Table of contents

The 11th General Conference of the Condensed Matter Division of the European Physical Society was held in Exeter from 8–11 April 1991. The annual Condensed Matter meeting of the UK Institute of Physics, which would have been held in December 1990, was not scheduled in order that there should not be two similar meetings too close together.

The Exeter EPS conference followed the traditional pattern for Condensed Matter Division conference by covering a very broad range of topics and including several plenary lectures. In addition, there was a lecture from one of the joint Hewlett-Packard prizewinners, Professor D Jerome, and the annual Mott Lecture was presented by Professor R G Clark. The invited lectures were divided into 5 parallel sessions, in part because of lecture theatre sizes, in which the topics roughly divided into semiconductors (2 sessions), metals and magnetism, high T_c superconductivity and heavy fermions, and soft matter and polymers. A number of contributors of abstracts for poster presentation were offered the opportunity of oral presentation. The three, very full poster sessions, were of a high standard and generated much interest and discussion. One can conclude that condensed matter physics is strong and active in Europe.

The papers of the invited talks contained in this volume will allow conference participants the opportunity for further study of the work presented and will also allow those unable to attent the meeting to learn of the interesting results presented. With such a broad subject coverage it is difficult to order the papers in a wholly rational way; according they have been brought together under five broad headings.

It is a pleasure to thank all those involved in the Organising and Programme Committees (see PDF file for detail) for their contributions to the Conference. The generosity of the Sponsors (see PDF file for list of sponsors) is gratefully acknowledged.

Layered materials constitute a large group of compounds, some of which are of large current technological interest. The weak interlayer forces offer the possibility of intercalation.

The properties of this particular class of solids are reviewed with the aims of a better understanding of the intercalation process and indicating guide lines for improving the performance of the materials.

Materials for high-performance permanent magnets must have a high Curie temperature, a large spontaneous magnetization and strong uniaxial magnetocrystalline anisotropy. Binary rare earth iron intermetallics are deficient in one or more of these respects. New series of rare-earth intermetallic compounds may be generated from existing series by a process known as gas-phase interstitial modification, whereby interstitial atoms such as nitrogen or carbon are introduced into the structure by reaction of a powder of the intermetallic with an appropriate gas. The interstitials occupy sites adjacent to the rare earth, thereby creating a crystal field that reflects the new local symmetry. They also expand the lattice, which greatly enhances the Fe-Fe exchange interactions, and may increase slightly the iron moment. These effects are discussed in detail for four systems, R₂Fe₁₇N_3-δ, R₂Fe₁₇C_3-δ R(Fe₁₁ Ti)N_1-δ and R(Fe₁₁ Ti)C_1-δ (R = rare earth), all of which include compounds with potential for making new parmanent magnets.

The increased succes in growing and manufacturing short period superlattices and laterally patterned structures has allowed physicists to demonstrate many of the properties first predicted when work on superlattices began. In addition new phenomena have emerged, such as coherently propagating edge states. Magnetic fields play a vital role in demonstrating and studying these phenomena. The construction of new superlattice band structures now consists in using far more properties than merely layer thickness. Strain, magnetic exchange, spin and electric field can all be built into individual layers. This allows us to look at zone-folding and mini-band formation, metal-semiconductor transitions, spin alignment, piezo-electric band modification, modified electron-phonon coupling and may other effects. Cyclotron motion through the structures and the resulting energy quantisation provide the means to map out these phenomena. The most direct spectoscopic information comes from studies such as inter and intraband magneto-optics, which allow us measure mini-band widths and zonefolding. Magnetotransport gives us information on the charge transfer and energy levels in structures, and can also demonstrate radical changes in the superlattice properties when large magnetic fields are applied. Lateral patterning, particularly when magnetic fields are applied, allows us to study 1-D and 2-D superlattice potentials in a plane. This can be used to study both individual quantum dots or connected lines or arrays which can exhibit controlled tunnelling or metal-insulator transitions.

High temperature superconductivity has been a strong booster in the early seventies for the search of molecular crystals which may conduct electricity like metals. After the stabilization of superconductivity in 1980 in the cation radical salt (TMTSF)₂PF₆ under pressure with a modest T_c of 1.2 K remarkable progresses have been achieved in chemistry since a decade of critical temperatures has been gained ten years later. Furthermore, besides superconductivity a wealth of new and fascinating phenomena have been observed, for example: the stabilization of spin density wave (SDW) states by a large magnetic field applied to a quasi-one-dimensional electron gas, the existence of quantized Hall resistance in these field induced SDW states, and a new channel of collective conduction provided by the coherent motion of a SDW condensate.

A review is given of selected collaborative experiments that we have carried out at Oxford, Saclay, Leuven, Grenoble and most recently Sydney, centred on the integer and fractional quantum Hall effects and the search for the electron solid in GaAs. The electronic ground state configurations is samples of the highest purity that have been fundamental to this programme, prepared by Philips Research Laboratories, UK, have been probed by a variety of novel techniques under extreme experimental conditions.

