Table of contents

The annual General Conference of the Condensed Matter Division of the European Physical Society has grown into a leading forum for discussion and exchange of the latest scientific developments. The 9th General Conference was held in Nice from 6 to 9 March 1989; it was attended by about 600 participants coming from more than 27 countries of Europe and overseas.

The scientific programme contained 75 invited speakers distributed in plenary lectures, invited talks in parallel sessions, and one lecture given by the Hewlett Packard Europhysics Prize winner. Numerous contributions (more than 450), ranging over the entire field of Condensed Matter Physics, were presented in posters sessions.

The editors of the Proceedings have decided to publish them as soon as possible after the Conference. So the authors were requested to follow a rigorous dead line. The present volume collects all the papers orally presented at the Nice Conference and which manuscripts were received in time. The presentation of the papers in this volume follows the pattern of the Conference as closely as possible.

The abstracts of all the contributions (lectures, parallel sessions and posters sessions) are published by the European Physical Society in the Europhysics Conference Abstracts volume 13A. We wish to thank the Conference Committees and all the people who helped in the choice of such stimulating material, and all the contributors to this volume for their cooperation in the timely preparation of manuscripts. A special acknowledgement is also due to the Sponsors, whose generous help has made this manifestation possible.

Since their discovery by Bednorz and Müller [1], the high T_c oxides have offered us the puzzle of a mix of conventional and unconventional properties both in the normal state and in the superconducting state. This is why some of the fundamental questions such as the applicability of the Fermi liquid description and BCS s-wave superconductivity are still open to discussion. Experimental results bearing on these questions will be reviewed. Special emphasis will be placed on the role played by the short coherence length in determining some of the unconventional properties of the oxides.

A brief survey is given on the physical properties of heavy-fermion metals, i.e. certain lanthanide- and actinide-based intermetallic compounds. The origin of the heavy electron masses and the formation of both magnetically ordered and coherent Fermi-liquid states, including the Fermi surface properties of UPt₃, are discussed. Particular emphasis is put on the phenomena and problems associated with the occurrence of superconductivity in CeCu₂Si₂ and UBe₁₃ as well as their counterparts UPt₃ and URu₂Si₂.

Relative abundances of metal clusters containing up to 100 alkali atoms show magic numbers for clusters of special stability, in agreement with a shell model analogous to the nuclear shell model. Observed ionization energies and photo-emission spectra also reflect the single particle features of the electronic shell structure. The long range Coulomb screening among the delocalized metallic electrons determines the static and dynamic electric polarizability, from which the plasma resonance absorption is derived. The photoabsorption cross sections show peaks in the visible, depending on the shapes of the clusters, and on the oscillator strengths. Experimental values of the peak cross sections are ~1 Ų per electron, consistent with free electron calculations.

The study of neural networks by physicists started as an extension of the theory of spin glasses. For this approach to be valid, one had to assume symmetric interactions between the neurons. The true interactions between neurons are non symmetric. In order to treat this asymmetry, new techniques have been developed which should be useful in the study of systems far from equilibrium.

One can construct a class of models (diluted asymmetric networks), the dynamical properties of which can be calculated analytically. These networks have many properties in common with symmetric networks: Retrieval of stored patterns, maximum storage capacity, mixed states. They also have several properties specific to the asymmetry: Chaotic behaviour and continuous distribution of activities.

Single crystal multilayer films with alternate heavy rare earth and yttrium layers have been shown by neutron diffraction to exhibit long-range magnetic order. Analysis of the neutron results on Dy|Y and Er|Y superlattices shows that the phase of the modulated magnetic structures in the Dy and Er is preserved across the intervening Y non-magnetic layers and corresponds to a "pseudo turn-angle" near 51 degrees in the Y, which is in accord with theoretical calculations from the band structure. This suggests that the exchange coupling is via a conduction band spin density wave in both Y and Dy, stabilized by the 4f spins of the Dy. The magnetic coherence length, determined from the neutron linewidth, exhibits a 1/r dependence on the Y layer thickness for a fixed number of Dy planes. The ferromagnetic transitions occurring in the pure elements are completely suppressed in the multilayers due to epitaxial "clamping" by the Y layers which exhibits the development of sufficient magnetostrictive strain to induce the phase transition. The temperature of the intermediate transitions in Er|Y multilayers is also modified by magnetostriction and evidence is found for different turn angles for c-axis and basal plane moment components.

