMAINS IN HYDROGENEOUS MATERIALS P. Hautle1, B. van den Brandt1, J.A. Konter1, S. Mango1, H.B. Stuhrmann2 and O. Zimmer3 1Low Temperature Facilities, Paul Scherrer Institute, CH-5232 Viiiigen PSI, Switzerland 2lnstitut de Biologie Structurale, F-38027 Grenoble, France 3University of Mainz, D-55099 Mainz, Germany Clouds of highly oriented protons have been created around paramagnetic centers in alcohols by rf irradiation. The decay of the polarization gradient has been indirectly observed by NMR methods. Dynamic nuclear polarization (DNP) is an efficient tool opens a path for the penetration of the diffusion to create contrast in SANS on hydrogeneous samples barrier. by exploiting the strong spin dependence of neutron A much weaker influence of this system can be scattering from protons. The nuclear polarization expected in biological samples, in which a much arises from paramagnetic centers distributed in the smaller number of well localised paramagnetic centers samples, which transfer their spin ordering upon is present. Preliminary measurements on tyrosil microwave irradiation to the nearby protons. In a radicals in catalase appear to support this assumption. second step this ordering spreads out into the bulk of the sample through spin diffusion. The DNP mecha- nism does not a priori lead to a homogeneous polarization distribution, as the two processes might proceed with different time constants. Situations can be imagined in which domains of highly polarized protons around the paramagnetic centers survive long enough to be observed by neutron scattering [1,2]. Such local contrast could highlight radicals, which play a key role in the biological activity of certain mole- cules, e.g. the tyrosyl radical in catalase [3]. In this study we tried to find an optimum procedure to create and hold polarization gradients in samples of propanediol with different isotopic substitutions (98%, 90%, 60%, 0% deuterated), doped with EHBA-Cr(V) 1.5 2.0 2.5 3.0 3.5 4.0 paramagnetic centers. The samples have been investigated by NMR methods in our polarized target Distance between neighbouring protons (normalized to 0% deuteration, a = 4.3 A) system within a temperature range of 100 mK to 1 K at magnetic field values of 2.5 T and 3.5 T. Fig. 1: The decay time of the polarization gradient The cw-NMR measurements performed with Q-meters shows a quadratic dependence on the distance could only monitor changes in the average polarization between neighbouring protons. of the protons located outside the sphere of influence of the paramagnetic centers, i.e. of protons in the bulk. In conclusion, we created polarization clouds around By applying microwave pulses to the sample no paramagnetic centers in standard polarized target characteristic signatures for the creation of polarization materials and observed their decay. In view of the clouds around the paramagnetic centers were seen in expected much longer persistence of polarization the evolution of the bulk proton signal. However, by gradients in biological samples, higher resolution creating polarization gradients by rf irradiation, e.g. SANS experiments [4] should be well feasible. destroying the bulk proton polarization, reversing it by AFP or saturating the protons near the centers, the [1] J.B. Hayter, G.T. Jenkin, J.W. White, Phys. Rev. coupling times and mechanisms between the "two Lett. 33, 696(1974) sorts of protons" could be deduced by the evolution of [2] H.B. Stuhrmann, B. van den Brandt, P. Hautle, the bulk protons NMR signals. J.A. Konter, T.O. Niinikoski, M. Schmitt, The decay of the polarization gradient shows a R. Willumeit, J. Zhao, S. Mango, J. Appl. Cryst. quadratic dependence on the distance between 30,839(1997) neighbouring protons, as is expected for a spin [3] D.C. Bicout, M.J. Field, P. Gouet, H.M. Jouve, diffusion dominated process (see Fig.1). It is slower at Biochemica et Biophysica Acta 1252,172 (1995) lower temperature and higher magnetic field strength, [4] Experiments Proposed at the ILL what hints at the involvement of a spin system in contact with the lattice, most likely the electron non- Zeeman system of the paramagnetic centers, which 73 SAMPLE SYNTHESIS LABORATORY AT LNS K. Conder, P. Allenspach, A. Furrer Laboratory for Neutron Scattering, ETHZ and PSI, CH-5232 Villigen PSI, Switzerland The sample synthesis laboratory at LNS has been reorganized at LNS. A high temperature furnace (up to 1750°C), an oxygen-isotope exchange apparatus and equipment for a gas volumetric oxygen-content determination in HTC superconductors have been installed. Chemical reactions performed in the solid state The principle of the determination of the oxygen sometimes require very high temperatures. Therefore, content in cuprate superconductors by the gas we decided to install a chamber furnace volumetric method is given by the following reaction: (NABERTHERM HT08/17) with the MoSi2 heating elements. The furnace can operate in air at + temperatures up to 1750°C. It is also possible to use it n-Cu H2O 74 O2T n-H (aq) + n-Cu (1)- as a tubular furnace, with different kind of the flowing (aq) q) gases: inert, oxidizing and reducing. An application of vacuum is also possible. When e.g. YBa2Cu307.x containing copper at the formal Synthesis of air-sensitive samples requires that all average valence above 2+ (oxygen contents the preparation work has to be done in an inert 6.5<7-x<7) is dissolved in the nitric acid, the atmosphere. The equipment existent at LNS was stoichiometric amount of gaseous oxygen is evolved. found to be not sufficient. Therefore, a glove-box The amount of this oxygen (i.e. volume at known (MBRAUN Unilab) have been ordered and will be temperature and pressure) can be measured. installed in February 2000. A gas purification system, Consequently (comp. eq.1) the concentration of the which is a part of this glove-box, enables to achieve Cu3+ cations and the oxygen content balancing their H2O< 1 vpm, O2< 1 vpm. charge can be calculated. Usually, for one Oxygen-isotope effect studies on properties of determination 0.1 g of the powder sample is used and HTC superconductors are important for a theoretical about 1ml of oxygen is evolved which can be understanding of the superconductivity mechanism. measured with a reproducibility of ±0.002ml (0.2%). The oxygen isotope-exchange process can be Thus, a reproducibility error of the oxygen- performed (Fig. 1) in a closed system. In order to stoichiometry determination of Ax=±0.001 (0.2% of 18 ensure the same thermal history of the O-substituted A(7-x)=0.5 for the fully oxidized sample) can be 16 and a reference O-sample, two experiments are reached which is by far the best result obtained up to performed simultaneously (chamber A and B). A liquid now. Direct calibration by the coulometric nitrogen trap is used to condense (and recycle) the decomposition of H O [1] shows that also the absolute 18 2 expensive O2 after the exchange process is finished. accuracy of the determination is Ax=±0.001. If the volume of the gas in the chamber is not sufficient to obtain a desired isotope ratio in a one-step process, the chambers can be evacuated and filled again. A BALZERS mass spectrometer Prisma™ is connected to the system (not shown in Fig.1) which enables in- situ determination of the achieved isotope exchange ratio. [1] K. Conder, S. Rusiecki and E. Kaldis, Mat. Res. Bull., 24(1989)581. Fig. 1: Schematic setup of the equipment used for the oxygen isotope exchange. NEXT PAGE(S) left BLANK 75 Condensed Matter Theory NEXT PAGE(S) left BLANK 77 ACOUSTIC PLASMONS AND SUPERCONDUCTIVITY IN LAYERED CONDUCTORS A Bill1, H. Morawitz2 and V.Z. Kresin3 1 Condensed Matter Theory, PSI, CH-5232 Villigen PSI 2IBMAImaden Research Center, 650 Harry Rd., San Jose, CA 95120, USA 3Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720, USA Layered conductors are characterized by the presence of low energy electronic collective excitations, named acoustic plasmons. We show that such collective modes contribute to superconductivity in a layered electron gas. The study is done in the strong-coupling phonon-plasmon scheme. Recent years have seen an increased interest in the interactions was studied at length and the knowledge study of superconductors with strong anisotropic phys- accumulated on this mechanism allows thus to single ical properties. Organic, high-temperature or multilayer out the effect of plasmons. conventional superconductors are examples of such We determined the superconducting critical tempera- systems. These materials have characteristic features ture Tc in the above scheme [3]. This requires a careful that are markedly different from pure two-dimensional account of the frequency and wave-vector dependence (2D) or 3D systems. For example, they may display of the Coulomb interaction (treated in RPA approxima- anisotropic magnetic correlations, nesting properties, tion), since the same interaction is responsible both for additional phonon branches, etc. One further property an attractive, low energy contribution to the pairing and that distinguishes layered structures (LS) from most 3D for the repulsive interaction between charge-carriers. metals is the existence of low-energy electronic collec- Furthermore, since some of the materials to which the tive modes. theory should be applicable have a rather high Tc, it is The presence of electronic collective motion in a imperative to take the full temperature dependence of conductor is tied to the screening of the long-range the dielectric screening function into account. Neither Coulomb interaction. In a conventional 3D metal, of the two were fully taken into account in past investi- plasmons have a characteristic dispersion of the form gations. 