Table of contents

The 15th General Conference of the Condensed Matter Division of the European Physical Society took place in Baveno- Stresa (Lake Maggiore, Italy) from April 22 to 25, 1996. It was the 15th of a series that started in 1980 in Antwerp and whose last edition took place in Madrid in March 28–31, 1994.

More than 800 scientists from 37 countries attended the Stresa Conference. Through the efforts of the Organizing Committee and the support of several national and international Agencies, a large participation of scientists from Eastern European Countries (more than 100) was attained. On the other hand the participants from outside Europe were 30.

In such a frame, the Conference established itself as a scientific ground for a worldwide discussion of results in the field of condensed matter physics, in a way comparable to the March meeting of the American Physical Society.

The structure of the Conference allowed for a relatively large number of invited speeches, which are collected into this special issue of Physica Scripta.

The Conference was partitioned into six sessions (Semiconductors and Insulators, Magnetism and Metals, Superconductivity, Liquids and Statistical Mechanics, Polymers and Soft Matter, Surfaces and Interfaces), 14 Symposia and a Forum on Large Facilities. Two special Plenary Sessions were added to the customary structure of the Conference, namely: (i) Applied Physics, (ii) Science and Society, in order to emphasize the importance of Condensed Matter Physics for high-tech industrial applications and, more generally, for the development of our Society.

In this special issue, the Articles derived from Plenary (11) and Invited (41) speeches are published in the order of the Sessions. The reference to a given Symposium is specified in the contents.

Processible conjugated polymers can be used to fabricate a range of thinfilm diodes which can be designed to show good characteristics both as electroluminescent diodes and also as photoconductive diodes. We consider here the present understanding of the operation of a range of semiconductor devices. Field-effect transistors are readily fabricated, but are limited by low carrier mobilities. However, these can be improved by ordering the polymer films. For light-emitting diodes we highlight the improvement to the electroluminescence efficiency that can be produced by the use of two polymer layers selected so that the heterojunction between the two layers is able to confine charge and thus bring about electron-hole capture to generate excitons at this interface. Photon absorption produces excitons which are considered to be bound at room temperature. Charge generation requires ionisation of these excitons, which can be achieved efficiently at heterojunctions between layers with different electronegativities, and we discuss how this can be achieved at the interfaces between interpenetrating networks of electron- and hole-accepting polymers.

In recent years, a level of nanoscale fabrication has been achieved where dimensions and energy scales are such that quantum effects are dominant and can even be controlled in a variety of condensed matter systems. In this paper, several examples are given to illustrate the influence of nanostructuring on the physical properties of superconducting and normal magnetic systems. For superconductors we will discuss quantum interference effects in mesoscopic single loops and multiloop structures and the confinement of flux lines by a lattice of artificial submicron antidots. For magnetic layers we will analyze the size dependence of the Kondo and spin-glass effects.

The development of high resolution x-ray scattering techniques, particularly using synchrotron sources enables very detailed measurements of the properties of the fluctuations associated with continuous phase transitions. Surprisingly, the results of almost all high resolution studies of structural and magnetic phase transitions in crystals show that two length scales are needed to describe the experimental results in striking contradiction to the basic hypothesis of the scaling theory of continuous phase transitions. The measurements further suggest that the longer length scale arises from a region of the crystal close (50 μm) to the surface.

Possible explanations for the two length scale behaviour will be discussed including the effects of surface damage, the effects of surface strains, or intrinsic behaviour and some experiments outlined which might distinguish between these different possibilities.

Bose–Einstein condensation in a dilute gas of sodium atoms has been observed. The atoms were trapped in an optically plugged magnetic trap or in a cloverleaf magnetic trap. Rf induced evaporative cooling increased the phase-space density by six orders of magnitude. We summarize the different techniques used, and discuss recent studies of properties of Bose condensates with an outlook on future developments.