We propose that the binding of O(1) atoms in YBa₂Cu₃O_6+x is responsible for double valence fluctuations between the Cu(1)2O(4)2O(1) units in the ribbons and the Cu(2)2O(2)2O(3) unites in the adjacent CuO₂ plans. The onset of these dynamical charge fluctuations above a certain critical concentration of O(1) doping leads to the breakdown of the charge transfer insulator state and is connected with drastic changes in the lattice properties such as abrupt changes in bond lengths, changes in the thermal factors associated with the mean square displacements of atoms and positional fluctuations of atoms.

The high temperature superconducting materials are considered as being composed of CuO₂-layers (having rather ordinary metallic character) coupled to negative U centers located on the ribbons. The normal state of such systems are characterized by a two-hole resonant state near the middle of the band of charge carriers in the CuO₂ planes and is responsible for the anomalous properties of this state. At low temperature the charge fluctuations become correlated and lead to a superconducting ground state with features which are intermediary between a BCS superconductor and a superfluid state of bosons like ⁴He II.

In the present paper, the theory of the large bipolaron is reviewed and the possibility of bipolaron formation in the high-T_c superconductors is indicated. Operator and path-integral formulations of the large bipolaron problem are compared. In the strong-coupling limit, the effect of non-optimal upperbounds to the single-polaron groundstate energy is emphasized. The fact that the interaction with multiple phonon branches enhances the electron–photon interaction and might result in a larger stability region for bipolaron formation is indicated. Experimental values for the static and high-frequency dielectric constants are used to discuss the relevance of bipolaron formation as a pair-forming mechanism in the high-T_c superconductors.

The bipolaron ground state binding energy and the effective masses are calculated self-consistently in a scheme where the electron-phonon interaction is described by the Fröhlich interaction. We explicitly use the total linear momentum conservation and both two-and three-dimensional systems are considered. We review results for binding energies and show that the bipolaron effective mass increases with the electron-phonon coupling constant α more rapidly than two free polaron masses. As expected, the increase is greater in two than in three dimensions.

We estimate the screening effects due to an electronic or hole density n in a range of values such that nR²_b ≪ 1 (here R_b is the bipolaron radius). We find that the bipolaron binding energy decreases with n and eventually becomes positive indicating the existence of a metastable bipolaron state.

Finally we discuss the possible connections between our results and high T_c superconductivity.

The vortex lattice in extremely anisotropic superconductors has several intriguing properties. These peculiarities are very predominant in the oxidic cuprates, especially the Bi and Tl compounds, where they determine new regimes in the field-temperature phase diagram. They are also responsible for the weak flux pinning in extended regions of the mixed state, and cause a relatively large flux creep decay of the magnetization even at low temperatures. A recently discovered phenomenon is the strongly deviating behavior of the magnetization from the well-known mean-field (Abrikosov) characteristics near the transition temperature.

Some isomorhous materials of the type A₂BX₄ (K₂SeO₄ being the archetypical example) exhibit a structural instability leading to an incommensurate phase with common superspace symmetry. A clear soft-mode mechanism has only been observed in potassium selenate, other compounds having probably an order-disorder mechanism. From the analysis of the incommensurate structures, the eigenvector of the unstable frozen mode can be determined in each case, being similar in all investigated compounds. After revising some structural features of these materials, the results of a lattice dynamics analysis, within an empirical rigid-ion model, are reported. The model of interatomic forces used has only three adjustable parameters and was optimized using only static structural data for each compound. The tetrahedral BX₄ groups are reduced to rigid bodies. The model is sufficient to reproduce the incommensurate lattice instability. The calculated eigenvectors of the soft or unstable modes fairly agree with the experimental ones, being rather unsensitive to the details of the interactions and explaining their strong similarities in most compounds. On the other hand, the form of the soft branch strongly depends on the material, and clearly distinguish those materials having the BX₄ groups disordered in the normal phase, from those having a soft-mode mechanism. The results indicate that any empirical model consistent with the structural data easily reproduces the essential features of the incommensurate instability, indicating that its origin is to be directly connected with structural fearutes, the relative size of the cations playing an essential role.

We discuss the modifications that quantum mechanics introduces on the general picture of classical chaos. In particular we show the connection between the suppression of classical diffusion and the Anderson localization. This connection is used to study the Anderson transition in three dimensions.

The transition to chaos manifests itself in quantum systems through a change in spectral statistics due to level repulsion as a result of the interaction of nonlinear resonances. In quantum systems the transition leads to the extension of the wave function in Hilbert space (although it may be limited by dynamic Anderson localization). We have found that a similar transition occurs in the Fokker-Planck equation of a system whose Langevin equation exhibits a transition to a type noise induced deterministic chaos.

The magnetic structure of a single crystal of the rare earth random alloy Ho_50%Er_50% has been investigated by elastic neutron diffraction measurements in the temperature range 120-10 K. Three distinct magnetic phases are indentified below the Néel temperature of 104 K. The high- temperature phase observed between 104 K and 47.5 K is a binary magnetic structure where the holmium and erbium moments belong to different modulated c-axis spirals. The intermediate-temperature phase between 47.5 K and 35 K is a simple basal plane spiral. Below 35 K, the measurements suggest a ferrimagnetic stucture in which the holmium and erbium moments are indistinguishable and form a conical spiral with a half-cone angle of 20°.