2 fip;(q) = Qpi + O(q ) with a large gap {Qpi ~ 5-30eV). Fig. 1 displays the main preliminary result of the study. On the other hand, pairing in conventional systems oc- It shows that, in the case of a layered superconduc- curs near the Fermi surface, and the energies involved tor, the account of low energy collective modes can are of the order of a characteristic phonon energy ttD. contribute constructively to superconductivity. To a first Because of the relation fiD « Qpi one can consider approximation, one can interpret the effect of acoustic the screening of the Coulomb interaction between elec- plasmons as a negative contribution to /u* for a layered trons as static: in Eliashberg's theory the Coulomb re- electron gas. pulsion between electrons is represented by a single number (j,* which is extracted from isotope effect or tun- 0.3 neling measurements. The previous reasoning fails, no plasmons however, in systems where low energy electronic col- with plasmons lective excitations are present (acoustic plasmons, ex- citons...) For example, the theory is inadequate to de- scribe 3D materials with two types of charge carriers (light and heavy electrons) or layered structures. In .3 0.2 such cases, the screening of the Coulomb interaction is truly dynamic in the energy range relevant to super- conductivity. In the same way as phonons in conventional super- conductors, low-energy electronic collective modes can provide for the exchange bosons leading to an attrac- tive interaction between quasiparticles. The purpose 1 1.5 2 2.5 electron-phonon coupling X of our study is to determine if such a mechanism of superconductivity is viable in a layered electron gas. Fig.1: Tc with and without acoustic plasmons. Previous investigations made on 3D and layered ma- terials have come to contradictory conclusions on this question (see, e.g., Refs. [1,2]). In contrast to these earlier studies, we assume that the main channel for superconductivity is provided by phonons. The rea- [1] H. Rietschel, L.J. Sham Phys. Rev. B 28, 5100 son for this is twofold. First, the problem to solve is (1983) highly non-linear. If, as some researchers claim, the [2] G.D. Mahan and Ji-Wei Wu, Phys. Rev. B 39, 265 superconducting transition can indeed not be achieved (1989); Y.M. Malozovsky et al., Phys. Rev. B 48, solely via the exchange of acoustic plasmons, it does 10504(1993) not exclude the possibility for them to contribute to su- [3] A. Bill, H. Morawitz, and V.Z. Kresin, J. Low perconductivity when the main cause is due to another Temp. Phys. 117, 283 (1999); V.Z. Kresin, mechanism. Secondly, the effect of electron-phonon H. Morawitz, Phys. Rev. B 37, 7854 (1988) 78 Li,CuO2 IN THEORY B. Delley H.B. Braun(FUN), B. Roessli(LNS), A. Amato (/J.SR) and U.Staub (SLS) The CuO title compound with ferrormagnetic chains in AF alignment is shown by local spin density functional theory to possess a number of unexpected properties. Its moment per formula unit is unusually large for a cuprate and there is very significant spin delocalization onto oxygen. Copper oxide compounds show a large variety of un- netic states as compared to the paramagnetic calcula- usual properties. Much of this depends on the many tions is the opening of a gap. States further away from possibilities of linking their fundamental structural unit, the Fermi energy show only marginal changes. The ex- a CuO4 square. The distorsion of the CuO4 structural istence of the unoccupied states right above Ef and of unit is of importance as the antiferromagnetic (AF) su- density of states peaks at 3 and 6 eV above E/ is in ex- perexchange coupling vanishes as the Cu-O-Cu angle cellent agreement with recent experimental observation approaches 90 degree and a small ferromagnetic (FM) by x-ray absorption spectroscopy [2]. In the analysis of coupling prevails. U2Cu02 has a Cu-O-Cu angle of the spin density for the FM chain in the AF cell one 94 degrees. In our present calculations we find that may assign 0.55/IB to Cu and a very sizeable 0.19/i.e the ferromagnetic state is significantly lower in energy to O. Although such density assignments to ionic sites (140meV per formula unit) than the paramagnetic state. are somewhat arbitrary, it remains a question for future The state with AF aligned FM chains is found a few meV investigation if moments on the O sites are found ex- lower in energy than the FM state. The paramagnetic perimentally. We have also studied larger cells with up state is metallic as a consequence of the odd electron to 80 atoms per cell with various spin arrangements. It number in the primitive cell. Fig 1. shows bandstructure emerges clearly that the Cu spin polarizes the oxygen and density of states for the paramagnetic calculation. atoms. The spin density on the O atoms is strongly de- The self-consistency was calculated using 256 k points pendent on X,-- -2. -4. -6. z RX r YT z A Z RX T YT Z Wavevectors and Density-of-States Wavevectors and Density-of-States Fig.1 Bandstructure and density of states for Fig.2: Bandstructure and density of states for Li2Cu02 in a paramagnetic calculation (con- Li2Cu02 with ferromagnetic chains in an anti- ventional cell). ferromagnetic alignment. in the full Brillouin zone of the conventional cell. In the spin orientation of the neighboring Cu atoms. The metallic compounds the Fermis surface is found by the results of the present spin density functional calcula- tetrahedron method with Bloechl corrections. For basis tions are consistent with an Ising picture and a spin ex- sets and static potential representations the usual de- citation gap in qualitative agreement with experiment 3 faults from DMol were used. A very small gap is found [3]. for the FM state, a few tens of meV, and a small gap of less than half eV for the AF cell. The existence of a gap is consistent with the experimental finding that this compound is insulating. The bandstructure and density [1] R. Weht and W.E. Pickett, Phys. Rev. Lett. 81, 2502, of states for the AF calculation are shown in Fig 2. Our (1998). present results agree excellently with the calculations [2] R.Neudert et al., Phys. Rev. B60, 13413, (1999). by Weht [1]. The most important difference of the mag- [3] M.Boehm et al., Europhysics. Lett. 43, 77, (1998). 79 SODIUM-NITRO-PRUSSIDE LATTICE VIBRATIONS B. Delley1, J. Schefer2 and Th. Woike? 1 Condensed Matter Theory, PSI, CH-5232 Villigen PSI, Switzerland 2 Laboratory for Neutron Scattering, ETHZ and PSi, CH-5232 Villigen PSI, Switzerland 3 Kristallographie, Universitat Koln, D-50675, Koln 1, Germany The lattice vibrations of sodium-nitro-prusside crystals are studied from first principles. The accuracy of the calculations is sufficient to take the spectroscopic properties as a fingerprint of the unusual structures in the metastable states. The title compound sodiumnitroprusside (in short SNP) Fig. 1 shows the calculated structure for state S2, there (Na2[Fe(CN)5NO]*2H2O) shows extremely long lived is also another long lived excted state S1. The calcu- metastable states which can be excited and de-excited lation suggests that S2 is characterized by a side on optically. An interesting question is if the metastable or kinked structure for the nitrosyi ligand, while S1 has states have such small transition probability for de- NO upside down. Vibrational spectroscopy, either in- excitation into the ground state (GS), because struc- frared or Raman, can provide a fingerprint for different tures in the metastable states are very different from the structures. We have studied lattice dynamics from first GS structure. While it has been difficult to obtain direct principles, using Perdews 1991 GGA functional, by cal- structural information about such changes by diffrac- culating the vibrational modes at the Gamma point for tion methods, an earlier theoretical study [1] has given the solid, which contains 84 atoms per cell. The elec- strong hints that important structural rearrangements tronic bandstructure has been integrated using a 2x2x2 occur on excitation. unshifted kspace mesh. The vibrational modes have been found in harmonic analysis by diagonalization of the weighted Hessian. A selection from the results for the GS and S1 S2 is shown in Table 1. Contrary to free anion calculations, the idealized symmetry Civ is broken in the lattice. As a consequence the modes are split. The present calculations show a significant low- ering of the NO stretch frequency by 56 cm"1 from GS to S1 which is much larger than the shift found for the free ion model. This demonstrates that this shift is de- pendent on the weak interactions of the nitrosyi group with the lattice environment. It appears that the func- tional we have chosen for this study overestimates the NO stretch frequency by about 60 cm"1. This suggests that the balance between the NO double bond and the strong Fe-NO bond is critically tested by the vibrational properties. The match between experimental and cal- culated frequencies is. sufficiently close to suggest that the vibrational spectrum presents a useful fingerprint of the unusual structures in S1 and S2. [1] B. Delley, J. Schefer and Th. Woike, J. Chem. Phys. Fig.1: The SNP unit cell with all four formula units in 107, 10067(1997) state S2. Table 1. Vibrational frequencies for NO modes [cm"1] Mode: GS experiment S1 experiment S2 experiment NO stretch 1985-1987 1947 1929-1931 1835 1711-1721 1664 Fe-NO stretch 683-684 667 604-632 587 629-631 Fe-NO bend 678-681 657 601-603 565 579-618 595 Fe-NO bend 667-672 657 590-598 565 553-566 80 SURFACE RELAXATION OF THE HEMATITE (0001) SURFACE B. Delley, A. Chaka1-2 and M. Scheffler1 1 Fritz-Haber Institut der Max-Plank-Gesellschaft, Berlin-Dahlem 2 The Lubrizol Corporation, Cleveland Ohio Using spin density functional theory we investigate the structure of a Fe2O3 hematite surface. As compared to the interlayer spacing the surface relaxation is extremely large. Furthermore surface relaxations are large for several layers deep in this compound. Hematite is a model compound for rust and is of obvi- this study hematite was used as a benchmark system. ous relevance to technology. Despite the widespread Fig 1. shows the hematite slab unit cell, the surface availability of rust, the surface of hematite remains a is in z direction. This compound has an antiferromag- challenge for experimental characterization with atomic netic structure with pairs of Fe layers ordering parallel resolution, especially when not the surface under vac- and an antiferromagnetic coupling across the oxygen uum is of interest. Bandstructure surface codes are be- layers. The local moment for the slab is near the bulk ginning to be able to supply complementary theoretical value of 3.4/iB. The Iron terminated surface exhibits a insights. While first results on this particular surface large number of dangling bonds, as shown in Fig 1. system have been Iterlayer Relaxations relative to bulk spacing [%] Interlayer: Wang [1] present Fe-O3 -57 -57 O3-Fe 7 8 Fe-Fe -33 -29 Fe-O3 15 14 O3-Fe 5 5 Fe-Fe -3 -1 Relaxations have been calculated using analytical en- ergy derivatives and a hessian eigenvalue following al- gorithm applied to cartesian coordinates. Small rela- tivistic corrections for the Fe ion core have been intro- duced via the semilocal pseudopotential developed by the Stuttgart and Dresden quantum chemistry groups. The calculated relaxed surface matches closely with the results from the full potential linearized augmented plane wave (FLAPW) method used by [1]. This demon- strates that our method is capable of giving state of the art results for such challenging systems. For systems with less than 100 atoms, the method scales linearly with the number of orbitals. This is an important ben- efit of using localized basis sets. The range parameter for the local basis sets is of significance to computa- tional speed. Compute time for a bulk lattice scales with the sixth power for the range parameter. Above results were obtained with a conservative setting of Rcut = 9 Bohr units. With this setting the present calcu- Fig.1: The hematite slab unit cell (hexagonal), sur- lation was approximately an order of magnitude faster face with dangling bonds on top and bottom. than FLAPW. If Rcut is reduced to 8, with a correspond- ing speedup factor of 1.8 for the hematite slab studied published recently [1], we have investigated the per- here, forces change less than 2.1Cr4 au, which is less spectives of doing similar studies with the DMol3 ap- than the usual threshold for termination of the relax- proach. While such calculations remain at the forefront ation calculation. In summary, the present method is a of todays computational possibilities, because treat- promising tool for density functional studies of surface ment of large unit cells, d-states and oxygen together systems. with a sizeable vacuum region is required. The promise is to soon consider also molecules at such surfaces. In [1] X.