Intensity anomalies in the mass spectra of (C₆₀)_n, (C₆₀)M_x and (C₆₀)_nM_x clusters can be used to identify particularly stable fullerene and fulleride units. Small, n < 147, fullerene clusters appear to have icosahedral symmetry. Multiple layers of alkaline earth metals can be deposited on single C₆₀ molecules. The first four layers of metal are well described in terms of a periodically reoccurring icosahedral motif. The stability of alkali metal coated C₆₀, on the other hand, is better described by the completion of shells of electrons rather than shells of atoms. Finally, data are presented for shrink-wrapping fullerenes around alkali halide molecules.

Inelastic x-ray scattering experiments have been pushed to the meV energy resolution range at the European Synchrotron Radiation Facility in Grenoble. This opens the possibility to measure the high frequency dynamics in liquids and disordered systems. This technique has been used to study the collective dynamics in liquid water in the high frequency limit with the aim to investigate the long-standing issue of the fast sound. Propagating collective excitations have been found at momentum transfers between 1 and 14 nm⁻¹, and the transition from a velocity of sound close to the hydrodynamic value to the fast sound value of ≈3200 m/s is observed between 1 and 4 nm⁻¹. At Q larger than 4 nm⁻¹ the excitations propagate with the fast sound.

Quantum cascade lasers are new intersubband light sources in which the wavelength can be tailored over a broad range of the infrared spectrum using the same heterostructure material by changing the active layer thickness. Room temperature operation at 5 and 8.5 μm has been achieved. These are the first semiconductor lasers to operate at 300 K and above in this wavelength region.

Uses of permanent magnets are expanding rapidly with volumes doubling every five years. Hard ferrite and rare earth magnets account for most of the market, with bonded magnets of both types taking a growing share. Some typical static and dynamic applications with mechanical or active recoil are presented. Prospects for further materials development and new applications are discussed.

Magnetoencephalography (MEG) and magnetocardiography (MCG) are two completely non-invasive imaging techniques, suitable for basic and clinical studies of human subjects. Large SQUID arrays, operating at liquid helium temperatures, are employed for detecting and localizing magnetically the active regions, modelled by current dipoles, in the working brain or heart. The measurements must be performed in magnetically shielded rooms. Time resolution of both methods is 0.5 ms and spatial accuracy 5-6mm. Commercial multi-SQUID instruments are now available from several manufacturers. An adequate account of the theory and practice of both techniques is given. Three examples of brain studies and two investigations of the heart are described. The advantages and drawbacks of modern imaging techniques, including MRI and PET, are compared and the future of MEG and MCG is discussed.

Recent progress in the simulation of complex fluids is reviewed. It is shown that the use of recently developed Monte Carlo techniques allows the simulation of systems that several years ago were considered impossible to study via molecular simulations only. It is now possible to apply simulation techniques, which used to be limited to systems containing small molecules, to systems which are of interest for petrochemical applications. In this review, this development is illustrated by two examples of practical importance: the adsorption of alkanes in zeolites and the phase behavior of longchain alkanes.

Highly competitive world markets are drastically affecting research in industry, which is becoming more focussed on near-term results. This in turn has equally drastic implications for academic research. These issues are discussed for physics in general and solid state physics in particular.

Within Europe, plans are well advanced for the construction of a 100 Tesla prototype magnet. The availability of such fields, having a duration of several milliseconds, creates an opportunity for much new science. By considering the new regimes of magnetic length and cyclotron energy, some promising areas for such research are identified, with examples drawn from the fields of semiconductors, metals, magnetism and superconductivity.

A recent upgrade of the 590 MeV proton cyclotron at the Paul Scherrer Institute resulted in a high proton current of 1.5 mA which makes it attractive for neutron production by spallation. This has been realized with the spallation source SINQ which will produce the first neutrons in late 1996. Great emphasis is put on the optimum extraction and transport of cold neutrons for neutron scattering experiments. The key components include a cold deuterium source placed at the maximum of the flux distribution in the moderator as well as a sophisticated neutron guide system. Most of the instruments for neutron scattering are served by neutron guides which are based on novel neutron optical components such as supermirrors (with twice the critical angle of natural nickel) and beam focussing devices. All these features are extremely beneficial to gain intensity for the cold neutrons and enable first-class experiments in that spectral range. Future instrumental developments as well as their impact on condensed matter research are discussed.