The various approaches proposed so far to describe the atomic structure of quasicrystals are discussed together with some of the descriptions of the newly observed defects in these structures.

The Shubnikov-de Haas oscillations have been measured on single crystals of the organic metal β-(ET)₂IBr₂ as well as the angle and field dependences of the resistance. The electron orbits have been found to be open in the c*-direction and closed in the ab-plane, so that the main motif of the Fermi surface is a slightly warped cylinder with the axis parallel to the c*-direction. The transverse magnetoresistance exhibits a strong anisotropy (> 10) in the ab-plane revealing that the Fermi surface cylinder is far from circular. For the magnetic field along the c*-direction two types of Shubnikov-de Haas oscillations have been found corresponding to the extremal ab-plane cross-sections values of 0.6 and 50% of the Brillouin zone area and to the effective masses of 0.5 and 4.5m_e, respectively. The classical part of the magnetoresistance exhibits the strong angular oscillations with rotating the field in the ac*- and b'c*-planes that may be an evidence of existing small necks between the FS cylinders in neighbouring unit cells of the reciprocal lattice.

In this short review we discuss the main differences observed in the physical properties of (TMTSF)₂X and (BEDT-TTF)₂X families of organic conductors. The special role played by quasi-one- or quasi-two-dimensionality in the magnetic and superconducting properties of these compounds is analyzed in the light of some recent experimental data.

The properties of charge density wave (CDW) low dimensional conductors are reviewed, with special reference to the molybdenum bronzes. For quasi-one-dimensional materials showing non linear transport due to the depinning of the CDW, the origin of metastability in the low electric field state as well as some aspects of CDW dynamics, are discussed. Results obtained by local techniques such as EPR and proton channeling may be consistent with the creation, annihilation and motion of CDW structural defects, such as phase dislocations, under applied electrical fields or through thermal treatments. We also discuss the properties of quasi 2D CDW materials such as the purple Mo bronze, which may show SDW and magnetic field induced transitions.

Real charge density wave systems do not show generally a perfect collective motion, in contrast to known macroscopic fluids. The usual reasons for this are the incomplete electron-hole condensation and/or local deformations of charge density waves due to the inhomogeneities on various spatial scales. In this text the short review of possible semi-metallic and insulating ground states is followed by the discussion of topological defects in the static and the non-linear dynamic regimes.

In semiconducting linear chain compounds with charge-density wave ground states a novel type of collective excitation appears at low temperatures. In the low field, insulating states the response for small amplitude a.c. excitations reveals a frequency independent dielectric constant up to a few GHz (with magnitude of ε ≃ 10⁶), and a narrow resonance peak in the millimeter wave frequency range. The collective electron transport due to the sliding condensate is characterized by a rapid depinning process at a sharp threshold field and by an unusually low damping in the current carrying state. Our observations suggest that the internal degrees of freedom play minor role in the CDW dynamics when normal carriers are frozen out. These results are contrasted to the conventional CDW conduction at higher temperatures.

The case in favor of the strong correlated one band or t-J model as the key to understanding superconductivity and magnetism in high T_c materials is presented. Some recent results on the t-J model which are encouraging in this regard are reviewed.

Recent experiments revealing strong electron-phonon coupling in high-T_c superconductors are reviewed and the possibility of phonon-mediated superconductivity in then is discussed. Special emphasis in enhancing T_c is put on the lattice instability due to highly anharmonic oxygen vibrations in perovskite-type compounds. To describe the latter, a pseudospin model of the lattice dynamics is proposed and electron-phonon interaction in ion-type crystals is considered. It is argued that a strong coupling and high-T_c can be obtained due to instability of the ion lattice at a mode/rate value of electron-phonon interaction.

Our opinion is that, in the high T_c copper oxides, the electronic correlations are not large enough to allow the localization of the electrons of the half-filled d-p sub-band. Thus, we treat them as itinerant electrons, in a bidimensional structure. And we show that, contrary to a widely held opinion, the electron-phonon interaction can induce high T_c superconductivity in these compounds, even in the weak coupling limit. This is due to the fact that, because of the bidimensionality, the electronic density of states is sharply peaked in the neighbourhood of the Fermi energy. A small coupling between nearest neighbouring CuO₂ planes is sufficient to prevent a very large reduction of T_c by the critical fluctuations. The calculated isotope effect is much smaller than usually in the BCS theory. And, in our weak coupling theory, the antiferromagnetic (AF) phase is much more rapidly destabilized by dopping or internal charge transfer than the superconducting phase, which takes place when the AF phase has vanished.