-G. Wang et al. Phys. Rev. Lett. 81,1038 (1998) 81 RANDOM MAGNETIC FLUX PROBLEM IN A QUANTUM WIRE C. Mudry1, P. W. Brouwer2 and A. Furusaki1 1 Condensed Matter Theory Group, PSI, CH-5232 Villigen PSI 2 Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, NY 14853, USA 3 Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto 606-8502, Japan The random magnetic flux problem on a lattice and in a quasi-one-dimensional (wire) geometry is stud- ied both analytically and numerically. The first two moments of the conductance are obtained analytically. Numerical simulations for the average and variance of the conductance agree with the theory. Both in strongly correlated electronic problems such as problem with a bipartite symmetry, changing the sign of the fractional quantum Hall effect [1] and high Tc su- the wave function on one sublattice implies a change perconductivity [2], it has been proposed to approxi- in sign of the random Hamiltonian (cf. Fig.1). The con- mate the strong interactions by a quenched (i.e., time- sequences of the chiral symmetry are dramatic at the independent) random magnetic flux. In other words, band center s = 0. one is interested in the localization properties of a s- The dependence on the length L of the quantum wire ingle particle constrained to move in a plane and sub- for the first two moments of the conductance g in the jected to a static magnetic flux that varies randomly in geometry of Fig.1 is calculated analytically at and away space, in short the random magnetic flux (RMF) prob- from the band center. Away from the band center, lem. the moments are the ones expected for a quantum The RMF problem is a special case of the problem of wire without time-reversal invariance (standard unitary Anderson localization. It has eluded a qualitative solu- class) [5]. At the band center, however, all moments tion until now [3]. Here, we report about a quantitative undergo a striking parity effect as a function of the num- solution to the RMF in a quantum wire (cf. Fig.1) [4]. ber Ar of channels in the quantum wire [4]. This parity effect is displayed in Fig. 2 where it is seen that expo- nential decay of the mean/variance for an even number (a) 1/2 of channels is replaced by algebraic decay L~ for an J =• odd number of channels. ©1,2 ©2,2 ©3,2 ©4,2 20 J = 0 ©0,1 ©1,1 ©2,1 ©3,1 ©4,1 3 = 1 m = 0 rn = 5 (b) Fig.2: Numerical simulations for the average (a) and L the variance (b) of g at s = 0 for N = 15 (circle) and N — 16 (diamond). The disorder strength p = 0.2. For Fig.1: (a) Lattice with N = 3 threaded by ran- these parameters, the crossover length scale £ = 280a r dom magnetic fluxes 0TOii in the disordered region for A = 15 and the localization length £ = 283a for 0< m< M. The fluxes that thread the elementary pla- N = 16. The solid (dashed) lines in (a) are the theo- quettes are independently and uniformly distributed in retical result for (g) for N = 15 (16); the solid (dashed) the interval [-pit,pit]. The RMF Hamiltonian represents lines in (b) are the large odd (even) N analytical results a particle undergoing nearest neighbor hopping. The for var#. phase of the hopping amplitude is random in the disor- dered region, (b) Quantum wire made of N channels [1] V. Kalmeyer and S. C. Zhang, Phys. Rev. B 46, labelled by v and v' with a disordered region of length 9889 (1992); B. I. Halperin, P. A. Lee, and N. Read, L = Ma. Here, a is the lattice spacing. Incoming ampli- Phys. Rev. B 47, 7312 (1993); D. K. K. Lee, tudes are c£ and c$ _, whereas outgoing amplitudes + Phys. Rev. B 50, 7743 (1994); C.B. Hanna, DP. are c£_ and c^, +, in the left and right leads, respec- Arovas, K. Mullen, and S.M. Girvin, Phys. Rev. B tively, the dimensionless conductance g is obtained by 52,5221 (1995). calculating the transmission probability of an incoming [2] N. Nagaosa and P. A. Lee, Phys. Rev. Lett. 64, 2450 plane wave in channel u and summing over all channel- (1990); B. L. Altshuler and L. B. loffe, Phys. Rev. Let- s. In a quasi-one-dimensional geometry, M > N. t. 69, 2979(1992). [3] A. Furusaki, Phys. Rev. Lett. 82, 604 (1999). The essential property of the RMF problem is the exis- [4] C. Mudry, P. W. Brouwer, and A. Furusaki, tence of a chiral symmetry. The chiral symmetry refers Phys. Rev. B59, 13221 (1999). to the fact that for any lattice regularization of the RMF [5] M. R. Zirnbauer, Phys. Rev. Lett. 69, 1584 (1992). 82 HIGHER ORDER FRACTIONAL QUANTUM HALL STATES R.H. Morf Condensed Matter Theory, PSI, CH-5232 Villigen PSI The microscopic study of edges of fractional quantum Hall states requires study of large systems. Accurate trial wave functions are needed for this purpose. We propose new wave functions for higher order fractional states at fillings v = n/(2n +1), which are almost as easy to evaluate as Laughlin's wave function atv = 1/3. First calculations have demonstrated the validity and accuracy of these wave functions. The great success of Laughlin's wave function for states dicted and a few years later observed experimentally. at filling v = 1/3,1/5,.. is due to both the physical va- We then discovered a way to modify these SU(n) sym- lidity as well as the simplicity of this wave function. In metric wave functions such that antisymmetrization (as particular, it is easily amenable to Monte Carlo calcu- required for fully spin polarized electron states) be- lations. Furthermore, it has been used successfully by comes possible. This modification disturbs the zeros Laughlin [1 ] to demonstrate that the charged excitations of the SU(n) symmetric wave function in a minimal way carry fractional charge, a prediction which has recently and so minimizes the energy cost of antisymmetriza- been confirmed by experiment studying the shot noise tion. For small systems, the antisymmetrizer can be in quantum Hall devices with a tuneable constriction. evaluated exactly, while for large systems (N > 12) it However, equally simple wave functions for higher order can be calculated perturbatively using a method devel- fractional states at filling fractions v = n/(2n + 1) have oped earlier [4]. not yet been constructed. The most succesfull ones In Figure 1, we show a first example of calculations have been the Composite Fermion wave functions in- based on our new wave functions. The electron den- troduced by Jain [2]. Their main disadvantage is that sity of a 72 electron system in the disk geometry is they contain states from higher Landau levels. Thus shown for filling fraction v = 2/5 both in the SU(2)- the total energy of these wave functions includes kinetic symmetric (spin singlet) and in the antisymmetric (spin energy which is proportional to the cyclotron energy polarized) state. Notice the significant difference in the %UJC ~ B and therefore large in high magnetic fields B. microscopic structure of the edge for the different sym- To avoid this, Jain proposed to use the projection of this metries. wave function onto the lowest Landau level. For small The validity of our new trial wave functions was tested systems, this projection can be calculated exactly, and by comparing exact diagonalization results for small these projected wave functions indeed exhibit a large systems with those obtained with the new trial wave overlap with the exact ground state. However, for larger functions. Their accuracy is similar to that of Laughlin's systems the projection can only be calculated in an wave function for v = 1/3. approximate, non-systematic way. Thus, the main ad- vantage of trial wave functions - the possibility to study large systems - is lost. In recent experiments, tunneling into quantum Hall edges has been explored. These have yielded results antisymmetric - that are in conflict with present edge state theory. It ap- pears now that a microscopic understanding of quan- tum Hall edges is required. Exact calculations cannot SU(2) symmetric c yield this information as the system sizes that are re- -a quired are substantially larger than can be handled by 0.5 exact diagonalizations. It appears that only trial wave v=2/5 state functions can provide this insight. We have found a new way to generate microscopic wave functions for higher order fractional states that are built entirely from lowest Landau level states, with- out any need of projection. Starting point for our new trial wave functions was the observation that Laughlin's Fig.1: Density of system at v = 2/5 in disk geometry: wave function could be generalized in an exact manner SU(2) symmetric vs. antisymmetric state. to states at filling fraction v = n/(2n + 1) provided that electrons occurred in n flavours and formed a SU(n) singlet state. The special case of Laughlin's wave func- [1] R.B. Laughlin, Phys. Rev. Lett. 50,1395(1983). tion ^3 is obtained for n = 1, while for n = 2, Halperin's [2] J.K. Jain, Phys. Rev. Lett. 63,199(1989). wave function V3,3,2 is recovered, which had been pro- [3] B.I. Halperin Helv. Phys. Acta 56,75(1983). posed as a trial state for a spin unpolarized v = 2/5 [4] R. Morf and B.I. Halperin, Phys. Rev. B33, 2221 fractional quantum Hall state [3], that was then pre- (1986). 83 INTERFACE WAVE FUNCTIONS AND ENERGY GAPS OF QUANTUM HALL STATES R.H. Mori1, N. d'AmbrumeniP and S. Das Sarma5 1 Condensed Matter Theory, PSI, CH-5232 Villigen PSI 2 Department of Physics, University of Warwick, Coventry, England 3 Department of Physics, University of Maryland, College Park, Maryland, USA We have accurately determined the electronic wave function (wf) at the interface between GaAs andAIGaAs by a variational method. We find that the most often used approximation, based on the Fang-Howard wf, greatly overestimates the width of the electronic density at the interface. As a result, the interaction between electrons in the layer is softened at short separations much more than realistic. This, in turn, leads to an overestimate of the reduction of energy gaps in fractional quantum Hall states by finite width effects. We find that for quantum Hall state calculations, a Gaussian approximation for the interface wf allows a simple parametrization of the problem, which is much more accurate than the traditionally used Fang-Howard wf. Energy gaps of quantum Hall states have been deter- duces width values that are in good agreement. mined experimentally by transport measurements and Contrary to previous claims [2,3], we conclude that our by inelastic light scattering. On the other hand, the the- results for the width of interface wf's lead to gap reduc- ory of the fractional quantum Hall (FQH) effect allows tions that are substantially too small (by a factor« 2) to a first principles calculation of energy gaps. Several lead to agreement with experiment [4]. difficulties have to be resolved for an accurate deter- mination of these gaps: finite size effects due to the use of small systems, the effects of curvature (when us- ing the spherical geometry) and effects due to the finite > 2 width of the interfacial wave function (wf) of the elec- 0 trons. The latter results in a softening of the Coulomb interaction at short