We have recently devised a new method for ab initio simulation of solid-solid phase transformation under pressure. The method is here applied to the study of ice at Mbar pressure, the solid-state polymerization of acetylene under pressure and the generation of ta-C: H from compression of polyacetylene.

High brilliance and high stability make the European Synchrotron Radiation Facility an ideal instrument for structural studies at very high pressures, especially for low-Z materials were high compressibility and poor scattering power are associated with very small sample volumes. We review here measurements on hydrogen, oxygen and ice VII to pressures beyond 100 GPa using single crystal and powder techniques. A brief discussion of future prospects is also included.

The ability to synthesize diamond and germanium single crystals of high quality having controlled isotopic compositions provides a unique opportunity to study a variety of physical properties of these materials depending on their atomic masses. One of the effects studied arises from the zero point motion of the atoms due to their confinement in the solid phase giving rise to distinct molar volumes for chemically identical crystals with different atomic masses. The changes in molar volume have ben observed experimentally using precise x-ray measurements of their lattice constants. The elastic constants experience small changes which have not been detected despite precise Brillouin scattering measurements. The electronic energy levels experience shifts due to the virtual emission and absorption of phonons which depend on the isotopic mass. A discussion of the theory of these effects and of their experimental verification in diamond and germanium is presented.

This paper reviews recent optical studies of isotope effects on the electronic states in the elemental semiconductors diamond, silicon and germanium. The effects of lattice isotope substitution on band states and on point defects are shown to be understandable simply from the temperature dependence of the optical transitions. Attention is drawn to current studies of H in silicon, and to the increasing use of unstable isotopes to characterise the chemistry of point defects.

Si 2p core-level shifts for a series of molecules are calculated within the local density approximation to density functional theory. Using a pseudopotential approach, the shifts are calculated both in the initial-state approximation and including core-hole relaxation effects. Overall, calculated Si 2p shifts show good agreement with experiment.

Recent progress in semiconductor microcavities is reviewed. In the weak-coupling regime of light-matter interaction photon-mode control leads to high-efficiency light emitters. In the strong-coupling regime coupled mode behaviour between resonant cavity-modes and excitons is observed. Both regimes have numerous applications through novel quantum optics phenomena.

A new type of microcavity system which allows a control of the resonances by tuning the Fabry-Perot resonator on a transition of the cavity material itself is presented. The resulting interaction between semiconductor excitations and the electromagnetic radiation can be manipulated according to the laws of QED as in usual QW microcavities. We have examined experimentally and theoretically two very different materials: bulk GaAs and porous silicon. In the first case one has to deal with a 3D exciton, whose center of mass is delocalized over the radiation wavelength, while in the second case the excitations are bound to small silicon nanoclusters. The mixing of exciton-photon modes and their coherent dynamics as well as satellite effects due to spatial dispersion are observed in GaAs cavities. Reduction of linewidth and emission intensity enhancement are obtained in the case of porous silicon. Non-linear behaviours originating from high carrier densities are also studied in both systems.

Employing deep reactive ion etching, we have realized narrow doped quantum wire arrays with lateral dimensions down to 60 nm and wires with periodic modulation of the width along the wire axis. The wire systems are characterized by means of inelastic light scattering from confined plasmons. These modes display unique Raman selection rules which we explain by taking into account both the polarization and the parity of the local electric field distribution in the wires. Lateral electric widths and carrier densities of the wires are deduced from the comparison of the plasmon dispersion with classical models. The electric widths are found to equal the geometrical ones, and the densities are comparable to the initial one of the unpatterned sample. Opening of forbidden gaps in the plasmon dispersion is observed in the presence of a width modulation and is quantitatively described in the frame of a classical model. This constitutes clear evidence, in contradiction to previous reports based on magneto-transport and microwave transmission, of the absence of lateral depletion in our experimental conditions.