Single crystals of Y-Ba-Cu-O compounds with various additions have been studied by x-ray topography, polarization optics technique and electron microscopy. During cooling due to phase transition from tetragonal into rhombic structure occurring by martensite mechanism, domains are formed with different orientation of structure directions a and b in basal planes. Diverse types of domain boundaries such as thin coherent and extended ones have been observed. The anisotropy of the polarized light scattering by different domains in the visible range of wave lengths enables the observation of domain boundaries. The integration of decoration technique with electron microscope makes it possible to observe the anisotropic lattice of the magnetic flux vortices and the interaction of the latter with the domain boundaries. The application of this technique permits to determine the pinning force in the domain boundaries as well as the anisotropy of the magnitude of the magnetic field penetration depth and the anisotropy of the magnitude of the current carriers effective masses. Single crystals of thallium-based compounds have been investigated.

Granular superconductors screen out an applied magnetic field only imperfectly. Fluxons enter the intergranular spaces at very low fields and their presence can be easily detected by microwave absorption. Experiments on high T_c superconductors have allowed the identification of two absorption processes, (a) viscous fluxon motion in the microwave field and (b) fluxon nucleation and annihilation by the induced microwave current. Both processes are also observed in conventional superconductors but not always with the same characteristics.

The structure of superconducting La₂CuO_4.032 has been determined by single-crystal neutron diffraction data. The excess oxygen, O4, is located between two adjacent LaO1 layers. The presence of O4 distorts the apical-oxygen O1 sublattice in such that each O4 forms a short bond (1.64 Å) with one O1, which indicates the formation of a strong covalent bond between these two atoms. The Cu and O2 sublattices as well as that of La are not affected by O4, however, 1.6% of the La cations increase their coordination from 9 to 10.

It is now well established that oxygen plays a crucial role in the structural, magnetic and transport properties of high T_c materials. We present here a brief review of the effect of the oxygen deficiency and vacancy ordering on the superconducting and magnetic properties of various superconductors.

First we present the principles involved in the study of the two-photon momentum distribution: The method requires deconvolution of the positron wavefunction and the estimation of matrix elements effects. Single crystal samples must be of sufficient quality to avoid positron trapping (tested by positron lifetime measurements). In ordinary metals (alkalis, transition- and rare earth metals and compounds) two-photon momentum distribution studies have given results in close agreement with relevant band structure calculations. Discrepancies have been successfully described as enhancement effects due to correlations. In the superconducting oxides, measurements are more difficult because there are fewer conduction electrons and more trapping. Correlation effects of a different nature are expected to be important and might render the band picture inappropriate. Two-photon momentum distribution measurements have now been made by several groups, but have been interpreted in different ways. We relate the current state of affairs, and our present interpretation, to the latest available results.

Results of neutron scattering studies of the magnetic properties of YBa₂Cu₃O_6+x materials are reviewed. At low oxygen concentration, anti-ferromagnetic ordering in the CuO₂ sheets has been evidenced. The Néel temperature is around 415 K. With increasing oxygen content, the ordered magnetic moment and the Néel temperature are reduced. Finally, the long range magnetic order disappears for x_c = 0.41, which indicates the appearance of a large number of holes in the CuO₂ layers. Inelastic neutron scattering measurements of spin waves have established the two-dimensional character of the magnetic couplings in the antiferromagnetic phase. Increasing the hole concentration reduces the spin wave stiffness and close to x_c the in-plane spin fluctuations become diffusive.

Inverse photoemission spectroscopy using photons in the energy range 10-50 eV is now the preferred method for mapping the unoccupied bands of metals. With suitable apparatus it is possible to measure the energy, momentum and spin of bulk and surface states from the Fermi energy to above the vacuum zero. A wide range of clean transition metal single crystals and of metal overlayers on crystals have been studied in this way and the band dispersions compared with first principles band structure calculations. Tungsten and tantalum are typical of metals with a high density of unoccupied states that have been measured at several photon energies. Calculations have been made of the photon yield from single crystal tantalum using a relativistic version of the multiple scattering theory of Pendry. The agreement between the calculated and measured spectra is good up to over 5 eV above E_F indicating that the one electron eigenstates derived in band structure calculations may be used with confidence for quasi-particles in transition metals.