separation. A realistic calculation of the electron-electron inter- CD 5 Fang-Howard action must be based on a accurate calculation of 0) z xGaussian the electron wf perpendicular to the interface between Gaussian the higher and lower bandgap semiconductors forming a) energy 10u the heterostructure. The interfacial wf can be com- Fang-Howard puted by numerical solution of the Schrodinger equa- z xGaussian tion in which the electrostatic potential is determined Gaussian self-consistently by solution of the Poisson equation. Ortalano et al. Exchange-correlation effects can be incorporated by using density functional theory in a local density ap- i=ns/20 proximation. This approach has been used by Stern and Das Sarma [1], and for the calculation of energy gaps of FQH states, by Ortalano et al. [2]. b) Alternatively, the Schrodinger equation can be used to formulate a variational calculation using suitable trial If/' 5 10" 2 wave functions. Such a calculation can be carried out n [1011/cm2] analytically, if the electronic wave function is approxi- s mated by one of the following functions: (1) z exp(-z/s) 2 Fig.1: a) Ground state energy of single electron at for z > 0 (known as Fang-Howard wf), (2) ex-p(z-zo)/s GaAs-AIGaAs interface as function of the areal (Gaussian) and (3) zexp(-z2/sz for z > 0 (referred to electron density ns. Different variational inter- in the following as z*Gaussian). While functions (1) and face wf's are compared in Figure a. z*Gaussian (3) have only one variational parameter, i.e. their width exhibits the lowest energy in the experimentally s, in the Gaussian both its width s and position z0 from most relevant density range. For the deple- the interface can be adjusted. tion density ndepi the values ns/5 or ns/20 are In Figure 1, we show the results of our analytical vari- used, b) The width w = \/< z2 > - < z >2 is ational calculations. In Figure 1a, the ground state plotted as function of ns. Exact results by Or- energy of the three types of trial wf's is shown as a talano et al. [2] are compared with results of function of the areal electron density ns- Clearly, for variational calculations. Agreement with results 11 2 ns < 2 - 3 x 10 /cm , z*Gaussian has the lowest en- based on z*Gaussian is best, as expected from ergy, while for larger ns the Gaussian wins. The tradi- results of Figure a. tional Fang-Howard wf is always inferior. The situation is even worse when we look at the width of the inter- [1] F. Stern and S. Das Sarma, Phys. Rev. B30, 840 facial wf, Figure 1b: The width estimated on the basis (1984). of the Fang-Howard wf, is too large by almost a factor [2] M.W. Ortalano et al., Phys. Rev. B55, 7702 (1997). 2, when compared to the results of Ortalano et al. [2]. [3] K. Park et al., Phys. Rev. Lett. 81, 4100 (1998). By contrast, the energetically superior z*Gaussian pro- [4] R.H. Morf, Phys. Rev. Lett. 83, 1485 (1999). 84 CROSS-OVER FROM UNIFORM MAGNETIZATION REVERSAL TO DOMAIN NUCLEATION IN MAGNETIC NANOSTRUCTURES Hans-Benjamin Braun (Condensed Matter Theory) Conventional magnetic data storage is limited by thermally activated magnetization reversal in magnetic nanostructures. Explicit expressions are derived which determine the cross-over from uniform (Neel-Brown) reversal in magnetic nanos- tructures to nonuniform nucleation of soliton-antisoliton pairs in quasi ID structures. The results are in agreement with recent Monte-Carlo simulations. There has been considerable recent progress in explor- domain wall width and 0 < h < 1 is the reduced exter- ing the physics of magnetic nanostructures. The rea- nal field. Above this critical length, the energy barrier sons are both considerable advances in sample prepa- approaches the value of the soliton-antisoliton pair in ration and novel detection techniques such as ultra- an infinite sample quickly, as is seen from Fig 1 ii). sensitive SQUID technology and magnetic dichroism These results have important implications: The ana- spectromicroscopy using x-ray synchrotron radiation. lytical field-theoretic approach employed for the com- These techniques are likely to significantly deepen our putation of the switching rate has been tested inde- understanding of magnetic nanostructures in the near pendently by Monte-Carlo simulations and agreement future. within the errortolerance has been found. We have There is also considerable practical interest in this field: established a clear criterion which characterizes the Current magnetic data storage technology will be fac- regime of soliton-antisoliton nucleation which is un- ing considerable challenges within the next few years. wanted for applications as it reduces thermal stability The size of the individual storage elements will soon be for a nanoparticle of a given volume. approaching the superparamgnetic limit where thermal fluctuations destroy the long term stability of one indi- vidual bit. uniform nonuniform In order to compete favorably with this limit, a de- tailed understanding of the reversal mechanism in mag- netic nanostructures is necessary. In particular, parti- cles which allow for nonuniform nucleation provide un- necessarily small energy barriers towards thermally in- duced magnetization reversal, it is therefore important to know the conditions which separate the regimes of uniform and nonuniform magnetization reversal. The analysis of the nonlinear field equations governing the stationary magnetization configurations yields the following "critical nucleus" for magnetization reversal [3] in a system of finite length w f's = 2 arctan ( (1) This configuration describes the exact interpolation be- Fig.1: Rates for thermally induced magnetization re- tween uniform reversal and nonuniform nucleation of versal in magnetic nanostructures in the presence of soliton-antisoliton pairs [4]. dn is a Jacobian elliptic an external field h not exceeding the anisotropy field, function and its ellipticity parameter k is related to the i) Rates for uniform nucleation in small samples with lengths L < L [S = \/20]. ii) Rates for reversal system length L, with w depending on the value of the crit o via nonuniform configurations. Symbols denote recent external field and the ellipticity parameter. This config- Monte Carlo results from [1], while the lines are ana- uration is shown in Fig. 1 ii) for various system lengths lytical results from Ref. [3], [4]. iii) shows the gener- at fixed fields. The corresponding energy can be ex- alization of soliton-antisoliton configuration for samples pressed in closed form through elliptic integrals [3] of finite length, interpolating between uniform (dashed Using the stochastic magnetization dynamics at the line) and inifinite size (dotted line) configurations, iv) saddle point, the total nucleation rate has the form [2] indicates the corresponding barrier energies. r = Q e-xp{-E[ MAGNETIC CORRELATIONS IN NANOSTRUCTURED FERROMAGNETS H.B. Braun1, J.F. Loftier2 and W. Wag net3 1 Condensed Matter Theory, Paul Scherrer Institute, CH-5232 Villigen PSI 2 W.M. Keck Laboratory, California Institute of Technology, Pasadena, California 91125 3 Spallation Source, Paul Scherrer Institute, CH-5232 Villigen PSI Magnetic correlations in nanostructured ferromagnets show a clear minimum at grain sizes of the order of the domain wall width. The experiments are explained within a new theoretical model which predicts the correlation length as a function of the grain size distribution. Nanostructured ferromagnets constitute a novel class regimes: For large grains, the magnetization rapidly ap- of materials whose properties can be tailored by an ap- proaches the easy-axis orientation away from the grain propriate choice of size and geometry of the individual boundary. This occurs for strong intergrain coupling grains. Such magnets significantly differ from the bulk either within a layer of width <50, or via an abrupt in- material: The coercivity may change by a factor of 100- terface phase slip for small coupling whose strength is 1000 upon variation of the grain size in the range of a controlled by the ratio g = I/dVAK. For small grains, few tens of nanometers. This sensitivity to grain size is we observe that the magnetization starts to override the mainly a consequence of a surprising drop of the co- anisotropy of the individual grains as indicated in Fig. 1 ercivity below a grain size of a few nm, which is not i), even for small intergrain coupling. A magnetic cor- related to superparamagnetism. relation length that may considerably exceed the grain If materials should be designed to the specific needs size results from the balance between the random ex- of applications, it is important to know how the macro- cess anisotropy, -K/y/N, within a finite region of size scopic properties arise from the interplay of micro- L3 = ND3, and the exchange energy density to adjust 2 scopic parameters such as grain size, intergrain cou- over this length scale, AeR/L , with Aei?, the effective pling and anisotropies. Small angle neutron scattering exchange constant. In contrast to the traditional ran- provides a first step in this direction as an extremely dom anisotropy model, the resulting exchange is here useful tool to investigate the magnetic correlations as determined by both interface and intragrain exchange. a function of the material parameters. However, for a Figs 1 iii) and iv) demonstrate that the transition be- deeper understanding a theoretical approach is needed tween these two regimes changes relatively abruptly that links the observations to the microscopic parame- as a function of grain size around D = 60. Using this ters of the system. In particular, it is a priori not clear fact together with the measured log-normal distribution, why the correlation length should increase at small par- yields the solid curve in Fig 1 ii). ticle sizes and why there is a cross-over of magnetic properties at a length scale that approximately equals i) magnetization anisotmpy the domain wall width of the bulk material. SZ"' In order to explain the properties of nanostructured Fe leng t •k magnets we have introduced the following energy c •• .9 80- A a t £ 40- v grain size H = - A R • (Vmf f ~YuY 10 30 50 70 nm i, a m)2. (l) Here m^x) denotes the magnetization unit vector field within grain i of volume Vi, and rij is the unit vector of the random anisotropy of strength K > 0 which is assumed to be uncorrelated between adjacent grains. A is the intragrain exchange constant and 1^ > 0 characterizes the (reduced) random, ferromagnetic ex- Fig.1: i) Schematic view of grains with the magne- tization correlation length substantially larger than the change across the interfaces 5^ of width d. In the limit random anisotropies. ii) magnetic correlation length as IijD/d < KD2 < A, the grains play the role of ran- a function of average grain size, the solid line is a pre- domly coupled large spins. Introducing the projection of diction of our model with domain wall width DIPOLAR INTERACTION IN 2D HONEYCOMB MAGNETS H. B. Braun1, B. RoesslP and K. Kramer3 1 Condensed Matter Theory, Paul Scherrer Institute, CH-5232 Viiiigen PSI 2 Laboratory of Neutron Scattering, Paul Scherrer Institute, CH-5232 Viiiigen PSI 3 Department of Chemistry, University of Bern, Bern