Lateral superlattices with very small lattice periods approaching the Fermi wavelength in size are fabricated on AlGaAs heterostructures by electron beam lithography. The potential modulation is induced by a gate voltage applied to a laterally modulated gate electrode. In our samples the two-dimensional electron gas is located very close to the sample surface, thus allowing us to induce very small lattice periods with a relatively large amplitude of the potential modulation. For weak potential modulation, which can be tuned by the gate voltage, we observe a modulation of the amplitude of the Shubnikov-de Haas oscillations arising in the magneto-resistance at quantizing magnetic fields. For more negative gate voltages, i.e. larger amplitudes of the potential modulation, certain maxima of the Shubnikov-de Haas oscillations split up reflecting the energy spectrum that arises when a given number of flux quanta penetrates a unit cell of the lattice. We compare our experimental results with theoretical predictions for the so-called Hofstadter butterfly energy spectrum and we conclude that we can experimentally detect the major energy bands and gaps.

We studied weak localization corrections to spin and particle transport parameters of III-V semiconductor quantum wells. The intrinsic spin splitting of electron energies in the conduction band was taken into account. At the region of times larger than the D'yakonov-Perel' spin relaxation time we found drastically different localization behaviours of the spin and particle diffusion coefficients. We also have shown that the weak localization corrections are strongly dependent on the crystal orientation of material within the quantum well. In particular, no antilocalization behaviour was found for the particle diffusion in case of [011] orientation in contrast to [111] and [001] oriented quantum wells. Possible experimental detection is suggested.

Rapid advances in the growth techniques of multilayer structures incorporating molecular as well as covalent crystals allow room for designing novel systems with great flexibility. We have recently considered the hybrid combination consisting of an organic and an inorganic quantum well in which the fundamental electronic excitations have a mixed character of Frenkel and Wannier excitons. In such a structure, the properties of both molecular and covalent crystals conspire to overcome the basic limitations of the individual constituents and a greatly enhanced nonlinear optical response can be achieved. Beside a discussion of the theoretical model, we present here the calculated spectra of the hybrid exciton polariton resonances for specific experimental configurations: attenuated total reflection in the non-radiative regime and normal incidence transmission and absorption in the radiative regime.

An overview of the theoretical and experimental efforts in obtaining a photonic band gap, a frequency band in three-dimensional dielectric structures in which electromagnetic waves are forbidden, is presented.

The integration of high-speed lasers with electrical switches on a semiconductor chip is now occurring in commercial devices. The evolution of these devices during the last decade is described, with emphasis on both circuit engineering and materials design and processing. These devices have recently been used to transmit 10¹² bits/sec through a single optical fiber.

The ground state of a two-dimensional electron gas at Landau level filling factor ν near 1 is studied in the Hartree-Fock approximation. For Zeeman coupling different from zero, the ground state is a Skyrme crystal with both spontaneous long range order in the charge density and spontaneous long range order in the transverse spin density. The energy of the Skyrme crystal is lowest for a square lattice structure with opposing postures for topological excitations on opposite sublattices. We discuss interpretations of our results in terms of non-linear σ and generalized spin models for quantum Hall ferromagnets. For zero Zeeman coupling, we find that the ground state can be interpreted as a meron crystal with two interpenetrating sublattices, each supporting quasiparticles with charge e/2.

Recent developments in the theory of hyperfine interaction between the nuclear spins and the spins of two dimensional electrons under strong magnetic field and its relevance to the rapidly growing experimental activity in this field is discussed. The crucial role of electron correlations on the nuclear spin-lattice relaxation time and on nuclear spin diffusion is described in the formalism where the elementary excitations are the spin-excitons and skyrmions.

The low-lying bulk excitations of two-dimensional electron systems subject to a strong perpendicular magnetic field have been studied using phonon absorption. Results reported earlier lead to the conclusion that the magnetoroton minimum energy in the fractional quantum Hall effect can be measured by phonon absorption. Recent time-resolved measurements have demonstrated fundamental differences in phonon absorption between fractional quantum Hall states, non-rational filling factors and filling factors close to 1/2. At non-rational filling factors the response is bolometric, whereas at filling factors corresponding to FQHE states the response also depends on the spectrum of the heater pulse. Around ν = 1/2 the response is largely independent of magnetic field with a weak maximum at ν = 1/2.

Long wavelength collective neutral excitations of the fractional quantum Hall liquid at filling factor v = 1/3 have been observed by inelastic light scattering. We will discuss the neutral excitation spectrum of this liquid as well as the theory of the light scattering. We will then consider how these ideas may be used to interpret recent experimental results.