Some of our recent local spin density electronic structure calculations for a number of ferromagnetic rare-earth systems are reviewed. A simplified model of the level densities for rare-earth (R) transition metal (M) intermetallic compounds, R_mM_n, is used to describe in a simple way the main features of their basic electronic structure. Explicit calculations for LuFe₂ and RFe₂ (R = Gd-Yb) systems are presented, where a method to treat simultaneously the localized 4f and the conduction electron spin magnetism is introduced. Thereby it becomes possible to calculate the K_RM exchange coupling constant. This method is also used to study theoretically the permanent magnet material Nd₂Fe₁₄B.

The electronic structure of the anomolous ferromagnets CeFe₂ and CeCo₅ is discussed and an induced 4f itinerant magnetism is predicted. The γ-α transition in cerium metal is considered, and result from calculations including orbital polarization are presented, where a volume collapse of 10% is obtained. On one side of the transition the 4f electrons are calculated to be essentially non-bonding (localized) and on the other side they are found to contribute to the metallic bonding and this difference in behaviour gives rise to the volume collapse. Recent calculations by Wills, Eriksson and Boring (Ref. [5]) for the crystal structure changes in cerium metal under high pressure are discussed. Their successful results imply an itinerant picture for the 4f electrons in α-cerium. Consequently this strongly supports the view that the γ-α phase transformation is caused by a Mott transition of the 4f electrons.

U_1-xSb_x amorphous ferromagnets were prepared that show a magnetoresistance anisotropy (AMR) reaching 26% at 10 K, in a 50 kOe field. This is a striking result, since the AMR of typical amorphous ferromagnets does not exceed 1%.

These materials are random anisotropy ferromagnets, with large coercive fields reaching 20 kOe at 10 K and with Curie temperatures ranging from 92 to 132 K.

Magnetization reversal processes at mean-free path level are discussed based on magnetoresistance hysteresis cycle data. The local magnetization reversal processes are found to be highly dependent on the magnetic history of the sample.

First principle calculations of ground-state charge and spin distributions of 3d and 4f ferromagnets are reported. While the metallic 3d-states are well described in the itinerant picture, 4f-states are better described in a localized picture of an "open core shell". The inter-relation between intra-atomic exchange splitting, moment formation, charge transfer, and structure is considered yielding essential contributions to an understanding of magnetic behaviour.

Recently, increasing interest has focused on DMS materials in which Mn is replaced by other transition metal ions: iron and cobalt. In Section 2 we will present particularly interesting properties of HgSe:Fe, connected with the peculiar energy position of d-derived Fe states in its band structure. Section 3 will be devoted to description of some optical and magnetic properties of Cd_1-xFe_x Te. The Jahn-Teller coupling piays an important role in the explanation of the observed effects. In Section 4 we will discuss some result for Co-based DMS. Finally, the study of Fe²⁺ in lowered symmetry environment, which is realized in alloys like (Cd_1-xFe_x)(Te_1-ySe_y), will be presented in Section 5.

We review magnetic properties of Mn, Co and Fe based Semimagnetic Semiconductors. Different energy structures of Mn, Co and Fe ions results in different magnetic behaviour (Brillouin-type or Van Vleck-type paramagnetism). In spite of that, for all the materials, specific heat, susceptibility and magnetization reveal d–d exchange interaction between the magnetic ions. We examine the interaction strength and range. Finally possible mechanisms behind the ion–ion interaction are discussed.

Iron-based diluted magnetic semiconductors having the cubic zinc-blende structure display unusual responces to external magnetic fields. For low fields, B, the magnetic susceptibility is isotropic as expected for cubic crystals. However, when the magnetic field is sufficiently high so that the Zeeman energy cannot be neglected in comparison to the spacing of the low lying levels of Fe²⁺ (owing to the spin orbit interaction), the nonlinear magnetization exhibits a striking anisotropy. This phenomenon is related to the Van Vleck paramagnetism primarily due to the nature of the ground state of the (3d)⁶ configuration of Fe²⁺, namely a singlet in the crystal field of tetrahedrally coordinated semiconductors. We are concerned with Fe²⁺ in a tetrahedral environment and the concomitant Van Vleck type of paramagnetism. We present an analytic investigation of the magnetic moment of a Fe²⁺ ion in a T_d field. In this approach the spin-orbit interaction and the Zeeman energy are treated as small compared to the crystal field spitting but are themselves viewed as of comparable magnitudes. We obtain expressions for the effective number of Bohr magnetons per ion for B along the ⟨001⟩, ⟨111⟩ and ⟨110⟩ directions. The anisotropy of this quantity is of the order of 15% at liquid helium temperatures in a magnetic field of 150 kG. Corrections proportional to the ratio of the spin-orbit interaction and the crystal field spiltting are also evaluated in first order.