We have measured the Macroscopic Quantum Tunneling (M.Q.T.) rate out of the zero voltage state of a Josephson junction shunted by a delay line terminated by a resistor. As the length of the line is increased in situ, thereby increasing the delay with which the resistor damps the junction, the M.Q.T. rate increases until it almost recovers its value in the absence of damping: This constitutes the temporal decoupling effect. We compare our results to the theoretical predictions.

Generalized Gutzwiller-type trial ground states for the Hubbard model are considered, which include extra variational parameters to control the nearest-neighbour charge-charge, charge-spin, and spin-spin correlations. The correlation between doubly occupied and empty sites is found to be particularly important. The results for the large-U regime show qualitative improvement when compared to the predictions of the simple Gutzwiller Ansatz: We get the correct order of magnitude for the energy density, and the band state occupation number is found to be a monotonically decreasing function, with the leading term corresponding to that of the t/U-expansion. We find that to get the small-U limit right, the pair-correlated Ansatz has to be used in conjunction with a cluster Gutzwiller approximation, and introduce a four-site cluster approximation which gives promising results. We argue that small-cluster Gutzwiller approximations get accurate in the limit of large lattice dimensionality.

A description of quantum transport for electrons in the presence of a random impurity potential is provided in terms of a kinetic equation, which is obtained from the diagrammatic structure for the linear response functions. When the electron-electron interaction is neglected, the kinetic equation is of the Boltzmann type, with an effective scattering kernel that signals localization effects. With the further inclusion of the interaction, a suitable distribution function for the energy of the quasiparticles can be introduced even in the presence of strong disorder. In this case the kinetic equation has the diffusive form, with diffusion coefficient and Landau interaction parameters that get renormalized by the interplay of disorder and interaction. The quasiparticles kinetic equation is also shown to contain all known results for the hydrodynamic correlation functions.

A review is given on the optical spectroscopy of electronic states in conduction subbands of III-V compound quantum well structures. By appropriate choice of well and barrier material, and in particular the well thickness, the intersubband transition can be tuned from about 3.5 μm to longer wavelengths over the entire infrared range.

The dependence of the absorption energy and lineshape on temperature has been studied for both well and modulation doping in GaAs/AlGaAs quantum wells. The lineshape analysis gives the single energy state width by deconvoluting the width due to the level-dependent dispersion.

Various application schemes of intersubband transitions in infrared photodetectors and lasers are discussed.

Low temperature electrical measurements on particular semiconductor structures can exhibit effects due to the spatial confinement of carriers and reduced dimensionality. Examples are provided by studies of tunnelling in confining structures, transport in narrow channels, ballistic transport through constrictions and quantum interference in rings. These observations rely on the growth of layers by molecular beam epitaxy (MBE) to provide both two-dimensional electron gases (2DEGs) and the very high material quality essential for the observation of some quantum effects. To define the specific geometries necessary for the above experiments electrostatic "squeezing" can be used to confine the planar carrier gas to regions of controllable extent. Recent results obtained from MBE GaAs/AlGaAs 2DEG structures can now be compared with measurements made on hole gases and the use of in situ ion beams in fabrication promises a new class of structures for investigation. However, it is rarely sufficient merely for MBE to provide a carrier gas with the highest maximum carrier mobility. The particular requirements of an experiment affect not only the design of the structure itself but also the MBE growth strategy and the optimum deposition conditions. Growth rate, substrate temperature and source material flux ratios are MBE variables and the suitability of layers for specific postgrowth physics experiments can be shown to depend upon these variables in a systematic manner. Moreover, further requirements for the grown material can include the need to provide specific sheet carrier concentrations, high mobilities in unilluminated samples and the complete absence of conducting paths in parallel with the 2DEG. There are often compromises inherent in attempting to simultaneously optimise several MBE variables and careful management of the MBE machine itself is essential.

The single crystal growth, by molecular beam epitaxy, of materials compatible with silicon is described, and device applications are given. Process temperatures are decreased to 240°C, heterojunction devices are fabricated, strained layer Si/Ge superlattices are investigated, and silicon based monolithic integration of heterojunction devices and conventional devices is suggested.

A dynamic systems approach to nonlinear charge transport and electrical instabilities in semiconductors is presented, and illustrated by specific examples of physical transport mechanisms which yield oscillatory instabilities: Impact ionization of shallow impurities at liquid helium temperature as a mechanism for selfgenerated chaotic dielectric relaxation oscillations; Poole-Frenkel emission from deep levels at room-temperature as a mechanism for low-frequency oscillations in semiinsulating GaAs; Impact ionization and optical phonon emission as a mechanism for global bifurcations and hysteresis of oscillations in p-Ge; Breathing current filaments.