We briefly review some recent photoelectron spectroscopy (PES) studies of quasi one- and two-dimensional metallic systems. We discuss the spectral features of a metallic system, the expected fingerprints of the metal-insulator transition and some experimental and conceptual limitations of the single particle framework. This approach gives a satisfactory interpretation of PES experiments in 2D charge-density-wave materials. The single particle picture is however incompatible with experiments on 1D systems.

Optical reflectivity measurements over a wide spectral range and at different temperatures together with self consistent electronic FLAPW calculations within LDA have been used to investigate the electronic structure of the compounds. A remarkable agreement between theory and experiment has been found which allows for an interpretation of the origin of the different degree of f-d hybridization in these Yb-based materials.

Intermetallic compounds of Mn and Group III elements constitute an interesting class of magnetic material, suitable for epitaxy on semiconductor substrates. By exploiting the epitaxial relationship to GaAs, e.g. metastable magnetic τ MnAl can be fabricated with the c-axis of the tetragonal crystal parallel to the substrate normal. Consequently, perpendicular magnetization due to the large magnetocrystalline anisotropy in τ MnAl is found. The resulting magneto-optic Kerr and extraordinary Hall effect has generated interest in applications such as optical storage and non-volatile magnetic solid state memories. Tayloring the magnetic properties of the epitaxial heterostructures can be done by post-growth annealing or micro-patterning. More flexibility is achieved with multilayers (e.g. τ MnAl/NiAl) and superlattices (e.g. τ MnAl/Co). In the latter all-magnetic superlattice on GaAs, a forced bcc Co structure and strong coupling effects are under study. Further, we recently introduced heterogeneous magnet-semiconductor materials based on GaAs with a high density of nanometer-scale spherical magnetic MnAs clusters grown by MBE. Some of the interesting physics to be explored in magnetic material and semiconductor combinations will be discussed.

Empty electron states in Fe-based systems have been studied by Inverse Photoemission spectroscopy excited by low energy polarized electrons. Along with bulk-like transitions, surface (or interface) features are clearly identified. Ultrathin epitaxial films, namely Fe/Au(100) and Ag/Fe(100), and the clean Fe(100) surface are considered as examples. In thin films, majority and minority quantum well states originated by the electron confinement within the overlayer are observed. For ferromagnetic films in the monolayer regime, data indicate an exchange splitting exceeding the bulk value. At the Fe(100)-vacuum interface image-potential derived surface states display an unexpected reversal of the ordering of the majority and minority levels. Spin resolution allows to unambiguously identify the spin character of the states.

We have investigated the relative change of sound velocity and internal friction in amorphous and polycrystalline solids at frequencies 0.1 kHz ⩽ v ⩽ 20 kHz and in the temperature range 0.04 mK ⩽ T ⩽ 10 K. We found that the temperature dependence of the acoustic properties at 0.1 K ⩽ T ⩽ 1 K can be well understood within the standard tunneling model assuming the existence of tunneling systems with a broad spectrum of energy splitting and relaxation rate. At T < 0.1 K the sound velocity is strongly strain amplitude dependent and the sound velocity and internal friction show substantial deviations from the standard tunneling model. The acoustic properties of polycrystalline metals resemble those of amorphous dielectrics with the same strain dependence. In contrast to amorphous superconductors no difference in the acoustic properties has been observed between the normal and superconducting states of polycrystalline superconductors. At T > 1 K the sound velocity decreases linearly with temperature. This dependence can be explained quantitatively by a new relaxation mechanism arising from overdamped tunneling systems.

After a brief description of the experimental technique used in the authors' diffusion studies of metallic glasses, a representative selection of their self-diffusivity data referring to both the unrelaxed and the relaxed glassy state is presented. On the basis of these observations a model of the diffusion mechanisms operating in metallic glasses is developed and considerably refined with the aid of what has been learnt on diffusive atomic-displacement mechanisms in Fe-Zr glasses from molecular-dynamics simulations. Finally, the strength of these ideas is demonstrated by straightforwardly explaining the extraordinary diffusion-enhancement behaviour of metallic glass under light-ion bombardment and by elucidating the diffusion mechanisms in supercooled molten alloys just above the glass transition temperature.