Heavy-fermion superconductivity and other cooperative effects have been explored by thermal expansion, specific heat and ultrasound measurements on CeCu₂Si₂ single crystals. Crystals annealed under Cu atmosphere show sharp superconducting transitions at T_c^max = 0.63 K. At the same temperature the "as grown", i.e., non-bulk-superconducting, crystals reveal a pronounced phase-transition anomaly, presumably of structural origin. This new transition is associated with an expansion of the volume upon cooling and gives rise to magnetic correlations. Our results indicate a complex interplay between lattice instability, magnetic phenomena and superconductivity in CeCu₂Si₂.

The one-and two-band Hamiltonians for CuO₂ layers of high-T_c superconductors are analyzed within the unrestricted Hartree-Fock Approximation, as a function of doping. It is shown that a large number of solutions exits, which correspond to possible metastable spin and charge configurations. A solutions in which the spins are coplanar and form a vortex-like pattern is described. The solution is shown to be favoured by the frustration effects induced by doping. The differences and similarities between the results for the two Hamiltonians are remarked on. The excitation spectrum and the response function of the textures induced by doping are calculated. The spin wave spectrum is shifted to lower frequencies, although the antiferromagnetic peak at zero energy is depleted, whereas charge excitations appear at energies comparable to the spin modes.

Melting of diamong at high pressure and the properties and of liquid carbon have been investigated with a first-principles molecular dynamics technique. The result indicate and increase of the diamond melting temperature with pressure, which is the opposite of the behaviour of Si and Ge. This is contary to long-held assumptions, but agrees with recent experiments. The structural and bonding properties of liquid carbon are found to vary strongly with pressure: At P ≃ 0 the systems has an average coordination slightly smaller than that of graphite, with a considerable amount of sp sites, whereas at P ⩾ 1 Mbar its coordination is close to that of diamond. Both at low and high P, liquid carbon is found to be metallic.

A new method of calculating the properties of correlated electrons propagting on D-dimensional lattices with N^D sites is used to study the analytic properties of the Hubbard model in one and two dimensions. The single-particle self-energy is evaluated in the weak-coupling limit around the Fermi energy, ω_F, and momentum, p_F, at T = 0. For D = 1, the imaginary part of the self-energy, around p_F and ω_F, behaves in the limit as Im Σ_pF (ω_F + ω) ~ |ω| for |ω| ~ 1/N. For D = 2 at half-filling and along the (1,0) direction in the Brillouin zone, the dominate contribution to the self-energy around ω_F is estimated as Im Σ_pF (ω_F + ω) ~ |ω| ln|ω|. For one and two dimensional lattices, the excitation spectra of the U = 0 and U → 0 models become qualitatively different as N → ∞ and can not be simply connected by the perturbation theory.

Recent progress of our understanding of the novel magnetoresistance oscillations observed on two-dimensional electron gases with a weak lateral superlattice is reviewed. The discussion is based on a quantum mechanical single-electron picture, taking consistently into account the effect of the superlattice on the energy spectrum and the effect of randomly distributed impurities on collision broadening and transport scattering rate. This, first, allows to explain the original results of Weiss et al. on high-mobility Al_xGa_1-xAsGaAs heterostructures with an unidirectional modulation potential produced by holographic illumination: the large-amplitude oscillations of ρ_xx, measured when the current flows in the direction of the modulation, are atributed to a bandconductivity σ_yy, and the weaker antiphase oscillations of ρ_yy (∝ σ_xx, Current parallel to equipotentials) are shown to result from quantum oscillations of the scattering rate. Second, for the case of a modulation with square symmetry, where the energy spectrum is given by Hofstadter's famous "butterfly", it explains the dramatic suppression of the bandconductivity, which has recently been observed on high-mobility samples, as a consequence of the splitting of Landau levels into the Hofstadter-type subbands.

Two-dimensional electron systems subjected to a periodic background potential usually show large positive magnetoresistance that breaks down at a particular value of the magnetic field. Magnetoresistance oscillations periodic in the ratio of the diameter of the cyclotron orbit to the modulation period have been found above this field. The semiclassical theory is used to explain both effects. We show that the oscillations may be thought of as oscillations in the probability of magnetic breakdown.

We have studied the magnetoresistance of a two-dimensional electron gas (2DEG) subjected to a periodic potential created using electrostatic gates. Periods vary between 100 nm and 500 nm. The detailed behaviour of the magnetoresistance, which includes a positive contribution at very low magnetic fields and oscillatory behaviour periodic in 1/B at higher fields, is explained successfully within a semi-classical framework. The dependence of the magnetoresistance on amplitude of potential and temperature is explained quantitatively. The limits to the validity of the semi-classical approach are derived.

By rotating the sample we are able to alter the magnetoresistance in an anisotropic fashion depending on the relative direction between the equipotentials and the axis of rotation. This is interpreted in terms of the in-plane magnetic field allowing the zero-point motion in the confinement direction to be observed in the plane of the 2DEG.

