europhysics BULLETIN OF THE EUROPEAN PHYSICAL SOCIETY news J. A. Volume 6 Number 12 December 1975

Europhysics News Condensation of Excitons in 1976 into Electron-Hole Drops Europhysics News in 1976 will be a little bigger and we hope a little M. Voos, Paris better. Joint winner with V. S. Bagaev, L. V. Keldysh and Three Meetings issues will be pu­ J. E. Pokrovsky of the 1975 EPS Hewlett-Packard blished instead of two — in March, prize. June and November. Moreover the June issue will be enlarged to in­ When a semiconductor is excited, semiconductors was the occurrence clude directory style information on for example with photons whose at low temperature of new radiative the Society, its Divisions and Advi­ sory Committees. The irregularity of energy is larger than its band gap, it recombination lines observed first in the sequence is not accidental but is well known that free electrons and Ge by Benoît à La Guillaume and Pa- has been chosen to correspond holes (vacant electron states) are rodi2. As far as we know, it was only best with the inflow of information created in its conduction and valence in 1969 that these lines were studied and the concentration of meetings and reported again3. These new ra­ which occurs in certain seasons. bands respectively. At low concentra­ Seven “green” issues will be pu­ tions the lowest electronic excitation diation peaks, which occur in the blished each of 12 pages instead in usual intrinsic semiconductors is Infra-red (~ 1.75 µ), arise below about of the traditional eight and will ap­ the free exciton which is a two-par­ 10 K and grow rapidly when the tem­ pear towards the end of the months perature is reduced. They are broader of February (issue combining Ja­ ticle complex analogous to positro- nuary and February) April, May, Au­ nium and resulting from the Coulomb and lie at lower energy than the well- gust (combining July and August), binding of a free electron and hole. known emission lines due to the ra­ September, October and December. With increasing density the excitons diative recombination of free excitons. Altogether EN will be publishing They have been attributed to EHD by 112 pages of information in 1976 as interact and, if their mutual interaction against 100 in 1975. is attractive, new bound states may be Pokrovsky and Svistunova3 who gave Copies are distributed free of expected. The first one which was a very strong argument in favour of charge to Individual Ordinary Mem­ proposed is the excitonic molecule. this interpretation. Indeed, it Is easy bers who are asked to recognize to calculate the shape of these new that these are essentially personal This complex, which has perhaps copies carrying the copyright of the been observed in several semiconduc­ lines if one assumes that they are due EPS. Members can help the EPS by tors (CdS, CdSe...), results from the to the radiative decay of EHD, and encouraging their institutions to sub­ binding of two excitons. The se­ the agreement between experiment scribe to the publication for display cond one was Introduced by Keldysh1 and theory is very good. This yields in their library. Contrast the subs­ cription price of Fr. 75.— for a com­ in 1968 and is a kind of electron-hole also the equilibrium density no of elec­ plete volume of green and yellow “liquid” corresponding to the conden­ tron-hole pairs in a liquid drop which issues with the effort that has gone sation of excitons into the so-called is typically for Ge 2 X 1017 cm-3 at 2 K. into, for example, the compilation of electron-hole drops (hereafter called Conclusive evidence for the exis- the meetings issues. Seventy five francs is about the salary of one EHD). This condensed phase looks person for one morning per year. like a liquid metal in many respects Few libraries would refuse to put in and Is constituted by a neutral two- that effort to produce the complete, component plasma of free elec­ CONTENTS checked out list of meetings that trons and holes. The equilibrium den­ Condensation of Excitons into the EPS is now publishing. As another encouragement to ins­ sity of electron-hole pairs in this Electron-Hole Drops .... 1 titutions to use EN more, an addi­ plasma is perfectly well defined and Chemical Physics Section of the tional 25 % discount is to be the plasma energy is lower than that Atomic Physics Division . . . 2 allowed from January 1 on adver­ Astronomy and Astrophysics : a tisements for situations vacant. of excitons. The decrease of energy European Journal, its History Conscious of the increasing pro­ in the liquid phase comes from ex­ and Present Status .... 3 blems of employment in the world change and correlation effects bet­ High Energy and Particle Phy­ of science, EPS is making this ween particles. Such a phenomenon sics Conference...... 5 gesture to help the flow of infor­ can be viewed as a phase transition mation on jobs. 2nd International Conference on Members are asked to draw the between gaseous excitons and liquid Phonon Scattering in Solids . 6 attention of their institutions to electron-hole drops analogous to or­ 7th European Conference on these changes and not to be hesi­ dinary gas-liquid transformations. Controlled Fusion and Plasma tant in promoting the well-being of The first evidence for the existence Physics ...... 8 their society. of a new bound state in intrinsic

Europhysics News is published monthly by the European Physical Society. © 1975. Reproduction rights reserved.