The classical ground state of a system of dipoles arranged on a 2D honeycomb lattice is rigorously shown to be a (1/3, -1/3) structure. This configuration exhibits a continuous degeneracy with respect to rotations of the two hexagonal sublattices in opposite directions. These results are in agreement with observations on ErBr3, suggesting that dipolar interactions are dominant in the latter compound. Frustrated magnetic systems are characterized by the a{V3m/2,m/2 + n) and R^n = a(\/3m/2 + fact that the classical ground state is unable to satisfy i/>/3,m/2 + n) are the lattice vectors of the hexag- the interactions on all bonds simultaneously. Frustra- onal sublattices A and B, respectively. Furthermore, tion can be caused by a special random arrangement a,a' - ± and da = \{dx - iady). Interestingly, the of pair interactions as in spin glasses, or due to the configuration characterized by -Q does not minimize special geometric arrangement of the nearest neighbor the dipolar interaction energy. As illustrated in Fig. 1, bonds. This latter situation occurs in 2D Kagome lat- this state can be regarded as a vortex lattice. It is in- tices or in 3D magnets with pyrochlore structure, the teresting to note that it is the particular geometry of latter having been termed "spin ice" since the ground the honeycomb lattice which leads to a favoring of the state is characterized by the same degeneracies as the (1/3, -1/3) structure. The same state does not mini- proton ordering in real ice. The perhaps simplest proto- mize the dipolar energy on a simple hexagonal lattice. type of a geometrically frustrated system is the 2D an- Using the specific symmetries of the honeycomb lattice tiferromagnet on a hexagonal lattice where the ground and the magnetization configuration (2) we can demon- state consists of a 120° ordering of adjacent spins — a strate that the ground state is invariant under the con- state that does not exhaust the minimal energy on each tinuous class of U{1) transformations bond. Related to this latter system are honeycomb mag- nets where the spins are subject to an easy-plane anisotropy. However, the lattice is bipartite and thus frustration may only occur in the presence of next- with an arbitrary angle ip. It is remarkable that the frus- nearest neighbor interaction. Such a compound, the tration of the dipolar interaction leads to an infinite de- generacy of the ground state on the honeycomb lat- novel rare-earth trihalide ErBr3, has recently been grown and investigated by some of us [1]. It showed tice. Such degeneracies are not known for other situa- an unusually low 2D in-plane ordering temperature of tions. The proof of the stability of the ground state relies on the duality trick of Ewald's summation technique by Tc = 400mK, while below Tc = 280mK a finite correla- tion length of 151 along c emerges, combining approx- which the slowly convergent dipolar sum is transformed imately 2 layers separated by 6.3A. In this tempera- into an exponentially convergent expression. For the ture range the dipolar interaction between the s = 15/2 ground state energy we obtain Em/jj? = -11.57 ± 0.01 spins can no longer be neglected, and thus frustration per site with fj, the magnetic moment. This result can occurs. be improved with exponential accuracy if desired. These experiments have motivated us to study the dipolar interaction on a 2D honeycomb lattice for easy- \|/=0 \|/=7l/6 plane spins. Due to the long-range character of the dipoiar interaction the system is inherently frustrated, , / \ / \ / \ it if if l and it is not even clear a priori whether a stable, or- \ / \ / \ / v. y -*, y -K. y dered ground state exists. / \ / \ / \ i »f i f t ! We have been able to demonstrate that the dipolar en- \ / v / \ / \ / >-. y -v y •»-. y -^ y ergy / \ / \ / \ / \ if if if if \ / \ / W \ / •«•- y ••-. y ^~ y 1 \Z - 2RR' """*/ \*~"/ \ ^V\*~ if' if' if' (1) sublattice A sublattice B is minimized by the magnetization configuration char- acterized by the azimuthal angle Fig.1: Two examples {ib = 0. w/6) of the class of con- L,A tinuously degenerate ground states of a dipolar magnet ,n) = Q ra.n' on a honeycomb lattice (2) ln) = -Q • K^n + 2TT/3. 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LOW TEMPERATURE FACILITIES Adams D; Adeva B; Arik E; Arvidson A; Badelek B; Ballintijn MK; Bardin G; Baum G; Berglund P; Betev L; Bird IG; Birsa R; Bjorkholm P; Bonner BE; de Botton N; Boutemeur M; Bradamante F; Bravar A; Bressan A; Bultmann S; Burtin E; Cavata C; Crabb D; Cranshaw J; Cuhadar T; Dalla Torret S; van Dantzig R; Derro B; Deshpande A; Dhawan S; Dulya C; Dyring A; Eichblatt S; Faivre JC; Fasching D; Feinstein F; Fernandez C; Forthmann S; Frois B; Gallas A; Garzon JA; Gaussiran T; Gilly H; Giorgi M; von Goeler E; Goertz S; Gracia G; de Groot N; Grosse Perdekamp M; Gulmez E; Haft K; von Harrach D; Hasegawa T; Hautle P; Hayashi N; Heusch CA; Horikawa N; Hughes VW; Igo G; Ishimoto S; Iwata T; Kabuss EM; Kageya T; Karev A; Kessler HJ; Ketel TJ; Kiryluk J; Kishi A; Kisselev Yu; Klostermann L; Kramer D; Krivokhijine V; Kroger W; Kurek K; Kyynarainen J; Lamanna M; Landgraf U; Layda T; Le Goff JM; Lehar F; de Lesquen A; Lichtenstadt J; Lindqvist T; Litmaath M; Lowe M; Magnon A; Mallot GK; 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11 Oct. 1999; p.354-74 92 Adeva B; Arik E; Arvidson A; Badelek B; Baum G; Berglund P; Betev L; Birsa R; De Botton N; Bradamante F; Bravar A; Bultmann S; Burtin E; Crabb D; Cranshaw J; Cuhadar T; Torre SD; Van Dantzig R; Derro B; Deshpande A; Dhawan S; Dulya C; Eichblatt S; Fasching D; Feinstein F; Fernandez C; Frois B; Gallas A; Garzon JA; Gilly H; Giorgi M; von Goeler E; Goertz S; Garcia G; de Groot N; Perdekamp MG; Haft K; von Harrach D; Hasegawa T; Hautle P; Hayashi N; Heusch CA; Horikawa N; Hughes VW; Igo G; Ishimoto S; Iwata T; Kabu EM; Karev A; Kessler HJ; Ketel TJ; Kiryluk J; Kisselev Yu; Kramer D; Kroger W; Kurek K; Kyynarainen J; Lamanna M; Landgraf U; Le Goff JM; Lehar F; des Lesquen A; Lichtenstadt J; Litmaath M; Magnon A; Mallot GK; Marie F; Martin A; Martino J; Matsuda T; Mayes B; McCarthy JS; Medved K; Meyer W; van Middelkoop G; Miller D; Miyachi Y; Mori K; Moromisato J; Nassalski J; Niinikoski TO; Oberski JEJ; Ogawa A; Ozben C; Pereira H; Perrot Kunne F; Peshekhonov D; Piegaia R; Pinsky L; Platchkov S; Pio M; Pose D; Postma H; Pretz J; Puntaferro R; Radei G; Reicherz G; Roberts J; Rodriguez M; Rondio E; Sabo I; Saborido J; Sandacz A; Savin I; Schiavon P; Sichtermann EP; Simeoni F; Smirnov Gl; Staude A; Steinmetz A; Stiegler U; Stuhrmann H; Tessarotto F; Thers D; Tlaczala W; Tripet A; Unel G; Velasco M; Vogt J; Voss R; Whitten C; Willumeit R; Windmolders R; Wiślicki W; Witzmann A; Zanetti AM; Zaremba K; Zhao J SPIN ASYMMETRIES IN A, OF THE PROTON AND THE DEUTERON IN THE LOW X AND LOW Q2 REGION FROM POLARIZED HIGH ENERGY MUON SCATTERING Physical Review D. 60, no.7; 1 Oct. 1999; p.072004/1-9 Zimmer O; Muller TM; Hautle P; Heil W; Humblot H HIGH PRECISION NEUTRON POLARIZATION ANALYSIS USING OPAQUE SPIN FILTERS Physics Letters B. 455, no.1 4; 27 May 1999; p.62-8 Suft G; Beulertz W; Bock A; Frank M; Glombik A; Hey J; Kowalzik B; Kretschmer W; Merz S; Meyer H; Sozuer L; Weidmann R; Boschitz E; Brinkmoller B; Meier R; van den Brandt B; Hautle P; Konter JA; Mango S; Mertens G; 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Casalbuoni S; Czapek G; Diggelmann U; Ebert T; Huber D; Janos S; Kainer KU; Knoop K M; Konter JA; Mango S; Moser U; Palmieri VG; Pretzl K STATUS REPORT ON THE ORPHEUS DARK MATTER DETECTOR AND ON ITS SQUID READOUT SYSTEM Nuclear Physics B, Proceedings Supplements. 70; Jan. 1999; p.101-5. Suft G; Amaudruz P; Beulertz W; Bock A; Boschitz E; van den Brandt B; Brinkmoller B; Frank M; Glombik A; Gruebler W; Hautle P; Hey J; Konter JA; Kowalzik B; Kretschmer W; Mango S; Meier R; Mertens G; Merz S; Meyer H; Sözüer L; Tacik R; Weidmann R POLARIZATION TRANSFER OBSERVABLES IN PION-DEUTERON ELASTIC SCATTERING Few-Body Systems Suppl. 10, 459-462 (1999) 93 Addendum 1998 C. M. Freeman, J. W. Andzelm, C. S. Ewig, J.-R. Hill and B. Delley THE STRUCTURE AND ENERGETICS OF GLYCINE POLYMORPHS BASED ON FIRST PRINCIPLES SIMULATION USING DENSITY FUNCTIONAL THEORY Chem. Commun., 2455 (1998). M. Gamier, D. Purdie, K. Breuer, M. Hengsberger, Y. Baer and B. Delley GARNIERETAL. REPLY Phys. Rev. Lett. 81, 1349 (1998). R.H. Morf TRANSITION FROM QUANTUM HALL TO COMPRESSIBLE STATES IN THE SECOND LANDAU LEVEL: NEW LIGHT ON THE -=5/2 ENIGMA Phys. Rev. Lett. 80, 1505 (1998). Addendum 1997 O. Oleksyn (Zaharko), T. Haibach, S. Kek, W. Morgenroth, ORDERING IN HoFe6Ge6 STUDIED BY SYNCHROTRON X-RAY RADIATION, Proceedings of Aperiodic '97, Alpe d'Huez, August 27-31, 359-363 (1997) 94 INTERNAL REPORTS NEUTRON SCATTERING LNS-198 UNTERSUCHUNG LOKALER INHOMOGENITAETEN IN HOCHTEMPERATURSUPRALEITERN MITTELS NEUTRONENSTREUUNG Matthias Gutmann Ph.D. Thesis (Januar 1999) LNS-199 KRISTALLFELD- UND DIMERAUFSPALTUNG IN Cs3Er2X9 (X=CI,Br) UND Cs2ErCI5 Dominik Schaniel Diploma Thesis (Winter 1998/99) LNS-200 ELEKTRONISCHE KUEHLUNG DURCH ADIABATISCHEN DRUCK Thierry Strässle Diploma Thesis (Winter 1998/99) LNS-201 MAGNETIC PROPERTIES OF THE RARE EARTH BOROCARBIDES RNLB2C. A NEUTRON SCATTERING STUDY Urs Gasser PH.D. Thesis (October 1999) 95 CONFERENCE, WORKSHOP AND SEMINAR CONTRIBUTIONS NEUTRON SCATTERING P. Allenspach RARE EARTH MAGNETISM IN HIGH-TEMPERATURE AND BOROCARBIDE SUPERCONDUCTORS 5th IUMRS International Conference on Advanced Materials IUMRS-ICAM'99, Beijing, China, 13.-18.6.99. P. Allenspach INTRODUCTION TO NEUTRON SCATTERING 7th Summer School on Neutron Scattering, Zuoz, Switzerland, 8.-13.8.99. P. Allenspach MARS: INVERTED TIME-OF-FLIGHT BACKSCATTERING SPECTROMETER AT SINQ European Conference on Neutron Scattering ECNS'99, Budapest, Hungary, 1 .-4.9.99. P. Allenspach EINFUHRUNG IN DIE NEUTRONENSTREUUNG PSI-Herbstschule fur Maturanden, PSI Villigen, Switzerland, 4.-8.10.99. P. Allenspach DAS EXPERIMENT IN DER FORSCHUNG DEMONSTRIERT AM BEISPIEL DER NEUTRONENDIFFRAKTION Lehrerfortbildung des Kantons Aargau, PSI Villigen, Switzerland, 30.10.99. F. Altorfer TRIPLE AXIS SPECTROSCOPY 7lh Summer School on Neutron Scattering, Zuoz, Switzerland, 7.-13.8.99. F. Altorfer, H. D. Lutz, S. Peter INVESTIGATION OF OH DISORDER IN Sr(OH)Br 2nd European Conference on Neutron Scattering, Budapest, Hungary, 1 .-4.9.99. P. Boni 10 JAHRE MULTISCHICHTEN Conference of the PSI Department F3A, Villigen, Switzerland, 18.1.99. P. Boni SPIN-DENSITY WAVES IN INCOMMENSURATE ANTIFERROMAGNETIC CHROMIUM: A NEUTRON SCATTERING INVESTIGATION Seminar, Universitat Hamburg, Hamburg, Germany, 26.1.99. P. Boni FERROMAGNETE UND ANTIFERROMAGNETE - UNTERSUCHUNGEN MIT POLARISIERTEN NEUTRONEN Kolloquium, Technische Universitat Munchen, Garching, Germany, 30.4.99. P. Boni NOVEL CONCEPTS IN NEUTRON INSTRUMENTATION Plenary Talk, European Conference on Neutron Scattering ECNS'99, Budapest, Hungary, 1.-4.9.99. P. Boni SPIN FLUCTUATIONS IN MAGNETS Annual Focussed Session on "New Concepts in Reflectometry" Max-Planck-lnstitut fur Metallforschung, Schloss Ringberg, Germany, 4.-6.9.99. P. Boni HERSTELLUNG UND ANWENDUNG VON SUPERSPIEGELN PSI-Herbstschule fur Maturanden, PSI Villigen, Switzerland, 4.-8.10.99. 