Electrical transport properties of the two-dimensional electron gas are studied in the presence of a perpendicular magnetic field B = B and of a weak one-dimensional electric (V₀ cos (Kx)) or magnetic (B₀ = B₀ cos (Kx)) modulation where B₀ ≪ B, K = 2π/a, and a is the modulation period. In either case the discrete Landau levels broaden into bands whose width: (1) is proportional to the modulation strength, (2) it oscillates with B, and (3) it gives rise to magnetoresistance oscillations, at low B, that are different in period and temperature dependence from the Shubnikov-de Haas (SdH) ones, at higher B. For equal energy modulation strengths, V₀ = heB₀/m*, the magnetic bandwidth at the Fermi energy is about one order of magnitude larger than the electric one. The same holds for the oscillation amplitude of the electrical magnetoresistivity tensor. For two-dimensional modulations the energy spectrum has the same structure but with different scales. For weak magnetic fields and equal modulation strengths the gaps in the spectrum can be much larger in the magnetic case thus making easier the observability of the spectrum's fine structure.

A comprehensive theory of shallow impurities in quantum wells (QW's) is presented. The energy levels of donor and acceptor impurities are calculated within the effective mass theory including the mismatch of the band parameters and of the dielectric constants between well and barrier materials. The theory also accounts for Coulomb coupling between different subbands and, in the case of acceptors, for valence-band mixing. The method is based on an expansion of the envelope functions into a basis set consisting of products of two-dimensional hydrogenic-like functions and impurity-free QW eigenfunctions at k_|| = 0. The present method is suited for the ground as well as for the excited impurity states and thus enables us to obtain the oscillator strengths of infrared transitions between these states. The results show a good agreement with recent experiments on both donor and acceptor impurities.

We have examined the effect of the confinement on the electronic structure of defect states in GaAs/AlGaAs quantum wells (QWs) by means of selective photoluminescence (SPL), excitation spectroscopy, resonant Raman scattering (RRS) and related methods. The energy levels for confined acceptors have been investigated by monitoring the two hole transitions (THTs) in SPL experiments and RRS measurements via the bound exciton. Confinement effects resulting from both the width of the QW and the acceptor position in the QW have been studied. A corresponding study on donor states is initiated and the first results on two electron transitions will be presented. Also satellite emissions associated with the free exciton (FE) have been observed in undoped QWs. The behaviour of these satellites is in many ways similar to the extrinsic THTs and are interpreted as the corresponding intrinsic THTs caused by the interaction between a FE state and an adjacent "free" hole. The prerequisite for such an interaction is an overlap of the involved wave functions, which is explained in terms of localization of these states due to interface roughness.

After briefly introducing the properties of strained layers, results of measurements of the strain, band-offsets and hole effective mass in InGaAs-GaAs quantum wells presented. Some advantages of employing compressively strained layers in quantum well lasers are described and then experimental results on the threshold current, quantum efficiency, temperature stability, differential gain, linewidth, maximum operating frequency and long-term structural stability are briefly reviewed. It is futher shown that some of the advantages described above may also be achieved in unstrained quantum well lasers grown on (111) oriented substrates. Finally, the use of plastically relaxed layers grown beyond their critical thickness is considered for the production of substrates and buffer layers which will enable a freer choice in the materials combinations and strain that can be employed.

We review in this paper some of the optical studies performed on CdTe/CdZnTe heterostructures which demonstrate interesting specific band structure effects in this strain-layer system. Due to the order of magnitude of the three physical parameters which determine the optical properties of such structure (bandgap discontinuity, lattice mismatch induced strain, electron-hole interaction), we observe (i) two types of quantization in CdTe/CdZnTe single quantum well: the first one, for well thickness smaller than 200 Å, is the usual description of a 2D-exciton, whereas the second one, for wider wells, corresponds to the quantization of the motion of the exciton centre-of-mass. (ii) The mixed type nature of the CdTe/CdZnTe superlattice: type I for heavy-hole excitons and type II for light-hole excitons with the possibility of changing the relative energy positions of the type I and type II transitions with the built-in strains. (iii) A dominant piezoelectric effect in the (111) oriented strain-layer superlattices which generates an internal electric field of the order of 10⁴ V/cm and modifies the optical properties of these Te-based heterostructures.

The dynamics of carrier recombination in GaAs/Al_xGa_1-xAs quantum well (QW) structures is investigated using continuous wave and time resolved photoluminescence (PL) spectroscopy. We find that the recombination occurs from excitons localized at crystal defects at low temperature, while both free exciton and free carrier recombinations are observed for T ⩾ 70–100 K. The relative weight of the PL contributions at high temperature only depends on T and results in agreement with the 2D law of mass action for the equilibrium population densities. The relevance of the non radiative processes in the determination of the photoluminescence decay time is stressed and the behaviour of the radiative recombination time constant is deduced from the simultaneous measurment of the PL decay time and the PL integrated intensity. A consistent picture of the radiative recombination dynamics is eventually achieved at least for high temperature and weak excitation; a more complicated phenomenology is observed after excitation higher than a few tens of W/cm².

Low temperature optical spectra of semiconductor microcrystals grown in a glass matrix demonstrate the effect of size quantization of the energy spectrum of Coulomb interacting electron-hole pairs. The phenomena and microscopic mechanism of ionization of microcrystals under optical excitation are regarded.