We present a theoretical investigation on point-defect formation, migration, and clustering in silicon by tight-binding molecular dynamics. We also provide, by means of first-principles Hartree-Fock calculations, a detailed chemical characterization of the Si-Si bond at the split ⟨110⟩ self-interstitial defect.

Based on macroscopic quantum coherence effects arising from pair tunneling and flux quantization, a series of tricrystal experiments have been designed and carried out to test the order parameter symmetry in high-T_c cuprate superconductors. By using a scanning SQUID microscope, we have directly and non-invasively observed the spontaneously generated half-integer flux quantum effect in controlled-orientation tricrystal cuprate superconducting systems. The presence or absence of the half-integer flux quantum effect as a function of the tricrystal geometry allows us to prove that the order parameter symmetry in the YBCO and Tl2201 systems is consistent with that of the d_x²−y² pair state.

Infrared reflectivity and/or absorption measurements in chemically doped perovskites, including high-T_c superconductors, show polaronic features similar to those observed in the photoinduced spectra of cuprates. These features, whose intensities depend on doping and temperature, are observed in both n-type and p-type materials. They include extra-phonon infrared-active vibrations (IRAV) and a broad band, centered at ~ 1000 cm⁻¹, which results from overtones of IRAVs and is well described in terms of photoexcitation of polarons. Most of the above features survive as extra-Drude peaks in the metallic phase of these materials. The strength of the polaron band increases for decreasing temperature, as expected for weakly bound polarons, in compounds which are parents of high-T_c superconductors. It decreases, instead, as expected for tightly bound polarons, in a Mn-based perovskite and in CuO, which do not become superconducting upon doping. A comparison with tunneling results shows that the superconducting carriers are strongly coupled to the IRAVs.

We present data on the copper isotope effect α_Cu = − ln T_c/Δ ln m_Cu for oxygen depleted YBa₂Cu₃O_7−δ as function of δ. α_Cu is negative for all δ. In the 60 K plateau, α_Cu reaches values of −0.14 or −0.34 for two comparison pairs. Outside the 60 K plateau, α_Cu increases in absolute value. Similarities with results on dT_c/dP in this system suggest that α_Cu is related to mass-dependent ordering in the chain system.

Surface superconductivity usually refers to the nucleation of superconctivity near the outer surface of a bulk specimen. The same intrinsic phenomena takes place in a patterned thin film giving rise to localized superconducting edge states near any free boundary, thickness jump or local variation of the vector potential. We have investigated a thin superconducting film with an array of patterned microholes. A splitting of the critical field line is found when the field is perpendicular to the film. The new transition line (H_c3) exhibits two types of fine structure due to the quantization of the multiply connected edge states. We discuss these states in terms of multiquanta vortices nucleated inside the holes. We report Bitter decoration of the the corresponding vortex configuration. Because of the enhancement of the order parameter near the hole edges, the interaction between the vortices and the microholes is repulsive slightly below the bulk critical field (H_c2) and is much stronger than the intervortex interaction. We find a "localization transition": all vortices drop inside holes when the coherence length becomes larger than the interhole spacing. This transition explains the two types of oscillation found in the transition line and leads to a qualitative change of the creep below the second critical field.

Mixed state properties of highly anisotropic layered superconductor are studied with the out-of-plane resistance in high magnetic field, up to 15 T, applied parallel to (ab)-plane of Bi₂Sr₂CaCu₂O_x (BSCCO-2212) single crystal with T_c > 90 K. Sharp (ΔT ⩽ 10 mK) hysteretic jumps in resistance temperature and field dependencies are discovered within the range of fields 5.5 ⩽ H ⩽ 11 T. Beyond that range of fields the jump-like features disappear while pronounced deviations from Arrenius law remain till 15 T. Jump height and its hysteresis are nearly sweep rate independent but could be eliminated by sufficiently high excitation current. The effect is found to be completely wiped out by a minor tilt of the crystal from the parallel orientation H||(ab), therefore it could be attributed to the Josephson vortex properties and possibly related with the first-order phase transition between the commensurable Josephson vortex lattices predicted by Bulaevskii and Clem.