96 P. Boni, A. Furrer and E. Jericha NEUTRON OPTICAL BENCH AT PSI TMR-Workshop on Perfect Neutron Crystal Optics, Atominstitut der Österreichischen Universitäten, Wien, Austria, 4.-6.3.99. P. Boni and Senthil Kumar M. LARGE-m SUPERMIRRORS XENNI Meeting, Ris0, Denmark, 5.5.99. G. Böttger, P. Fischer, J. Mesot, B. Roessli, P. Allenspach, A. Furrer, A. Donni, Y. Aoki and H. Sato 3+ ANTIFERROMAGNETIC ORDERING OF THE RARE EARTH IONS R IN R2Ba4Cu7015.6. (R = Er, Dy) lUCr Congress, Glasgow, U.K., 4.-13.8.99. N.Cavadini, G.Heigold, W.Henggeler, A.Furrer, K.Krämer, H.U.Güdel, H.Mutka and P. Vorderwisch SPIN GAP AND QUANTUM EXCITATIONS IN THE S=1/2 KCuCL/TICuCL,COMPOUNDS 2nd SINQ User Meeting, Villigen, Switzerland, 2.12.99. N.Cavadini, W.Henggeler, A.Furrer, K.Krämer, H.U.Güdel and H.Mutka MAGNETIC EXCITATIONS IN THE QUANTUM SPIN SYSTEM KCuCI3 Conference of the PSI Department F3A, Villigen, Switzerland, 18.1.99. N.Cavadini, W.Henggeler, A.Furrer, K.Krämer, H.U.Güdel, H.Mutka and P.Vorderwisch MAGNETIC EXCITATIONS IN THE DIMERIZED SPIN LIQUID KCuCI3 7th Summer School on Neutron Scattering, Zuoz, Switzerland, 7.-13.8.99. N.Cavadini, W.Henggeler, A.Furrer, K.Krämer, H.U.Güdel, H.Mutka and P.Vorderwisch MAGNETIC EXCITATIONS IN GAPFUL QUANTUM SPIN SYSTEMS 2nd European Conference on Neutron Scattering, Budapest, Hungary, 1 .-4.9.99. N.Cavadini, W.Henggeler, A.Furrer, K.Krämer, H.U.Güdel, H.Mutka and P.Vorderwisch MAGNETIC EXCITATIONS IN THE DIMERIZED SPIN LIQUID KCuCI3 Swiss Workshop on Superconductivity and Novel Materials, Les Diablerets, Switzerland 27.-29.9.99. D. Clemens AMOR - DAS NEUE USER-REFLEKTOMETER MIT DER RICHTIGEN EINSTELLUNG ZUR PROBE Deutsche Neutronenstreutagung, Potsdam, Germany, 25.-27.5.99. D. Clemens INTRODUCTION TO NEUTRON REFLECTOMETRY 7th Summer School on Neutron Scattering, Zuoz, 8.-13.8.99. D. Clemens REMANENT POLARIZING MIRRORS Workshop "Neutron Optics in the Next Millennium", Villigen, Switzerland, 25.-27.11.99. D. Clemens, P. Gross, P. Keller, N. Schlumpf and M. Könnecke AMOR - THE VERSATILE REFLECTOMETER AT SINQ 2nd European Conference on Neutron Scattering, Budapest, Hungary, 1 .-4.9.99. I. Detemple, S. Janssen, G. Meier, J. Kohlbrecher and R. Hempelmann UNTERSUCHUNG DERSTRUKTURBILDUNG IN NANOKRISTALLINEN ANORGANISCHEN KOLLOIDSYSTEMEN MITTELS NEUTRONENKLEINWINKELSTREUUNG Deutsche Neutronenstreutagung, Potsdam, Germany, 25.-27.5.99. A. Donni, P. Fischer, L. Keller. T. Herrmannsdörfer, F. Fauth and T. Komatsubara NEUTRON DIFFRACTION STUDY OF A Nd3Pd20Ge6 SAMPLE WITH THREE SUCCESSIVE MAGNETIC PHASE TRANSITIONS Conference on Strongly Correlated Electron Systems, Nagano, Japan, 24.-28.8.99. 97 P. Fischer COMPARISON OF D20 AND D1B TO DMC AND HRPT AT SINQ Review of Neutron Diffraction ILL Instruments for Powders and Disordered Systems, ILL, France, 22.-23.3.99. P. Fischer, G. Frey, M. Koch, M. Konnecke, V. Pomjakushin, J. Schefer, R. Thut, N. Schlumpf, R. Burge, U. Greuter, S. Bondt and E. Berruyer HIGH-RESOLUTION POWDER DIFFRACTOMETER HRPT FOR THERMAL NEUTRONS AT SINQ 2nd European Conference on Neutron Scattering, Budapest, Hungary, 1 .-4.9.99. A. Furrer INFORMATION ON THE EUROPEAN NEUTRON SCATTERING ASSOCIATION (ENSA) Huitiemes Joumees de la Diffusion Neutronique, La Grande Motte, France, 19.-21.5.99. A. Furrer THE NEW SPALLATION NEUTRON SOURCE SINQ AT PSI Huitiemes Journees de la Diffusion Neutronique, La Grande Motte, France, 19.-21.5.99. A. Furrer ON THE EUROPEAN NEUTRON SCATTERING ASSOCIATION (ENSA) AND THE EUROPEAN SPALLATION SOURCE PROJECT (ESS) III Curso de tecnicas de haces de neutrones, Oviedo, Spain, 2.-5.6.99. A. Furrer SCIENTIFIC AND TECHNOLOGICAL IMPACT OF THE PROJECT AUSTRON AUSTRON Meeting, Dresden, Germany, 8.6.99. A. Furrer WISSENSCHAFTLICHE PERSPEKTIVEN DER ESS Kolloquium am Institut fur Festkorperforschung, FZ Julich, Germany, 18.6.99. A. Furrer ENSA REPORT & ENSA NEUTRON SOURCE STRATEGY IN EUROPE 2nd European Conference on Neutron Scattering (ECNS'99), Budapest, Hungary, 1.-4.9.99. H. Grimmer REFLECTOMETRY Synchrotron Radiation Information Day "In Situ Surface Diffraction", Bern, Switzerland, 18.1.99. H. Grimmer, O. Zaharko, H.Ch. Mertins and F. Schafers OPTICAL COMPONENTS FOR POLARIZATION ANALYSIS OF SOFT X-RAY RADIATION SPIE Conference on X-Ray optics design, performance, and applications. Denver, Colorado, U.S.A., 20.-21.7.99. H. Grimmer, O. Zaharko, H.Ch. Mertins. and F. Schafers OPTICAL COMPONENTS FOR POLARIZATION ANALYSIS OF SOFT X-RAY RADIATION. SLS Workshop, Brunnen, Switzerland, 26.-30.10.99. F. Hegedus, P. Wobrauschek, Ch. Streli, D. Wegrzynek, R. Abela, P. Boni, J. van Aarle and M. Victoria SYNCHROTRON RADIATION INDUCED TOTAL REFLECTION X-RAY FLUORESCENCE ANALYSIS (SR-TXRF) OF TRACE ELEMENTS IN PURE METALS AND IN ALLOYS EU workshop on "Research with Synchrotron Radiation at Hasylab", Hamburg, Germany, 28.1.99. T. Herrmannsdorfer, A. Donni, P. Fischer, L. Keller, G. Bb'ttger, A. Furrer, M. Gutmann, H. Kitazawa and J. Tang SUCCESSIVE MAGNETIC ORDERING OF THE Tb SUBLATTICES IN Tb3Pd20Si6 Jahrestagung der Schweizerischen Physikalischen Gesellschaft, Bern, Switzerland, 25.-26.2.99. 98 T. Herrmannsdorfer, A. Donni, P. Fischer, L. Keller, G. Bottger, M. Gutmann, H. Kitazawaand and J. Tang SUCCESSIVE MAGNETIC ORDERING OF THE TB SUBLATTICES IN Tb3Pd20Si6 4th International Symposium on Advanced Physical Fields, NRIM Tsukuba, Japan, 3.99. T. Herrmannsdorfer, A. Donni, P. Fischer, L. Keller and H. Kitazawa TWO SUCCESSIVE MAGNETIC PHASE TRANSITIONS IN Dy3Pd20Si6 STUDIED BY NEUTRON POWDER DIFFRACTION Conference on Strongly Correlated Electron Systems, Nagano, Japan, 24.-28.8.99. T. Herrmannsdorfer, P. Fischer, T. Bonelli, L. Keller, A. Furrer, A. Donni, H. Kitazawa, M. Giovannini and E. Bauer MAGNETIC NEUTRON SCATTERING FROM THE STRONGLY CORRELATED ELECTRON SYSTEMS R3Pd20Si6 AND R2Pd2ln (R = RARE-EARTH) Swiss Workshop on Superconductivity and Materials with Novel Electronic Properties, Les Diablerets, Switzerland, 27.-29.9.99. T. Herrmannsdorfer, P. Fischer, G. Bottger, L. Keller, M. Giovannini and E. Bauer MAGNETIC ORDERING IN THE RARE-EARTH INTERMETALLIC COMPOUNDS Tb2Pd2ln AND Ho2Pd2ln 2nd European Conference on Neutron Scattering, Budapest, Hungary, 1 .-4.9.99. S.Janssen FOCUS: TIME-OF-FLIGHT SPECTROMETER FOR COLD NEUTRONS AT SINQ Seminar am Physik-Department der TU Munchen, Germany, 26.4.99. S. Janssen NEUTRON TIME-OF-FLIGHT SPECTROSCOPY 7th Summer School on Neutron Scattering, Zuoz, Switzerland, 7.-13.8.99. S. Janssen RESULTS OF THE YOUNG SCIENTIST PANEL WORKING GROUP 'SOFT CONDENSED MATTER, POLYMERS, BIOLOGY1 2nd European Conference on Neutron Scattering, Budapest, Hungary, 1.-4.9.99. S. Janssen, F. Altorfer, L. Holitzner and R. Hempelmann FLUGZEITSPEKTROMETER FOCUS: EIN NEUES VERBUNDINSTRUMENT STEHT ZUR VERFUGUNG Deutsche Neutronenstreutagung, Potsdam, Germany, 25.-27.5.99. S. Janssen, F. Altorfer, L. Holitzner and R. Hempelmann FOCUS: TOF-SPECTROMETER AT SINQ, FIRST RESULTS 7th Summer School on Neutron Scattering, Zuoz, Switzerland, 7.-13.8.99. S. Janssen, F. Altorfer, L. Holitzner and R. Hempelmann TIME-OF-FLIGHT SPECTROMETER FOCUS AT SINQ: FIRST RESULTS 2nd European Conference on Neutron Scattering, Budapest, Hungary, 1 .-4.9.99. S. Janssen, D. Rubio-Temprano and A. Furrer TIME AND MONOCHROMATIC FOCUSSING APPLIED IN TIME-OF-FLIGHT SPECTROSCOPY Workshop "Neutron Optics for the Next Millennium", Villigen, Switzerland, 25.-27.11.99. L. Keller, A. Donni and H. Kitazawa MAGNETISM IN PrPdAI: A NEUTRON DIFFRACTION STUDY 2nd European Conference on Neutron Scattering, Budapest, Hungary, 1.-4.9.99. A. Leineweber, M. W. Friedriszik, H. Jacobs, R. Essmann, G. Bottger, F. Fauth and P. Fischer DIAMMINMETALL(II)-HALOGENIDE M(NH3)2X2 (M = Mg, Mn, Fe, Co, Ni; X = Cl, Br, I) 7. Jahres-Tagung der Deutschen Gesellschaft fur Kristallographie, Leipzig, Germany, 8.-10.3.99. J. Mesot, J. C. Campuzano, H. M. Fretwell, A. Kaminski, M. R. Norman and M. Randeria HIGH RESOLUTION ARPES EXPERIMENTS ON BI2212 SUPERCONDUCTORS 2nd International Workshop on Synchrotron Radiations, Brunnen, Switzerland, 26.-30.10.99. 99 V. Pomjakushin, A. Balagurov, D. Sheptyakov, V. Aksenov, P. Fischer, M. Gutmann, L. Keller, et al. EFFECT OF OXYGEN SUBSTITUTION ON MAGNETIC STRUCTURE OF (La1.yPry)07Ca03MnO3, y = 0.75 International Union of Crystallography Congress, Glasgow, Scotland, 4.-13.8.99. V.Y. Pomjakushin, A.A. Zakharov, A.M. Balagurov, F.N. Gygax, A. Schenck, A. Amato, D. Herlach, et al. PHASE SEPARATION IN La2Cu04tx SINGLE CRYSTALS 8th International Conference on Muon Spin Rotation, Relaxation and Resonance, Les Diablerets, Switzerland, 30.8.-3.9.99. E.V. Raspopina, A.M. Balagurov, V.Yu. Pomjakushin, V.V. Sikolenko, A.V. Gribanov, A. Amato and A. Schenck MAGNETIC STRUCTURE OF U(Pd, Fe )2Ge2 STUDIED BY |xSR: COMPARISON WITH NEUTRON DIFFRACTION DATA 8th International Conference on Muon Spin Rotation, Relaxation and Resonance, Les Diablerets, Switzerland, 30.8.-3.9.99. D. Rubio Temprano, A. Furrer, K. Conder and H. Mutka LARGE ISOTOPE EFFECT ON THE PSEUDOGAP IN THE HIGH-TC SUPERCONDUCTOR HoBa2Cu408 22nd Rare Earth Research Conference, Chicago, USA., 10.-15.7.99. D. Rubio Temprano, A. Furrer, K. Conder and H. Mutka LARGE ISOTOPE EFFECT ON THE PSEUDOGAP IN THE HIGH-TC SUPERCONDUCTOR HoBa2Cu4O8 7'" Summer School on Neutron Scattering, Zuoz, Switzerland, 7.-13.8.99. D. Rubio Temprano, A. Furrer, H. Mutka and K. Conder PSEUDOGAP IN THE UNDERDOPED HIGH-TC COMPOUND HoBa2Cu4O8: A NEUTRON-CRYSTAL FIELD STUDY OF THE ISOTOPE EFFECT Annual Meeting of the Swiss Physical Society, Bern, Switzerland, 25.-26.2.99. D. Rubio Temprano, J. Mesot, A. Furrer, K. Conder and H. Mutka LARGE ISOTOPE EFFECT ON THE PSEUDOGAP IN THE HIGH-TC SUPERCONDUCTOR HoBa2Cu408 2nd European Conference on Neutron Scattering, Budapest, Hungary, 1 .-4.9.99. D. Rubio Temprano, J. Mesot, A. Furrer, K. Conder and H. Mutka LARGE ISOTOPE EFFECT ON THE PSEUDOGAP IN THE HIGH-TC SUPERCONDUCTOR HoBa2Cu4O8 Swiss Workshop in Superconductivity and Materials with Novel Electronic Properties, Les Diablerets, Switzerland, 27.-29.9.99. D. Schaniel, P. Allenspach, A. Furrer, K. Kramer and H.U. Gudel CLUSTER EXCITATIONS IN Cs3Er2X9 (X=CI,Br) Annual Meeting of the Swiss Physical Society, Bern, Switzerland, 25.-26.2.99. D. Schaniel, P. Allenspach, A. Furrer, K. Kramer and H.U. Gudel 3+ DIMER SPLITTING OF Er IN Cs3Er2X9 (X=CI,Br): MYSTERY SOLVED 2nd European Conference on Neutron Scattering, Budapest, Hungary, 1 .-4.9.99. J. Schefer NEUTRON DIFFRACTION 7'" Summer School on Neutron Scattering, Zuoz, Switzerland, 7.-13.8.99. J. Schefer, M. Bohm, L. Keller, M. Medarde, M. Horisberger, P. Fischer, V. Petrakovskii and A. Do'nni APPLICATION OF COMPOSITE GERMANIUM NEUTRON MONOCHRO-MATORS AT SINQ: NEUTRON POWDER DIFFRACTION (HRPT) AND SINGLE CRYSTAL DIFFRACTION (TRICS) Workshop "Neutron Optics in the next Millennium, Villigen, Switzerland, 25.-27.11.99. J.Schefer and P. Fischer FOCUSING GERMANIUM MONOCHROMATORS - RESULTS FOR THE TRICS AND HRPT DIFFRACTOMETERS AT SINQ Workshop "Neutron Optics for the next Millennium", Villigen, Switzerland, 25.-27.11.99. 100 J. Schefer, Th. Woike and M. Imlau LIGHT STORAGE MATERIAL Sr [Fe(CN)5NO]4H2O XVIIIth Congress of the International Union of Crystallography, Glasgow, Scotland, 4.