Photoluminescence measurements of two-dimensional electrons in GaAs/GaAlAs heterojunctions in the integer and fractional quantum Hall regimes are reviewed. Intensity and energy anomalies observed experimentally are discussed in terms of many-body effects in the recombination process. Optical evidence for the formation of an electron solid in the extreme quantum limit is presented.

Magnetotransport can directly measure the energy gap between the symmetric and antisymmetric electron states in double quantum wells (DQW). Additional quantum Hall states are observed at magnetic fields corresponding to the Fermi energy lying within the symmetric/antisymmetric (SAS) energy gap. Upon increasing the magnetic field beyond a threshold field, these quantum Hall states are absent, suggesting the collapse of the SAS energy gap in high magnetic fields. An increase of the electron density or interwell barrier thickness will destroy a larger number of quantum Hall states. Recent calculations find that coulomb interactions can drive the collapse of the SAS energy gap at high magnetic fields, in good agreement with experiment.

Some localization effects in a 2D electron gas are reviewed, emphasizing the effects of an external magnetic field and a hydrostatic pressure on modulation doped GaAs/GaAlAs heterojunctions with a spacer.

As a result of electron transfer in the quantum well controlled by the pressure, various effects are studied as a function of the surface electron density on the same sample.

Three regimes are discussed, depending on the magnetic field intensity: the ultra quantum limit, the quantum limit - and the weak magnetic field limit.

Nitrogen under high pressure due to its elevated thermodynamic potential can be used for the synthesis of semiconducting compounds. High temperature growth of III-group metal nitrides from metallic solution is discussed and illustrated by the results of crystal growth of GaN. The growth of III-V crystals from the melts under high N₂ pressure leads to high concentration of substitutional nitrogen. The results of crystal growth experiments are used in the analysis of mixing properties in III-V-N pseudobinary systems.

The presented paper is a short review of some interesting phenomena characteristic for the DX centre in GaAs and AlGaAs as well as the EL2 defect in GaAs. We mainly concentrate on pressure studies of metastable properties of the both defects trying to refer the experimental results to the existing microscopic models of DX and EL2 defects.

We review an analytical model for the interpretation of transport measurements on spatially inhomogeneous Schottky contacts. The comparison of barriers from current/voltage- as well as from capacitance/voltage-curves permits a quantitative analysis of spatially distributed Schottky barriers. We reveal that the ideality coefficient n of abrupt Schottky contacts reflects the deformation of the barrier distribution under applied bias; a general temperature dependence for the ideality n is predicted and observed. An extension of our model includes the so-called flat band barrier of current/voltage curves. Here we demonstrate the interdependence of flat band Schottky barrier, ideality n and the homogenization of the barrier distribution under the application of a bias voltage. Effective photoresponse barriers and electrical noise at inhomogeneous Schottky diodes are also discussed.

A high quality of epitaxial silicon layers grown on Czochralski single crystal substrates is required in semiconductor technology, since it directly influences device parameters and yield. Unintentional doping of these layers from the substrate is well documented in the literature. However, we found no reports on unintentional contamination of epi-layers due to the oxygen present in the substrate.

The use of micro Fourier transform infrared spectroscopy in transversal mode is shown to be a powerful technique for oxygen profiling and precipitation studies at the same time. It is found that oxygen behaviour during epitaxial layer growth, even for lightly B and P doped substrates, is significantly different.

Oxygen, that in some cases diffuses into the grown epitaxial layer, tends to form precipitates there. This is an important consequence of outdiffusion since it directly influences the quality of the epitaxial layer.

Recent applications of optical spectroscopy to the study of double barrier resonant tunneling structures are discussed. Particular topics emphasized are the determination of charge build up in the quantum well active region on resonance, determination of the nature of the tunneling processes and the detection of electroluminescence emission resulting from impact ionisation in double barrier structures.

An asymmetric double quantum well structure consisting of a wide and a narrow quantum well separated by Al_xGa_1-xAs barriers of various thicknesses is used to investigate resonant and nonresonant tunneling of electrons as well as holes through a single barrier. The carrier transfer times are obtained by tracing the luminescence decay of the two quantum wells with picosecond time resolution as a function of an applied electric field. For the first time, resonant and nonresonant electron and hole tunneling times in one single structure are reported. Our results are compared with those of other groups. Idealized theoretical models cannot explain the experimental values.

We present theoretical and experimental results bearing on the assisted transfer of electrons, holes and excitons in biased double quantum wells. We show that the exciton relaxation proceeds via a conversion of a "vertical" exciton into a "crossed" one.

The Hall voltage in narrow high-mobility samples at low magnetic fields depends strongly on the geometry of the junction used to measure it. It can be quenched, enhanced or even negative. At high fields and low temperatures, dips appear in quantum Hall plateaux when the junction is almost isolated. This can be explained as a magnetic diffraction effect. We report preliminary observations of large oscillations as a function of gate voltage when a junction is surrounded by tunnel barriers. These are probably caused by the Coulomb Blockade of single electron tunnelling.