1 tence of EHD was given by two expe­ investigations which have been done us note that this condensed phase is riments proving their macroscopic to understand this condensed phase. probably the simplest example of character, namely the photovoltaic However it may be interesting to note liquid metal which is available at the detection4 of EHD in p-n junctions and that, at least in Ge, many of the most present time. As such it provides a the scattering5 of 3.39µ light by EHD. important EHD parameters have been good opportunity to test theoretical In the first experiment, a Ge p-n measured. This is, for example, the methods of great interest in many junction photodiode is immersed in case of the following ones : density of body problems. Finally, it seems that pumped liquid helium at 2 K, and the electron-hole pairs in the liquid phase6 this field remains very attractive for n region is illuminated with a CW (no ~ 2 X 1017 cm-3), EHD binding many solid state physicists and is in mercury lamp for instance. The new energy6 with respect to gaseous exci- fact still developing. emission lines of Ge are easily seen tons (~ 2 meV), EHD lifetime6 to be present in these conditions of (40 µ sec), electric charge at the drop REFERENCES temperature and excitation. The fluc­ surface7, surface energy8 of drops 1. L.V. Keldysh, Proc. Ninth Intern. Conf. Semi­ tuations in the photovoltaic current (~ 2 X 10-4 ergs/cm2), critical tempe­ conductor Physics, Moscow (1968) p. 1303. 2. C. Benoît à La Guillaume and O. Parodi, J. are then analyzed with a rapid oscil­ rature9 (6.5 K). It should be pointed Electron. Control 6, 356 (1959). 3. Y.E. Pokrovsky and K.I. Svistunova, JETP loscope and are seen to have a spiky out that theory10 and experiment are Letters 9, 261 (1969) ; C. Benoît à La Guil­ structure similar to that of the current generally in very good agreement. laume, F. Salvan and M. Voos, Proc. Intern. Conf. Luminescence, Newark (1969) p. 315 ; from a photomultiplier. But at higher On the other hand, some interesting V.S. Bagaev, T.I. Galkina, O.V. Gogolin and L.V. Keldysh, JETP Letters 10, 195 (1969) ; temperature (~ 10 K), this structure aspects of EHD are not yet well known V.M. Asnin and A.A. Rogachev, Proc. Intern. is not observed. Each current spike or understood. Among others one can Conf. Photoconductivity, Stanford (1969) p. 13. 4. V.M. Asnin, A.A. Rogachev and N.I. Sablina, is interpreted as being due to the des­ quote the following ones : the ma­ JETP Letters 11, 99 (1970) ; C. Benoît à La Guillaume, M. Voos, F. Salvan, J.M. Laurant truction in the junction electric field gnetic properties of drops have not and A. Bonnot, Compt. Rend. 272B, 236 (1971); of an EHD containing N electron-hole yet been investigated ; it seems that T.K. Lo, B.J. Feldman and C.D. Jeffries, Phys. Rev. Letters 31, 224 (1973) ; J.C. Mc pairs. By integrating the current under EHD can move rather rapidly6, but this Groddy, M. Voos and O. Christensen, Sol. St. Commun. 13, 1801 (1973) ; J.M. Hvam and O. a spike, one easily gets N which is is not explained ; the nucleation pro­ Christensen, Sol. St. Commun. 15, 929 (1974). typically 107. This gives r ~ 2 µ for cess (or processes) involved in drop 5. Y.E. Pokrovsky and K.I. Svistunova, JETP Letters 13, 212 (1971) ; N.N. Sibeldin, V.S. the drop radius if we take no ~ 2 formation is still unclear and raises Bagaev, V.A. Tsvetkov and N.A. Penin, Sov. Phys. Sol. State 15, 121 (1973) ; M. Voos, K.L. x 1017 cm-3. Investigations at liquid very interesting and stimulating pro­ Shaklee and J.M. Worlock, Phys. Rev. Letters helium temperature of the scattering blems whose understanding might be 33, 1161 (1974) ; J.M. Worlock, T.C. Damen, K.L. Shaklee and J.P. Gordon, Phys. Rev. of 3.39 µ light by a crystal of pure Ge helpful in other areas. One could also Letters 33, 771 (1974). 6. See, for example, Y. Pokrovsky, Phys. St. excited optically by a lamp or a laser wonder whether this liquid phase may Solidi a 11, 385 (1972) ; M. Voos, Proc. Twelfth also gave good proof of the existence give rise, under conditions which are Intern. Conf. Semiconductor Physics, Stutt­ gart (1974) p. 33 ; M. Voos and C. Benoît à of EHD. These experiments show that to be defined, to superfluidity or su­ La Guillaume, in “Optical Properties of Solids, New Developments”, edited by B.O. electron-hole drops are certainly perconductivity. This may seem Seraphin, North Holland Publishing Cy. (1975); spherical and, from the angular dis­ highly speculative, but the answer to and references therein. 7. Y.E. Pokrovsky and K.I. Svistunova, JETP tribution of the scattered light, one this question is not clear. Letters 19, 92 (1974) 8. V.S. Bagaev, N.N. Sibeldin and V.A. Tsvetkov, gets their size which is of the order To conclude, the existence of elec­ JETP Letters 21, 180 (1975) ; B. Etienne, C. of some microns. tron-hole drops in semiconductors is Benoît à La Guillaume and M. Voos, Phys. Rev. Letters 35, 536 (1975) ; R.M. Westervelt, A lot of other data and arguments6 now well established. This liquid J.L. Staechli and E.E. Haller, Bull. Am. Phys. Soc. 20, 471 (1975). have been put forward to support the phase, which presents many interest­ 9. G.A. Thomas, T.M. Rice and J.C. Hensel, existence of electron-hole drops, and ing features, results from the conden­ Phys. Rev. Letters 33, 219 (1974). 10. The first theoretical studies on drops were it is generally admitted now that they sation of gaseous excitons, and this made by W.F. Brinkman, T.M. Rice, P.W. Anderson and S.T. Chui (Phys. Rev. Letters have been observed in Ge, Si and Ge- phenomenon corresponds to a first 28, 1961 (1972)), and by M. Combescot and Si alloys. More than 250 papers have order phase transition. Many aspects P. Nozières (J. Phys. C 5, 2369 (1972)). For a review of other theoretical works, see, for been published in this field during the of EHD are now understood, but a example, M. Voos and C. Benoît à La Guil­ laume in Ref. 6, and also T.M. Rice, Proc. last six years, so it is not possible to number of challenging experimental Twelfth Intern. Conf. Semiconductor Physics, give here an exhaustive list of all the and theoretical problems remain. Let Stuttgart (1974) p. 23.