-13.8.99. J. Schefer, M. Konnecke, A. Murasik, A. Czopnik, Th. Strassle, P. Keller and N. Schlumpf SINGLE CRYSTAL DIFFRACTION INSTRUMENT TriCS AT SINQ 2nd European Conference on Neutron Scattering, Budapest, Hungary, 1 .-4.9.99. F. Semadeni, J. Kohlbrecher and P. Boni SPIN DYNAMICS IN Ni WITH SANS: AN INDIRECT METHOD FOR INVESTIGATING DIPOLAR EFFECTS 7th Summer School on Neutron Scattering, Zuoz, Switzerland, 7.-13.8.99. F. Semadeni, J. Kohlbrecher and P. Boni SPIN DYNAMICS IN NI WITH SANS: DIPOLAR EFFECTS AND PARALLEL FLUCTUATIONS 2nd European Conference on Neutron Scattering, Budapest, Hungary, 1.-4.9.99. F. Semadeni, B. Roessli, A. Amato and G. Petrakovskii Magnetic Phase Diagram of Cu(Fe2xGaJO4 uSR Meeting, PSI Villigen, Switzerland, 1.-2.12.99. F. Semadeni, P. Vorderwisch, T. Chatterji, B. Roessli and P. Boni SPIN WAVE EXCITATIONS IN THE WEAK ITINERANT FERROMAGNET NLAL Deutsche Neutronenstreutagung 99, Potsdam, Germany, 25.-27.5.99. M. Senthil Kumar, P. Boni and D. Clemens INTERFACE ROUGHNESS IN Ni/Ti MULTILAYERS AS PROBED BY NEUTRONS 2nd European Conference on Neutron Scattering, Budapest, Hungary, 1.- 4.9.99. M. Senthil Kumar, P. Boni and M. Horisberger MAGNETIC PROPERTIES OF Fe^Co^V/TiN, MULTILAYERS AND ITS USE IN THE POLARISATION OF NEUTRONS International Magnetics Conference (INTERMAG 99), Kyongju, South Korea, 18.- 21.5.99. D.V. Sheptyakov, A. M. Balagurov, V. Yu. Pomjakushin, V. L. Aksenov, N. A. Babushkina, L. M. Belova, O. Yu. Gorbenko, A. R. Kaul, P. Fischer, M. Gutmann and L. Keller PECULARITIES OF ATOMIC AND MAGNETIC STRUCTURES OF (LalyPry)07Ca03MnO3 (0.50 • y • 0.75) UCr Congress, Glasgow, U.K., 4.-13.8.99. Th. Strassle KUHLUNG DURCH ADIABATISCHEN DRUCK Conference of the PSI Department F3A, Villigen, Switzerland, 18.1.99. Th. Strassle and A.Furrer COOLING BY ADIABATIC (DE)PRESSURIZATION -THE BAROCALORIC EFFECT 2nd International Seminar on Neutron Scattering at High Pressures, Dubna, Russia, 29.9.-1.10.99. Th. Strassle, A.Furrer, Ph. Lacorre and KAMiiller A NOVEL PRINCIPLE FOR COOLING BY ADIABATIC PRESSURE APPLICATION IN RARE-EARTH COMPOUNDS 22nd Rare Earth Research Conference, Argonne, U.S.A, 10.-16.7.99. Th. Strassle, A.Furrer and K.A. Muller COOLING BY ADIABATIC APPLICATION OF PRESSURE-THE BAROCALORIC EFFECT 2nd European Conference on Neutron Scattering, Budapest, Hungary, 1.-4.9.99. S. Wehrli and D. Clemens ABSORPTION LAYERS FOR SUPERMIRROR POLARIZERS Workshop "Neutron Optics in the next Millennium", Villigen, Switzerland, 25.-27.11.99. 101 Th. Woike, M. Imlau, V. Angelov, J. Schefer, B. Delley, M. Fechtelkord, U. Haeberlen, K. Schwarz and P. Blaha ELEKTRISCHE FELDGRADIENTEN IM GRUNDZUSTAND UND ELEKTRONISCH ANGEREGTEN ZUSTAND DESNa2[Fe(CN5)NO]2H2O 7. Jahrestagung der Deutschen Gesellschaft für Kristallographie, Leipzig, Germany, 8.-10.3.1999. O. Zaharko, H. Grimmer, H.Ch. Mertins and F. Schäfers MAGNETIC CIRCULAR DICHROISM IN HoCox AMORPHOUS FILMS XMCD workshop, ESRF Users' Meeting, Grenoble, France, 11.-13.2.99. O. Zaharko, H. Grimmer, H.Ch. Mertins and F. Schäfers SOFT X-RAY MAGNETIC CIRCULAR DICHROISM IN THICK' M/TI MULTILAYERS (M=Fe050Coo48V0o2) MEASURED IN TRANSMISSION SR workshop, satellite to the XVIII-th lUCr Congress, Daresbury, U.K, 1 .-4.8.99. O. Zaharko, H. Grimmer, H.Ch. Mertins and F. Schäfers SOFT X-RAY MAGNETIC DICHROISM IN R,JX AMORPHOUS FILMS (R=RARE-EARTH, M=FE, CO) XVIII-th lUCr Congress, Glasgow, U.K, 4.-13.8.99. O. Zaharko, H. Grimmer, H.Ch. Mertins and F. Schäfers SOFT X-RAY MAGNETIC DICHROISM IN R1o O. Zaharko, H. Grimmer, H.Ch. Mertins, F. Schäfers, D. Alliata and R. Kötz SOFT X-RAY MAGNETIC DICHROISM IN M/TI MULTILAYERS (M= Fe05oCOo48V0o2) MEASURED IN TRANSMISSION SLS Workshop, Brunnen, Switzerland, 26.-30.10.99. M.Zolliker: SAMPLE ENVIRONMENT AT SINQ 1st Workshop on New Techniques and Developments in the Field of Sample Environment at Neutron Research Facilities, HMI Berlin, Germany, 8.-10.4.99. CONDENSED MATTER THEORY A. Bill, H. Morawitz and V.Z. Kresin THE EFFECT OF LAYER-PLASMONLAYER-PLASMO S ON THE THE SUPERCONDUCTING Tc Centenial Meeting of the American Physical Society, Atlanta, USA, March 1999. A. Bill, H. Morawitz and V.Z. Kresin COLLECTIVE MODES AND SUPERCONDUCTIVITY IN LAYERED STRUCTURES International conference on Physics and Chemistry of Molecular and Oxide Superconductors (MOS99), Stockholm, Sweden, July 1999. A. Bill, H. Morawitz and V.Z. Kresin PLASMONS IN LAYERED SUPERCONDUCTORS 22nd International conference on Low Temperature Physics (LT22), Helsinki, Finland, August 1999. A. Bill, H. Morawitz and V.Z. Kresin COLLECTIVE MODES AND SUPERCONDUCTIVITY IN LAYERED STRUCTURES 1999 Swiss Workshop on Superconductivity and Materials with Novel Electronic Properties (SWS99), Les Diablerets, Switzerland, September 1999. H.B. Braun SOLITON-ANTISOLITON NUCLEATION IN MAGNETIC NANOWIRES International Symposium on Heterogeneous Systems, Gerhard Mercator Universität at Duisburg, Germany, February 1999. 102 H.B. Braun TOPOLOGICAL EXCITATIONS, QUANTUM SPIN PHASES AND CHIRALITY IN LOW DIMENSIONAL MAGNETISM Gerhard Mercator Universitat at Duisburg, Germany, May 1999. B.Delley PROPERTIES OF CONDENSED MATTER STUDIED BY DENSITY FUNCTIONAL THEORY Group Seminar p.SR Group at PSI, , Villigen PSI, Switzerland, 18.1.99. B.Delley DMOL DFT STUDIES: FROM MOLECULES AND MOLECULAR ENVIRONMENTS TO SURFACES AND SOLIDS invited talk at: EMRS'99 SPRING MEETING, Strasbourg, France, 1.-4.6.99 . B.Delley DMOL DFT STUDIES: FROM MOLECULES AND MOLECULAR ENVIRONMENTS TO SURFACES AND SOLIDS Instituts Seminar Fritz-Haber-lnstitut der Max-Plank-Gesellschaft, Berlin, Germany, 8.7.1999. B.Delley DMOL DFT STUDIES: FROM MOLECULES AND MOLECULAR ENVIRONMENTS TO SURFACES AND SOLIDS seminar Northwestern University, Evanston Illinois, USA, 1.12.99. B.Delley MULTIPLE K-POINTS THEORY AND ITS IMPLEMENTATION Plenary talk at Catalysis Consortium Meeting, MSI, San Diego, USA. 6-10.12.99. R.H. Morf KOLLOQUIUMSVORTRAG: 'FRACTIONAL QUANTUM HALL EFFECT: BASICS AND RECENT DEVELOPMENTS' Institut de Physique Theorique, Universite Fribourg, Switzerland, 24.3.99. R.H. Morf SEMINAR: "NEW LIGHT ON THE ENIGMATIC QUANTUM HALL STATE AT FILLING FRACTION 5/2' Institut de Physique Theorique, EPFL Lausanne, Switzerland, 11.5.99. R.H. Morf POSTER: 'NEW LIGHT ON THE ENIGMATIC NU=5/2 QUANTUM HALL PLATEAU' Workshop on 'Physics and Chemistry of Novel Materials: Strongly Correlated Electron Systems' Monte Verita, Ascona, Switzerland, 10.6.99. R.H. Morf SEMINAR: 'DER GEBROCHENZAHLIGE QUANTEN-HALLEFFEKT: GRUNDLAGEN UND NEUES ZUM RATSELHAFTEN NU=5/2 PLATEAU' Institut fur theoretische Physik, Universitat Augsburg, Germany 29.6.99. R.H. Morf VORTRAG 'NEW LIGHT ON THE ENIGMATIC NU=5/2 QUANTUM HALL STATE1 Workshop on 'Mesoscopic Physics and Transport Phenomena' Monte Verita, Ascona, Switzerland, 23.7.99 R.H. Morf KOLLOQUIUMSVORTRAG: 'DER GEBROCHENZAHLIGE QUANTEN-HALLEFFEKT: GRUNDLAGEN UND NEUERE ENTWICKLUNGEN' Universitat Wuppertal, Germany, 9.11.99. 103 LOW TEMPERATURE FACILITIES B. van den Brandt, E.I. Bunyatova, P. Hautle, J.A. Konter, S. Mango SIZEABLE NUCLEAR POLARIZATION IN A FULLY DEUTERATED SCINTILLATOR International Workshop on Polarized Sources and Targets 1999, Erlangen, Germany, 29.9. - 2.10.1999 B. van den Brandt, E.I. Bunyatova, P. Hautle, J.A. Konter, S. Mango OPERATION OF A SCINTILLATING PROTON TARGET International Workshop on Polarized Sources and Targets 1999, Erlangen, Germany, 29.9. - 2.10.1999 B. van den Brandt, E.I. Bunyatova, P. Hautle, J.A. Konter, S. Mango POLARIZED PROTONS IN SOLID BLOCKS OF POLYETHYLENE International Workshop on Polarized Sources and Targets 1999, Erlangen, Germany, 29.9. - 2.10.1999 J. Olsen, E.C. Kirk, K. Thomson, B. van den Brandt, Ph. Lerch, L. Scandella, A. Zehnder, S. Mango, H.R. Ott, M. Huber, G.C. Hilton, J. M. Martinis FIRST STEPS TOWARDS SMALL ARRAYS OF MO/AU MICROCALORIMETERS 8th Internat. Workshop on Low Temp. Detectors, 1999, Dalfsen, the Netherlands, 15.8 -20.8.1999 104 FUN SEMINARS AT PSI 11.01.99 Dr. Lixin Fan, Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, Dubna, Russia SOME RESULTS IN SMALL ANGLE NEUTRON SCATTERING 17.02.99 Prof. Keshav N. Shrivastava, School of Physics, Univ. of Hyderabad, PO Central University, Hyderabad, India SOFT VORTICES IN BOROCARBIDE SUPERCONDUCTORS 09.03.99 Dr. B. van den Brandt, Paul Scherrer Institut, Villigen PSI FUN WITH LOW TEMPERATURE FACILITIES 20.04.99 Dr. S. Rosenkranz, Materials Science Division, Argonne National Laboratory, Argonne, U.S.A. SPIN, CHARGE AND LATTICE CORRELATION IN THE NATURALLY LAYERED MANGANITES 27.04.99 PD Dr. Jurgen Rohler, Universitat zu Koln, Koln, Germany LOKALISIERUNG DER SAUERSTOFFLOCHER IN UBERDOTIERTEM Y1.yCayBa2Cu30x 03.05.99 B.Braun, Paul Scherrer Institut, 5232 Villigen PSI UBER DEN EINFLUSS EPITAKTISCHER VERSPANNUNGEN AUF DIE MAGNETISCHE ANISOTROPIE DUNNER METALLFILME 04.05.99 Dr. W. Reichardt, Institut fur Nukleare Festkorperphysik, Forschungszentrum Karlsruhe, Germany STUDIEN ZUR MESSUNG VON PHONONENSPEKTREN MIT NEUTRONEN- FLUGZEIT-SPEKTROSKOPIE 12.05.99 Dr. E. Clementyev, Paul Scherrer Institut, 5232 Villigen PSI MAGNETIC EXCITATIONS AND LATTICE DYNAMICS IN INTERMEDIATE-VALENCE CeNi 04.06.99 Prof. L.C. Gupta, Tata Institute of Fundamental Research, Bombay, India MAGNETIC ORDERING IN R2Ni3Si5 (R=RARE EARTHS) INTERMETALLICS - MATERIALS WITH ANOMALOUSLY LARGE MAGNETORESISTANCE (• 90%) 14.06.99 Dr. A. Zheludev, Brookhaven National Laboratory, Upton, NY, U.S.A. EXOTIC MAGNETIC INTERACTIONS IN Ba2CuGe207 17.06.99 Dr. M.R. Fitzsimmons, Los Alamos National Laboratory, Los Alamos, U.S.A. NON-PERPENDICULAR COUPLING IN EXCHANGE-BIASED FeF2 BILAYERS 22.06.99 Dr. M. Divis, Charles University, Department of Electron Systems, Prague, Czech Republic FIRST PRINCIPLE CALCULATIONS OF THE CRYSTAL FIELD INTERACTION IN RARE EARTH COMPOUNDS 06.07.99 Prof. E. Kaldis, Laboratorium fur Festkorperphysik, ETH Zurich THE T-x PHASE DIAGRAMS OF YBa2Cu30x AND Y1xCaxCu307 IN MACROSCOPIC AND MESOSCOPIC SCALES 30.07.99 PD Dr. Theo Woike, Kristallographisches Institut der Universitat Koln, Germany HOLOGRAPHISCHE DATENSPEICHERUNG 27.08.99 Dr. Michael James, Neutron Scattering Group, ANSTO Lukas Heights, Menai, NSW 2234, Australia NEUTRON SCATTERING DOWN-UNDER - THE RECENT PAST, THE PRESENT AND VERY NEAR FUTURE OF NEUTRON SCIENCE AT ANSTO 105 06.09.99 Prof. G. Petrakovskii, L.V. Kirenskii Institute of Physics SB RAS, Krasnoyarsk, Russia MAGNETIC AND RESONANCE PROPERTIES OF SINGLE CRYSTALS CuB2O4 AND Cu3B2O6 24.09.99 Dr. Myles Dean, EMBL Grenoble, Grenoble, France LADI- A NEW FACILITY FOR NEUTRON LAUE CRYSTALLOGRAPHY 20.10.99 Claus Urban, Departement de Physique, Universite Fribourg, Fribourg LIGHT SCATTERING IN TURBID COLLOIDAL SYSTEMS 21.10.99 Dr. J.