Various properties of the condensate wave function of a Bose gas confined in a magnetic trap are discussed. Special emphasis is given to the role played by the interaction in determining the wave function in both co-ordinate and momentum space. The predictions of the Thomas-Fermi approximation are compared with the solution of the Gross-Pitaevskii equation. Explicit results (both numerical and analytical) for the kinetic energy of the condensate are presented.

For the first time the classical hydrodynamics problem of stationary solutions in Rayleigh-Taylor instability (RTI) has been studied for three-dimensional space periodical flow. The existence of RTI stationary solutions multitude is shown for the general case of flow symmetry. The multitude dimension in functional space is studied ab initio by analysing an initial perturbation symmetry in the instability linear stage. The relation between flow symmetry and solutions multitude is established. The description of the stationary solutions for flows with five symmorphic plane groups symmetry are given. The RTI stationary flow free boundaries problem is derived in the first and second orders of approximations. Velocity and Fourier-amplitudes dependencies as parameters functions are obtained. The symmetry violations are examined, the limiting transitions to well known cases of plane and "high" symmetry flows and their accuracy are outlined. The physical reasons of the obtained incorrectness for the free surface problem in Rayleigh-Taylor instability are discussed.

It is a long-standing experience, as expressed by Ostwald's Rule of Stages, that metastable phases can play a dominant role in phase transformations. Presently the crystallisation of polymers, polyethylene in particular, has thanks to the special mesomorphic features of one of its phase variants, provided an example by which this old experience can be rationalised. In the course of it new perspectives have arisen in the field of polymer crystallisation in its own right, and beyond it new possibilities have opened up for the accounting for and hopefully control of polymorphism in crystal growth in wider generality.

This paper describes research activities aimed at developing polymer materials that will support and promote the transition from computing and communication based on electricity to optical handling of data streams. It deals with chemical synthesis of polymers possessing a well-defined, functional basic structure, and with physical characterization at the (sub)molecular, supramolecular and macroscopic levels, including surfaces and interfaces. The emphasis is on light-emission properties.

The depth of exciton and electron traps arising due to the presence of structural defects in organic crystals consisting of dipole molecules were calculated. In the model studied the structural defect appears if a molecule is rotated by 180° along the long axis as compared to the bulk molecules. The contribution of the electrostatic interaction to the depth of the trap state on such a defect is studied in detail. For crystals consisting of molecules which have large dipole moment in ground state and adopt large values in excited states this contribution may be dominant. The calculations are carried out for triplet excitons and electrons in benzophenone crystal.

In thin amorphous polymer films, the entanglement density, v_e, strongly influences the draw ratio during micronecking and the competition between cavitational and non-cavitational processes, often referred to as crazing and shear deformation respectively. TEM investigations suggest this also to be true of polymers capable of crystallizing, although modes of deformation involving chain slip may be suppressed owing to strain induced crystallization. In either case, v_e is determined not only by the intrinsic properties of the polymer in question (chain stiffness and packing density), but also by the sample history. In melt crystallized semicrystalline films, for example, if crystallization takes place relatively slowly compared with the rate of relaxation of chain conformations in the melt, v_e may be substantially reduced owing to a decrease in chain interpenetration. Evidence for this comes from observations of the draw ratios in micronecks in isotactic polypropylene films isothermally crystallized at different temperatures, which increase sharply in regimes where entanglement loss is predicted to occur during crystallization.

A review of the most recent experimental progress in the investigation of the dynamics of the interaction of dioxygen with Ag single crystal surfaces is given. In such studies the dioxygen molecules are dosed by a supersonic molecular beam, which allows for angle and energy selectivity. Moreover the background pressure remains very low during dosing thus minimizing surface contamination. Energy and angle dependence of the initial sticking probability was investigated for the different low Miller index surfaces. We find that non dissociative adsorption occurs below 150K only on Ag(110) and (001) with the same activation barrier. The dependence of the sticking probability on the angle of incidence is anisotropic with respect to surface azimuth for Ag(110) because of the anisotropic surface corrugation. Dissociation is always promoted by chemisorbed O₂ and, when heating the crystal, it prevails over desorption for Ag(110), while the contrary holds for Ag(001). For the latter case we find also evidence for subsurface diffusion at molecular dissociation. The coverage dependence of the sticking coefficient as well as of the TDS spectra are indicative of repulsive interactions between the admolecules, which are particularly strong for Ag(001).