The Chemical Physics Section of the Atomic Physics Division

The recently-constituted Chemical On this occasion the Section will The Provisional Board has decided Physics Section of the Atomic Physics hold its first General Meeting and the to couple each of its meetings with a Division will be holding its first confe­ Provisional Board has taken this scientific lecture. It is particularly ap­ rence from the 30th of August to the opportunity to announce the creation propriate that the first of these will 1st of September 1976 in Amsterdam. of the Section to the various Euro­ be given by Professor W. Kolos who, The theme is “Exploring the chemical pean Chemical Societies. Anyone together with Professor L. Jansen, bond. New developments”. interested in participating in the acti­ was largely responsible for the setting People interested in attending the vities of the Section should contact up of this Section. The title of the meeting should contact Dr. R. Block, either the Chairman of the Provisional talk, to be given in the Physical Che­ Congress Bureau (IBD) A-76, Vrije Uni- Board Professor E.A.C. Lucken, Dé­ mistry Department of the University versiteit, de Bœlelaan, 1105, Amster­ partement de Chimie Physique, Scien­ of Zurich on the 16th of January 76 dam, The Netherlands or the Confe­ ces II, 30, Quai E. Ansermet, 1211 will be “Ab Initio Calculations on the rence Secretary, Miss A.-M. Bonino, Geneva 4, Switzerland or the Secre­ Excited States of the H2 Molecule”. Batelle Institute, Advanced Studies tary, Professor G. Wagnière, Départe­ Center, 7, route de Drize, CH-1227 ment de Chimie Physique, Université E.A.C. Lucken Carouge-Geneva, Switzerland. de Zurich, Zurich, Switzerland. Geneva

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