Transport phenomena in small semiconductor structures are briefly reviewed including the effect of d.c. and high-frequency fields. Experiments concerning weak localization, universal conductance fluctuations, boundary scattering, and Shubnikov-de-Haas oscillations in InGaAs/InP and n⁺-GaAs are reported. Non-equilibrium effects in these phenomena due to the application of pulsed d.c. as well as microwave and millimeterwave fields are studied.

A unified theoretical description is given for the magneto-phonon resonance in: (1) the linear resistivity, and (2) the cyclotron resonance linewidth and frequency for electrons confined to two dimensions. The resonances occur for ω* ≈ Nω_c where ω* is slightly less than the LO-phonon frequency ω_LO. This can be explained as due to the broadening of the Landau levels.

We present a review of magneto-optical experiments and theoretical calculations which have been used to examine the influences of dimensionality and many electron effects on electron-optic-phonon coupling in low dimensional structures of GaAs-(Ga,Al)As.

Using selected examples of reconstructions induced by oxygen or sulphur adsorption on copper surfaces it is shown how Scanning Tunneling Microscopy can reveal the formation mechanisms of new structures, determine the unit cells for complicated phases and investigate the level of structural perfection. Some dynamic aspects including the possible influence of the tunneling tip are discussed as well.

Direct methods in LEED are an important challenge to research in the field of Surface Crystallography. In this paper we discuss a recent advance in this direction: the reinterpretation of a Diffuse LEED pattern as a hologram. This allows us to directly get the geometrical information without needing to use a trial and error search. Resolution is sufficient enough to obtain the structure (≈ 1 Å [16, 19]), but attempts to improve this value are presently in progress. We also discuss how different parameters can influence the holographic reconstruction process.

Recent progress in surface phonon spectroscopy using inelastic electron scattering is reviewed. For a number of adsorbate systems the dispersion curves of the adsorbate modes have been measured. It is demonstrated that important structure parameters can be deduced from the local symmetry of the adsorbate revealed by the absence of a disperison branch under particular scattering conditions. Among the adsorbates hydrogen is a particularly interesting case, since there the classical picture of vibrational motion must be replaced by a quantum mechanical description. For some adsorbate covered or metastable clean surfaces the dispersion of the Rayleigh wave indicates a compressive or tensile surface stress. Finally the significance of an experimental determination of the dispersion of surface phonons with odd symmetry will be pointed out.

We describe the application of the x-ray standing wave technique to investigating the structure of surface adsorbates and epitaxial interfaces. We outline the analysis of (i) Si(111)/Ga, √3 × √3 as an example of a commensurate system, (ii) Si(111)/Ga, 6.3 × 6.3 as an example of an incommensurate system, (iii) Si(111)/Ge/Ga, 6.3 × 6.3 as an example of a complex, composite adsorbate and finally (iv)the analysis of the interface structure of epitaxial NiSi₂ on Si(111). The main principles of the XSW technique relevant to surface structure analysis are described briefly.

Investigations of the properties of the metal-semiconductor (M-S) interfaces formed at low temperatures are of special importance due to the drastic reduction of many processes involved in the interface formation. In the present report we present latest results obtained for interfaces of this metal films with n- and p-type InAs(110) at two different substrate temperatures, 10 K (LT) and 300 K (RT). The measurements were done by ultraviolet photoelectron spectroscopy (UPS), low electron energy loss spectroscopy (LEELS), Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The investigations show that the interfaces between metals and InAs(110) present a variety of peculiar phenomena. This makes the systems markedly different from those with other A³B⁵ substrates. Very distinctive results are obtained at LT.

Valence electrons confined to a thin metal overlayer on a semiconductor or a metal can form discrete overlayer states which may be observed by photo-emission. Samples with overlayer states can be used to obtain information about the thickness dependence of various physical properties in a layer by atomic layer fashion and are well suited to study phenomena that can be observed only for well defined films. Here we show that overlayer states can be observed for Si(100)/Na. We furthermore use photoemission data for Cu(111)/Cs to determined the bulk band dispersion for the overlayer metal and to determine the thickness dependence of the desorption energy for Cs. Finally we note that for the Cu(111)/Cs system dynamic Friedel oscillations can be studied via optical second harmonic generation.

R⁺ rotons can be injected into liquid ⁴He and for ambient temperature T < 0.1 K the rotons have long lifetimes. This means that beams of ballistic rotons can be created. Such beams can be efficiently detected using quantum evaporation. The total time of flight of the R⁺ roton and atom is a function only of the roton wavevector and the path lengths, so by injecting short pulses of rotons it is possible to do roton spectroscopy. If two beams of ballistic rotons intersect there is scattering. Such an experiment is described and how the roton-roton cross-section may be obtained from it. It appears that the cross-section is strongly wavevector dependent. The scattering of high energy photons (ℏω/k ⩾ 10 K) from slow rotons is compared to roton scattering from similar slow rotons and the ratio agrees with that found by neutron scattering.