Ag on Al(111), Co on Cu(111) and Ag on Ag₂Al(0001) are studied by means of low-energy electron diffraction and secondary-electron imaging during the initial stage of growth. The latter technique reveals in real space the crystallography of a near-surface region of a solid even in the absence of long-range order. The results show that Ag initially forms disordered islands on Al(111), while perfect epitaxy develops after 6 monolayers (ML). Ag vapor deposited onto Ag₂Al(0001) grows in-registry, however, in the hcp structure up to 6 ML. The structure gradually, but not completely, changes to the fcc for thicker layers. Co predominantly grows on Cu(111) in-registry for the first 2 ML, and a mixed fcc-hcp stacking sequence is found in the next 2 ML. For thicker Co layers, the hcp sequence dominates. An unusually small energy difference between the fcc and hcp stacking sequence in Ag is responsible for this observation. The results are discussed in the light of theoretical predictions that relate the growth mode to the d-band occupation.

We report our optical investigations on the alkali-doped C₆₀ over a broad spectral range, extending from the far-infrared up to the ultraviolet. The occurrence of superconductivity in A₃C₆₀ compounds has raised quite a bit of controversy with respect to arguments, favouring pairing mechanism based on electron-phonon interactions with low frequency intermolar vibrations or with high frequency intramolecular modes. We discuss our experimental results within the standard Eliashberg electron-phonon theory of superconductivity, in order to single out the relevant phonon excitation for the pairing mechanism. The Eliashberg calculation strongly supports a pairing mechanism mediated by high-frequency intramolecular phonon modes, in accord with the implications of the weak coupling BCS limit. Moreover, we present the electrodynamic response of the quasi-one-dimensional A₁C₆₀ compounds. Our experimental findings demonstrate that Rb₁C₆₀ and Cs₁C₆₀ undergo a metal-insulator phase transition below about 50 K, while K₁C₆₀ remains, however, metallic at all temperatures. The possibility of a broken symmetry ground state due to the formation of a spin density wave (SDW) condensate for the Rb and Cs compound, as suggested by the magnetic properties, is discussed.

We study the origin of the nanometric resolution seen in BEEM experiments. The problem is separated in three steps: (1) tunnelling from the STM tip to the metal, (2) elastic propagation in the metallic layer, and (3) overcoming the Schottky barrier in the metal-semiconductor interface. For the first step we apply an LCAO model and a Green's functions formalism due to Keldysh. The second step is investigated extending our formulation for step 1. To compute the necessary Green's functions we use an asymptotic approximation for thick layers where metallic band structure effects are included, and a decimation technique for thin layers. The third step is analyzed by looking at the matching of the incoming electrons and the available states in the semiconductor. For thick metallic layers (≈100–150 Å), the elastic interaction between propagating electrons and the metallic periodic lattice results in a few focalized beams of nanometric size, providing a simple explanation for the observed resolution. For thin metallic layers (≈20–50 Å), we study the propagation layer by layer and we analyze the distribution of current as a function of the depth in the metal.

In this work we present an experimental method which allows to investigate experimentally the energy and momentum distribution of the carriers which are injected by an STM tip into a metallic sample. We perform in situ ballistic-electron-emission microscopy (BEEM) and spectroscopy (BEES) simultaneously with atomic-resolution STM. The experiments are done at 77K on epitaxial CoSi₂ films on Si(111) and Si(100). The potential of the method to improve on our understanding of tunneling on an atomic scale will be illustrated by two important findings: For the first time we have directly observed variations of the tunneling distribution on an atomic scale. The BEEM data also give evidence for a pronounced forward focussing of the angular distribution of the tunneling electrons.