-M. Mignod, Laboratoire Leon Brillouin, CEA-Saclay, Gif-sur-Yvette, France NEUTRON-DIFFRACTION STUDIES OF QUADRUPOLAR AND MAGNETIC ORDER IN Tm MONOCHALCOGENIDES 02.11.99 Dr. G.P. Felcher, Argonne National Laboratory, Argonne IL, U.S.A. HOW TO SOLVE THE MYSTERY OF EXCHANGE ANISOTROPY 08.11.99 Dr. N. Metoki. Japan Atomic Energy Research Institute.Tokai, Naka, Ibaraki, Japan NEUTRON SCATTERING STUDIES ON HEAVY FERMION SUPERCONDUCTORS 23.11.99 Dr. H. Lauter, Institut Laue-Langevin, France SPECULAR AND OFF-SPECULAR NEUTRON SCATTERING FROM MULTILAYERS AND SUPERCONDUCTING FILMS 06.12.99 Dr. D. Sheptyakov, Frank Laboratory of Neutron Physics, Dubna, Russia PRESSURED-INDUCED STRUCTURAL TRANSITION IN SAMARIUM HEXABORIDE 14.12.99 Ch. Beck, Universitat des Saarlandes und Paul Scherrer Institut, Villigen LIGHT AND X-RAY SCATTERING STUDIES ON SURFACE FUNCTIONALISED NANOPARTICLES 106 LECTURES AND COURSES NEUTRON SCATTERING Prof. Dr. A. Furrer ETH Zurich, SS 1999: • Neutronenstreuung in der Festkorperphysik II • Seminar uber Neutronenstreuung • Praktikum in Neutronenstreuung ETH Zurich, WS 1999/2000: • Neutronenstreuung in der Festkorperphysik I • Seminar uber Neutronenstreuung • Praktikum in Neutronenstreuung 107 MEMBERS OF SCIENTIFIC COMMITTEES NEUTRON SCATTERING Dr. P. Allenspach • Wissenschaftlicher Ausschuss der SINQ: Sekretar (seit 1995) • European Neutron Scattering Association (ENSA): Assistant Secretary (seit 1997) • Subcommittee "Structural and Magnetic Excitations" of the Scientific Council, Institute Laue-Langevin, Grenoble (seit 1998) • International Scientific Advisory Committee, Budapest Neutron Centre, Hungary (seit 1998) • Round-Table on Neutron Sources, EC "Large Scale Facilities" Programme (seit 1999) Dr. P. Boni • Vorstand der Schweizerischen Gesellschaft fur Neutronenstreuung: Sekretar (seit 1991) • Projektkomitee "Neue Technologien fur polarisierte Neutronen", Verbundforschung des Bundesministeriums fur Bildung und Forschung, BRD (1994-1999) • Nutzerausschuss des Berliner Neutronenstreucentrums, Hahn-Meitner-lnstitut, Berlin, BRD (1995-1999) • ENSA Working Group "Neutron Optics": Convenor (seit 1997) • Neutron Optics for the Next Millennium (NOP99): Conference Chairman (1999) • Polarized Neutrons for Condensed Matter Investigations (PNCMI-2000): Scientific Advisory Committee (seit 1999) • Optical Component Advisory Committee, Spallation Neutron Source, USA (seit 1999) • Instrument Subcommittee, Institut Laue-Langevin, Grenoble (seit 1999) • International Conference on Neutron Scattering ICNS 2001: International Advisory Committee (seit 1999) N. Cavadini • Working Group "Magnetic Excitations", Young Scientists Panel at ECNS'99 (1999) Dr. P. Fischer • Scientific Committee of HRFD Neutron Diffractometer, Frank Laboratory of Neutron Physics, Dubna, Russia (seit 1995) • Forschungskomitee |J.SR, Paul Scherrer Institut, Villigen (seit 1996) 108 Prof. Dr. A. Furrer • Internationaler Wissenschaftlicher Rat des Projektes "Spallationsneutronenquelle AUSTRON", Wien: Vorsitzender (seit 1993) • PSI Summer Schools on Neutron Scattering: Programme Chairman (seit 1993) • Projektkomitee "Räumliche und zeitliche Korrelationen in magnetischen Materialien", Verbundforschung des Bundesministeriums für Bildung und Forschung, BRD (1994-1999) • Projektkomitee "Lokalisierung leichter Atome und Bestimmung von magnetischen Ordnungszuständen in Strukturen neu synthetisierter Verbindungen aus dem Bereich der Festkörperchemie", Verbundforschung des Bundesministeriums für Bildung und Forschung, BRD (1994-1999) • Round-Table on Neutron Sources, EC "Large Scale Facilities" Programme (1996-1999) • Swiss Workshop on Superconductivity and Novel Metals: Steering Committee (seit 1996) • European Neutron Scattering Association (ENSA): Chairman (1997-1999) • IUPAP Committee on Neutron Sources, International Union of Physics and Applied Physics (seit 1998) • Expertenkomitee "Kollektive Quantenzustände in elektronisch eindimensionalen Übergangsmetallverbindungen", Deutsche Forschungsgemeinschaft, BRD (seit 1998) Expertenkomitee "Seltenerd-Übergangsmetallverbindungen", Deutsche Forschungsgemeinschaft, BRD (seit 1998) • 2nd European Conference on Neutron Scattering ECNS'99: Chairman of the Programme Committee (1998-1999) • Neutron School Les Houches (France), International Programme Committee (seit 1999) • Polarized Neutrons for Condensed Matter Investigations (PNCMI-2000): International Committee (seit 1999) • Conference on "Major Trends in Superconductivity in the New Millennium" (MTSC 2000): Program Committee (seit 1999) • International Conference on Neutron Scattering ICNS 2001 : International Programme Committee (seit 1999) • Wissenschaftlicher Beirat des Hahn-Meitner-instituts, Berlin (seit 1999) Prof. Dr. H. Grimmer • Schweizerische Gesellschaft für Kristallographie: Editor SGK Newsletter (seit 1997) • Schweizerische Gesellschaft für Kristallographie: Präsident (seit 1999) • Sekretär des Schweizer Landeskomitees für die International Union of Crystallography (seit 1999) • Delegierter der Sektion 1 der Schweizerischen Akademie der Naturwissenschaften (seit 1993) Dr. H. Heer ENSA Working Group "Software" (seit 1995) 109 Dr. S. Janssen ENSA Working Group "TOF devices" (seit 1995) • Working Group "Soft Condensed Matter, Polymers, Biology", Young Scientists Panel at ECNS'99: Convenor (1999) Dr. V. Pomjakushin • 8"1 International Conference on Muon Spin Rotation, Relaxation and Resonance, Les Diabierets: International Advisory Committee (1999) Dr. J. Schefer • ENSA Working Group "Monochromators" (seit 1995) 110 HIGHER DEGREES AWARDED NEUTRON SCATTERING Urs Gasser Title of Thesis: Magnetic properties of the rare earth borocarbides RNi2B2C- A neutron scattering study ETH Zurich No. 13380 Thesis Advisors: Dr. P. Allenspach (PSI) Prof. Dr. A. Furrer (ETH Zurich & PSI) Prof. Dr. P. Wachter (ETH Zurich) AWARDS RECEIVED NEUTRON SCATTERING Nordal Cavadini Young Scientists Award, 2nd European Conference on Neutron Scattering, 1-4 September 1999, Budapest Albert Furrer Medal of the Frank Laboratory of Neutron Physics, Dubna, Russia Daniel Rubio Premio Extraordinario Licenciado en Ciencias Fisicas, University of Santander, Spain Daniel Rubio Young Scientists Award, 2nd European Conference on Neutron Scattering, 1-4 September 1999, Budapest Thierry Strassle ETHZ-Diploma with distinction 111 GUESTS NEUTRON SCATTERING - Adams M.A., ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, U.K. - Anderson K., ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, U.K. - Balagurov A., Joint Institute for Nuclear Research, Dubna, Russia - Divis M., Charles University, Department of Electron Systems, Prag, Czech - Fan Lixin, Joint Institute for Nuclear Research, Dubna, Russia - Fitzsimmons M.R., Los Alamos National Laboratory, Los Alamos, U.S.A. - Golosovsky I.V., Petersburg Nuclear Physics Institute, Gatchina, Russia - Gupta L.C., Tata Institute of Fundamental Research, Bombay, India - Haussuhl E., Laboratorium fur Kristallographie, ETH Zurich, Zurich, Switzerland - James M., ANSTO, Lukas Heights, Australia - Jericha E., Atominstitut der osterreichischen Universitaten, Wien, Austria - Kozhevnikov, Joint Institute for Nuclear Research, Dubna, Russia - Lauter H.Jnstitut Laue-Langevin, Grenoble, France - Lazukov V., Institute Superconductivity & Solid State Physics, RRC "Kurchatov Instiute", Moscow, Russia - Medarde Marisa, Materials Science Division, Argonne National Laboratory, Argonne, U.S.A. - Meier G., Max-Planck-lnstitut fur Polymerforschung, Julich, Germany - Metoki Naoto, Japan Atomic Energy Research Institute, Tokai, Naka, Ibaraki, Japan - Mignot J.M., Laboratoire Leon Brillouin, Saclay, France - Nadal H., CERCA, Romans, France - Reichardt W., Forschungszentrum Karlsruhe, Karlsruhe, Germany - Rohler J., Universitat Koln, Koln, Germany - Rosenkranz S., Argonne National Laboratory, Argonne, USA - Sheptyakov D., Frank Laboratory of Neutron Physics, Dubna, Russia - Shrivastava K.N., School of Physics, University of Hyderabad, Hyderabad, India - Urban K., Departement de Physique, Universite Fribourg, Fribourg, Switzerland - Woike, Th., Crystallography, University at Cologne, Germany LOW TEMPERATURE FACILITIES - Bounyatova E., Dubna, Russia - Stuhrmann H., IBS, Grenoble, France 112 SINQ USER STATISTICS 1999 Czech Poland Australia Republic Japan 3o/o 2% 1% 3% Ukraine PSI Villigen France 14% 5% Austria 7% Germany 8% Switzerland Russia 27% 8% United Kingdom USA 11% 10% SINQ SCIENTIFIC COMMITTEE Prof. B. Dorner, ILL Grenoble Prof. H.U. Gudel, Universitat Bern Prof. R. Hempelmann, Universitat Saarbrucken Prof. G. Kostorz, ETH Zurich Prof. H. Rauch, Atominstitut Wien Prof. P. Schurtenberger, ETH Zurich Prof. D. Schwarzenbach, Universite de Lausanne Prof. W. Steurer, Universitat Zurich & ETH Zurich Prof. H. Stuhrmann, Forschungszentrum Geesthacht Prof. K. Yvon, Universite de Geneve 113 STAFF FUN DEPARTMENT Phone Head of Department: Fischer Walter 3412 Secretary: Bercher Renate 3402 LABORATORY FOR NEUTRON SCATTERING Head of Laboratory: Furrer Albert 2088 Secretaries: Braun-Shea Margit (since May 1999) 2087 Bucher Claudia (until February 1999) 2087 Castellazzi Denise 2087 Neutron Scattering: Adams Mark (since November 1999) 3176 Allenspach Peter 2527 Altorfer Felix 2086 Beck Christian (Univ. Saarbrucken, since November 1999) 4612 Boni Peter 2518 Braun Artur (until September 1999) Clemens Daniel 2925 Clementyev Evgeni (since March 1999) 4611 Fischer Peter 2094 Janssen Stefan 2875 Keller Lukas 4007 Manickam Senthil Kumar (until June 1999) 3176 Mesot Joel (since October 1999) 4029 Pomjakushin Vladimir (since February 1999) 3070 Roessli Bertrand 4192 Schefer Jurg 4347 Zolliker Markus 2089 X-ray Scattering Grimmer Hans 2421 Zaharko Oksana 4633 Computing: Heer Heinz 2093 Konnecke Mark 2512 Materials Synthesis: Conder Kazimierz (since June 1999) 2435 Technical Sections: Fischer Stephan 4118 Frey Gerhard (since February 1999) 4311 Halter Thomas (since May 1999) 3018 Holitzner Lothar (Univ. Saarbrucken) 2526 Horisberger Michael 2997 Isacson Anders 4023 Keller Peter 2052 Koch Max 2465 Schneider Roger 3726 Thut Rudolf 2465 114 Phone Ph.D. Students: Bohm Martin (at ILL Grenoble, since August 1999) Cavadini Nordal 4668 Detemple Ingrid (Univ. Saarbrucken, until June 1999) Gasser Urs (until October 1999) Gutmann Matthias (until January 1999) Herrmannsdorfer Thilo 4374 Rubio Daniel 4686 Schaniel Dominik (since May 1999) 4192 Semadeni Fabrizio 2091 Strassle Thierry (since April 1999) 2092 Diploma Students: Heigold Georg (since October 1999) Schaniel Dominik (until February 1999) 4192 Strassle Thierry (until February 1999) 2092 3179 Trainees: Bertschinger Rolf (February-March 1999) Burgy Raffael (March 1999) Heigold Georg (March 1999) Schaller Fredy (until June 1999) Trottmann Andreas (July-August 1999) Wehrli Samuel (July-October) Back: M. Adams, M. Konnecke, J. Mesot, P. Boni, F. Altorfer, R. Schneider, B. Roessli, L Keller, Th. Halter, G. Frey, Th. Strassle 3" row: V. Pomjakushin, G. Heigold, S. Janssen, O. Zaharko, N. Cavadini, D. Rubio Temprano, P. Allenspach, St. Fischer, P. Keller, M. Bohm, Th. Herrmannsdbrfer 2nd row: E. Clementyev, Ch. Beck, D. Clemens, R. Thut, M. Horisberger, H. Grimmer, M. Zolliker, L. Holitzner Front: A. Isacson, J. Schefer, H. Heer, A. Furrer, D. Castellazzi, M. Koch, K. Conder, P. Fischer 115 LOW TEMPERATURE FACILITIES GROUP Phone Group Leader: Mango Salvatore 3245 Arrigoni Willi 4042 Baines Christopher 4211 Bonier Josef 3238 Hautle Patrick 3210 Konter Ton 8383 Kwasnitza Kurt 3593 Van den Brandt Bernardus 4027 THEORY GROUP Group Leader: Morf Rudolf 4459 Bill Andreas (since 1.4.1999) 4515 Braun Hans-Benjamin 4515 Delley Bernard 3665 Mudry Christopher (since 1.9.1999) 4247 PAUL SCHERRER INSTITUT Paul Scherrer Institut Phone +41 56 310 21 11 CH-5232VilligenPSI Fax +4156 310 2199 Internet http://www.psi.ch