This paper is dedicated to recent experimental studies of the optical absorption edge in non-hydrogenated and hydrogenated amorphous tetracoordinated semiconductors. Attention is first paid to the respective contributions of the density of states near the band edges and of the matrix elements, and the derivation of a so-called amorphous "optical gap" is discussed. The sub-band gap absorption, which can now be determined accurately down to quite low levels by sensitive techniques like the photothermal deflection spectroscopy, is then analyzed in terms of disorder and defects. Problems related to hydrogen incorporation, doping, light-induced effects and alloying are considered in the Si case. The specificity of disorder and defects in amorphous III-V compounds is also pointed out.

The interaction of the radiation field with an elementary excitation of the polarization field in a solid gives rise to polariton modes. Examples are phonon-, exciton- and plasmon-polaritons. Such modes are unstable above a critical damping of the excitons; we concentrate on weakly damped polaritons in bulk material and discuss in detail the exciton-polariton, for which spatial dispersion is an inherent property. With a full quantum mechanical treatment we obtain the exact polariton states, which contain for all values of the wave vector a non linear contribution of photon and polarization states. Two- and three-photon components are explicitly singled out.

Optical spectroscopy of impurity levels in GaAs is discussed with special emphasis on acceptors. Photoluminescence, Fourier transform infrared absorption, and electronic Raman scattering spectra are presented of shallow acceptors like carbon and zinc as well as of the 78/203 meV double acceptor. In the latter case Raman spectroscopy enables to analyse the electronic structure of the neutral double acceptor in detail and to determine the energy level diagram. Possible models for the microscopic structure of this double acceptor are reviewed including the Ga_As antisite defect.

Spin dependence of the absorption of photons in the keV energy range has been studied using circularly polarized synchrotron radiation. It has been established, that the near-edge absorption coefficient contains a spin-dependent part, which is characteristic for the magnetic absorbing medium. The physical origin of this phenomenon is the ejection of a partially spin-polarized photoelectron from an unpolarized inner-core state by absorption of a circularly polarized photon. In the XANES region the spin-dependent absorption profile reflects the spin-polarization of the exchange-split final states. The observed spin-dependent EXAFS result from exchange scattering of the polarized outgoing photoelectron wave by the magnetized neighboring atoms.

The application of Grazing Incidence X-ray Diffraction to the structural analysis of clean reconstructed surfaces and heterointerfaces is presented. By clearly assessing the nature and position of the atomic species in the first layers, the method has brought a finer description of the bonding schemes at surfaces and interfaces. Examples of elemental and compound semiconductor reconstructed surfaces are discussed together with some metal-semiconductor interfaces.

Pulsed laser heating of solid surfaces is described as a novel technique to produce liquid-quenched layers of metastable materials. It is shown how the rapid solidification following the laser induced melting can be controlled in order to produce quench rate variable from 10⁹ to 10¹¹ K s^-1. Several examples are reported mainly related to the semiconductors field. This is because historically this technique has been developed as a tool to anneal damage in ion implanted semiconductors. Partitionless or partion-depressed solidification of Si-dopands solutions is discussed; then the amorphous silicon and amorphous Si–As solution solidification is shown and finally an example of metastable single crystal layer of Nickel monosilicide produced by rapid solidification is given.

It is demonstrated how nuclear hyperfine interaction measurements on implanted radioactive atoms in solids allow fundamental studies in the field of beam–solid interactions. The principles and relative merits are discussed of Mössbauer spectroscopy, time differential perturbed angular correlations and nuclear orientation at low temperatures. It is demonstrated how in many cases unique information can be extracted on the microscopic configuration around implanted atoms in solids, often in circumstances where the more classical techniques fail.

The driving force for miniaturisation is to achieve a very high integration density on a chip. Today the typical feature size of a device in a circuit is about 0.8 μm. In this presentation we will address some of the problems that are important in context with the modeling of these structures. In the first place we will give a brief discussion of possible extentions of the semiconductor equations suitable to describe high field phenomena in a MOSFET device. This will include the field dependent mobility, the carrier temperature, and field enhanced impact ionisation. In a second part the problem of leakage in a DRAM cell is presented as an example how computer simulation of complex structures is used to find an optimal design.