Crystallization waves observed on solid-superfluid ⁴He interface appear to be a unique "instrument" for studying thermodynamic and kinetic properties of the interface. Measuring of the spectrum and the damping of the waves allows us to determine directly important interface characteristics: surface stiffness and growth coefficient. Studying the spectrum and the damping as the functions of the interface crystallographic orientation allows us to draw the picture of both the surface energy and the growth kinetics anisotropy. The data help to clarify the nature of the roughening phase transitions which should be universal for all crystals.

A model of liquid-liquid interface (LLI) has been investigated by molecular dynamics (MD) technique. The model is made of two "Lennard-Jones" liquids. The interface has been characterized via thermodynamic properties: interfacial tension, density and pressure profiles.

This model has then been used to study the kinetics of adsorption of a simple amphiphilic molecule at the LLI. The kinetics of the adsorption of this molecule at the interface is studied by generating trajectories starting in the bulk of one liquid. We tried to interpret the behaviour of the amphiphilic molecule as a diffusive motion in a potential well. To achieve this, we have calculated the mean force acting on the molecule fixed at a given distance from the interface. The potential of mean force deduced from this calculation has then been used in a Langevin simulation. The agreement of the MD and Langevin approaches tend to support the idea of a diffusion controlled mechanism for the adsorption.

A similar study has been performed for the transfer of a solute across the interface. In this case a modified Lennard-Jones potential has been used to model a solute particle solvated in the two liquids. In order to evaluate the rate of transfer of the solute through the LLI we calculated the free energy barrier height. As in the case of the amphiphile, the mean force acting on the solute particle is plotted as a function of the distance to the average position of the interface. The potential of mean force deduced from this calculation gives the free energy variation for a solute crossing the interface. The profile of mean potential energy has also been calculated and compared to the potential of mean force. The comparison of the two profiles show that in this case the value of mean potential energy is a good approximation to evaluate the barrier height.

Recent work on the calculation of the electrical and thermal conductivity of ionic solids by molecular dynamics simulation is reviewed. A brief summary is given of the techniques that can be used for such calculations, and it is pointed out why equilibrium methods based on the Green-Kubo relations are generally to be preferred. The predictive power of the calculations is illustrated by recent results on the transport coefficients of alkali halides, calcium fluoride and uranium dioxide, and some potentially important applications are noted.

Many divalent and trivalent metal ions in stoichiometric liquid mixtures of their halides with alkali halides are four-fold or six-fold coordinated by halogens into relatively long-lived "complexes". The stability of these local coordination states and the connectivity that arises between them in the approach to the pure polyvalent metal halide melt determines the character of its short-range and possible intermediate-range order. The available evidence on local coordination in some 140 mixtures has been successfully classified by a structure sorting method based on Pettifor's chemical scale of the elements. Within the general phenomenological frame provided by structure sorting, main attention is given in this work to the liquid structure and melting mechanisms of trivalent metal chlorides. The liquid structure of YCl₃ is first discussed on the basis of neutron diffraction measurements and of calculations within a simple ionic model, and the melting mechanisms of YCl₃ and AlCl₃, which are structurally isomorphous in the crystalline state, are contrasted. By appeal to macroscopic melting parameters and transport coefficients and to liquid structure data on SbCl₃, it is proposed that the melting mechanisms of trivalent metal chlorides may be classified into three main types in correlation with the character of the chemical bond.

Anion clustering according to a rule discovered by E Zintl may occur in ionic alloys. The chemical bonds between the anions are predominantly covalent. The drastic consequences of this effect for the electronic and structural properties of liquid ionic alloys are demonstrated for alloys of alkali metals with Pb, Sn, Bi, Tl and In.

An account of complex liquids and the methods used for their study is presented. It is shown that modern techniques of diffraction combined with numerical methods allow such liquids to be studied at a microscopic level.

Molecular electronics in its broadest definition encompasses all actual and potential applications of organic molecular materials to electronics and opto-electronics in which the molecular material plays an active role. The macroscopic properties of organic materials are already utilised in xerography and liquid crystal displays. Advances in understanding of electrical and nonlinear optical properties offer prospects for new applications in the near future. Molecular scale electronics has been frequently discussed but in the absence of experimental tests remains speculative. Progress in physics, chemistry, engineering and biology is now providing the means to conduct experiments at the molecular scale. This has opened up a number of alternative routes which may lead to the realisation of molecular scale electronics.

Umklapp processes in two-fold commensurate compounds (TMTSF)₂X result in the coexistence of two spin density waves in the magnetic field. One of them corresponds to the vector of ideal nesting and causes known phase transitions. The other wave corresponds to the vector of direct nesting and causes fast oscillation of the physical values.

We report the observation of a very rich reorientational dynamics of the molecular director of a 5CB nematic film, induced by an elliptically polarized c.w.-laser beam above the threshold for the Optical Fréedericksz Transition. In despite of the very complicated character of the observed motions, a simple model, based on continuum theory and angular momentum conservation reproduces very well the experimental findings.

The elastic energy of topological points defects and strings connecting pairs of defects is discussed for the geometry of hybrid aligned nematic film with horizontally degenerated boundary conditions. Experimental determination of their structure enables to reach an estimation of the saddle-splay elastic constant.