The flow of fluids in porous media leads to displacement fronts that are fractal in many situations. We discuss results obtained in experiments on well defined models. The dispersion of tracers, the invasion percolation at low displacement rates and the viscous fingering at high displacement rates are discussed in detail.

A review is given on switching processes on a molecular level, as a first step towards future molecular electronics. Special emphasis is layed on conjugated polymers and the concept of soliton switching. For the sake of discussion a cellular automaton based on donor-acceptor-modified polyenes is sketched and the error probability due to stochastic quantum effects is estimated, using life time values derived from experimental and theoretical investigations of polyacetylene.

The physics associated with the formation of electrical barriers at metal–semiconductor interfaces has been the subject of investigation for a long time. The application of modern "surface science" techniques to probe ultra-thin metal layers on atomically clean semiconductor surfaces has recently led to considerable progress in identifying the relative importance of various mechanisms which contribute to Schottky barrier formation. Aspects of this recent work is considered.

The stability of steady state flow is investigated by means of a model based on dislocation population dynamics, wherein the equilibrium density ρ_e of procreative dislocations plays a central role. The temporal evolution of ρ_e is governed by a single system parameter α containing the loading conditions and structural parameters respectively. Below a critical value α = α* = ρ_e(α) is a single and above α* a multi-valued function of α. Correspondingly, depending of loading conditions, ρ_e(α) reveals bifurcations. These are suggested to generate instabilities of steady state deformation structures which macroscopically may reveal as instabilities in plastic flow. This behaviour is typical for the phenomenon of dynamic recrystallization (DRX). In the present paper α is connected with the ratio λ ≡ W/W_s (expended to stored energy). λ is an increasing function of strain for α = α* approaching a critical value λ_m*. λ is proportional to the stationary cell size d_s. The experimental λ_ex* values associated with DRX of Cu- and Ni-single crystals – in strong support of the present model – turned out to be independent of temperature and these, within the accuracy of the model prediction, agree with the model value λ_m*. By means of the rate of energy storage W_s' equivelence dW_s/dW a stability criterion is derived according to which the instability of plastic flow in DRX can be suggested as the consequence of insufficient entropy export by the deforming material.

Besides their well-known properties concerning optical waveguiding, optical fibers are also used for other important applications because of long interaction lengths allowing to take full advantage of physical phenomena. We review some effects occurring in optical fibers as passive or active media: Dispersion modified fibers used in advanced high bit rate and long haul transmission systems, rare-earth doped fibers to be used as optical amplifiers to boost the regeneration span in optical systems or as lasers to generate new wavelengths, non-linear phenomena due to the third order nonlinear susceptibility with potential applications in optical amplification (Raman or Brillouin effects) and in optical processing (self-phase modulation, cross-phase modulation, ultra-short pulse generation, soliton propagation, four-wave mixing).

Recent developments in multiple light scattering from disordered turbid media such as colloidal suspensions are outlined. Interferences between time reversed scattering paths give rise to a coherent enhancement of the back-scattered intensity α. Its dependence on angle and on absorption can be described, in the diffusion approximation, as a function of the length distribution of the scattering paths. Dynamic fluctuations of α arising from Brownian motion of the scatterers are described similarly. Our experiments demonstrate the quantitative analogy between the time autocorrelation function and the dependence of α on scattering angle and absorption. This makes possible to study single particles dynamics even under conditions of strong multiple scattering and to probe motion at small time and distance scales not accessible in single light scattering. The effects of interparticle interactions are briefly discussed.

In this paper new methods for generation and detection of terahertz-phonons and application of these methods for a study of dynamical properties of high-frequency acoustic phonons in a crystal at low temperature are discussed. Terahertz-phonons are generated via multiphonon absorption by use of pulsed infrared radiation. For phonon detection phonon-induced fluorescence is observed. We have investigated the anharmonic decay of acoustic phonons. The experimental results indicate that the decay of acoustic phonons in the linear part of the dispersion curves can be described by a phonon decay cascade that can be characterized by a shape function. The optical techniques allow also to study for tetrahertz-phonons elastic scattering at impurities and the behaviour at crystal surfaces. We will, finally, discuss the role of the dynamics of tetrahertz-phonons for particle detection via phonons.

An overview is given of the known relations between various dynamical properties of fractal structure, such as vibration density, diffusion and electrical resistivity. Examples are given from the family of walk-generated fractals: SAW with and without bridges, k-tolerant walks and random walks. Results are also discussed for a deterministic model, depending on a topological parameter f which describes the relative scaling between the sizes of clusters and bridges in the fractal.

We discuss the scaling properties of a scalar model for fracture: The random fuse network with randomly distributed breaking thresholds. We present the current-voltage breaking characteristics for the system and we discuss its universal scaling behaviour. We consider then the distribution of local currents in the network and its moments: This distribution has multifractal properties just before the system breaks fully apart, whereas it exhibits constant gap scaling at the maximum of the breaking characteristics, that is at the end of the disorder controlled regime when the catastrophic regime sets in. All the presented results appear to be universal with respect to different distributions of quenched disorder in the thresholds and other models for fracture.

Recent experimental studies on the structure and dynamics of silica aerogels are reviewed. These tenuous materials can be prepared with porosities ranging typically from 80% to 99%. Small-angle neutron scattering indicates that these monolithic solids can have fractal structures extending over more than two orders of magnitude in length. The structural picture which emerges is that aerogels are made of dense particles of mean size a, forming a fractal assembly up to a correlation length ξ, beyond which the materials become macroscopically homogeneous. To these three structural regions correspond distinct vibrational regimes. With increasing frequency, and decreasing length scale, these are the phonons, fractons, and particle modes. We show how Brillouin, Raman, and inelastic neutron scattering give information on the dynamics of aerogels over a frequency range sufficiently extended to cover all three regimes. Evidence is given of a crossover from phonons to fractons, and the results allow the determination of both the fractal and spectral dimensions. The former dimension characterizes the scaling of mass, and the latter that of the vibrational density of states.

The possibility to contrast given fractions of a polymer system at a cost of a low enthalpy difference is responsible for the success of the neutron method. This approach is especially useful for polymers as compared to colloids or simple liquids, because of the significative intra- and intermolecular correlations. In this respect, the pseudo diblock copolymer constitutes one of the best test molecule of homogeneous polymer system.

A review is given of main results: Polymer size in melts, true backbone conformation and universal constants related to polymer structures. Effects of mechanical stress, or mesogenic order have been revealed. Neutron spinecho experiments have given characteristic dispersion relations of intra- and inter-polymer diffusive motions.

The labelling method is however more powerful than first realized. The amplitude associated with a polymer structure at an interface has been obtained directly (as in ellipsometry) with the use of contrast variation. Such structures are currently investigated by neutron reflectivity.

Latest developments are found in the use of spin polarized targets, from which important cross correlations are derived.

We report an experimental study of surface waves generated parametrically on a layer of liquid under vertical vibration. We show that only standing waves can be generated with the parametric forcing, and study the secondary instabilities of the standing wave pattern. In particular, we show the existence of time-dependent compression modes. We next consider the situation where the "fluid" consists of a layer of solid particles, and study surface instabilities connected with subharmonic bifurcations of the layer bouncing motion.

Dynamics of Langmuir films formed by surfactant molecules on the liquid-gas interface is investigated. All possible homogeneous phases of such films (liquid, nematic, smectic, crystalline) are considered. Boundary conditions for hydrodynamic equations in the presence of a film are discussed. The main attention is paid to low frequency surface modes whose spectrum is directly related with the symmetry of the film. Analogous program may be performed for membranes and films on the glass substrate. In the first case the capillary waves transform into a overdamped mode. In the second case the appropriate mode disappears, the dispersion laws of other surface modes are modified.

Wetting, adhesion, and adsorption phenomena are considered which involve the mutual interaction of low-dimensional objects such as interfaces, membranes, and polymers. In general, the direct interactions arising from intermolecular forces are strongly renormalized by thermally-excited shape fluctuations. This renormalization leads to a variety of unbinding transitions which exhibit complex critical behavior. The corresponding renormalization group flow is governed by a line of nontrivial fixed points and has a parabolic character.

When a crystal grows at a non-zero velocity, its roughening temperature is lower than at equilibrium. A criterion for such a dynamic roughening is β²/(aϱ_sΔμk_BT) ≈ r_c/ξ ⩽ 1 where β is the step free energy, a the step height, ϱ_s the crystal density, Δμ the difference in chemical potential (per unit mass) between the solid and the fluid phase, r_c the critical radius for the nucleation of terraces and ξ the correlation length. The physical origin of this property is that the large scale fluctuations of the crystal surface sees a lattice potential which is averaged to zero in time by the growth. From the point of view of the renormalization calculation, the dynamic roughening is a finite size effect which broadens the roughening transition. A good agreement has been found with measurements of the growth rate of helium 4 crystals. The case of helium 3 crystals shows that the disappearance of facets on slowly growing crystals may happen well below the true static roughening temperature if the coupling of the interface to the lattice is weak enough.

We briefly review the basic principles governing the spreading of a simple liquid on a solid surface. If the liquid completely wets the solid surface, the macroscopic wedge is preceded by a microscopic precursor film. If the liquid incompletely wets the solid surface, the heterogeneities of this surface play a major role.

These results are applied to study the Rayleigh–Taylor instability in a Hele–Shaw cell with open boundaries.

A short discussion of magnetoencephalography is given, with emphasis on multi-SQUID instruments. Three examples of auditory measurements, performed in the author's laboratory, are described briefly.

Recent progress in the theory of liquid–solid coexistence as approached from the liquid phase in systems with Coulomb forces is reviewed. Main attention is given to (i) Wigner crystallization of the electron gas in the degenerate and classical limits, and (ii) localization of bond particles leading to freezing in a pseudoclassical liquid-state version of the bond-charge model for elemental semiconductors. These models serve to illustrate crystalization driven by pure Coulomb repulsions and crystallization resulting from the interplay of attractions and repulsions in multicomponent systems, respectively.

Glass transition singularities as obtained within mode coupling theories can be classified according to underlying topological singularities in a parameter space. The general equations for the β-relaxation process are characterized by a few relevant parameters characteristic for every singularity. The simplest singularity, the Whitney fold, gives a dynamical scaling law with a critical power law decay, and shows features which are in general agreement with those observed near structural liquid-glass transitions. The Whitney cusp and higher order singularities are characterized by two or more parameter scaling laws, where time t enters as ln t and where the scaling times has a Vogel-Fulcher like dependence on control parameters. The critical relaxation follows a 1/ln^x(t) law with x = 2/(k − 2), k ⩾ 3. In this case the scaling laws give a relaxation pattern with features which agree with those observed in spin glasses.

Real experimental data on liquids include effects due to both pair and many-body forces, while computer simulations of liquids are made usually with a model pair potential. If this model is a good one, the difference between the experimental and simulated data is due to many body forces. Such differences may be of almost any size depending on the property and state examined. Because neutron scattering results depend upon the details of the structure and dynamics of the fluid samples, they are an excellent subject for the application of this technique. Thus they may be used to examine many-body forces of several different kinds and examples taken from the study of noble gas fluids will be given.

We briefly review the field of spin polarized fluids. The study of spin polarized hydrogen was initially motivated by the hope to observed a Bose–Einstein condensation in a dilute gas. This goal has not yet been reached but, on the way, a lot of beautiful physics has been done, including atom–wall interactions studies and the observation of nuclear spin waves. Spin polarized helium can be studied in gas, liquid and solid phases. The two major polarization methods are fast melting of polarized solid or fast condensation of optically pumped helium three gas. We briefly discuss heat conductivity changes under the effect of nuclear spin polarization, sound velocity, and finally liquid–gas phase diagrams.

Polarized liquid ³He can give us important information concerning the physics of this intriguing quantum system. The small nuclear Pauli-spin susceptibility of the liquid allows only modest polarization with current equilibrium brute force techniques, (typically Δ ⩽ 0.04 at 9 T). Quick melting of polarized solid can produce much higher polarizations and is giving us the possibility of testing several theoretical predictions. One such prediction is that the superfluid A₁ transition should increase as the square of the polarisation, Δ². We report on the first observation of an enhancement of T_A1, to 3.3 mK, due to transient polarisation. This is the highest ³He T_c up to now. An accident on the viscosity curve, between the A₁ and the A₂ transitions, may indicate the presence of a new superfluid phase.

Experiments on rotating superfluid ³He have revealed unique vortex phenomena not observed in other known superfluids. In the A phase, vorticity can be supported by continuous "coreless" vortices. In the B phase, the vortices have an intricate core structure. The observed vortex-core transition in rotating ³He-B is to be understood in terms of a spontaneous bifurcation of vorticity. The superfluid A–B interface is interpreted as a "vortex sheet"; the free ends—monopoles—of the ³He-B vortices on the rotating A–B interface are also discussed and novel isolated half-quantum boojums are described for the B-phase surface layer.

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