Table of contents

The 13th General Conference of the Condensed Matter Division of the European Physical Society was held in conjunction with the Frühjahrstagung des Arbeitskreises Festkörperphysik der Deutschen Physikalischen Gesellschaft from March 29 till April 2, 1993, in Regensburg. The programme comprised 3,134 contributions : 8 Plenary Talks, 171 Invited Talks, 1,480 Contributed Talks, 1,441 Poster Presentations, 1 Public Evening Talk and 33 Exhibitors Reports. The abstracts have been published as Europhysics Conference Abstracts, Volume 17A/Verhandlungen der Deutschen Physikalischen Gesellschaft 5/1993. The table (see PDF file) shows the distribution of the Plenary and Invited Speakers as well as of the participants according to countries within and outside of Europe.

The conference was the largest meeting of physicists held in Germany to date. It was a manifestation of the enormous scientific activity in both basic and applied research in the fields of Condensed Matter Physics in Europe. Most of the research work, which was presented at the conference, was done by young physicists. They represent a large human capital in Europe. Most of the senior physicists and many of our young colleagues maintain scientific cooperations, and also personal friendships, which are and which have been almost independent of national barriers over the past three decades. The latter is to a large extent due to the European Physical Society which always cultivated these contacts, especially between the eastern and western parts of Europe.

We would like to express our sincere thanks to the members of the Programme Committee. By their intensive work, which was free from national interests, a scientific programme was prepared, which covered the entire field of Condensed Matter Physics. About 70% of the Plenary and Invited Speakers came from 20 different foreign countries and about 30% from Germany. The meeting therefore has been a truly European Conference. For the young physicists, the number of which traditionally has been very large, the conference was an excellent opportunity to discuss their own research work and to get an impression of the broad research field in Condensed Matter Physics on an international level and also to cultivate scientific and personal friendships.

We also would like to thank the city of Regensburg with their major, Mrs Maier, and the University of Regensburg with its Rektor, Professor Altner, for their overwhelming hospitality, Mr Ernst and his about 60 coworkers for their excellent local organization and Dr. Heinicke and his staff for the production of the abstract book. Finally we would like to thank the members of the Editorial Office of Physica Scripta for the smooth cooperation in the production of these proceedings, which comprise the papers of about 70% of the Plenary and Invited Speakers.

This survey shows experiments by means of high resolution neutron spectroscopy and small angle scattering of neutrons for the investigation of fluctuations and relaxation in macromolecular and molecular matter, in particular relaxation phenomena in different molecular glasses, polymer diffusion in the molten state, relaxation of concentration fluctuations, and critical phenomena in polymer blends.

Quasicrystals uncovered in aluminium alloys are still a matter of intense research regarding their structure which does not obey the rules of classical crystallography. Today, they also offer a new challenge in relation with their specific electronic properties and surprising mechanical behaviour. These properties are summarized in this review in the light of some structural data. The technological relevance of these results is briefly advocated.

The present status of applications of high-T_c superconductivity (HTSC) is reviewed. Rather than discussing selected topics in detail, an overview is given over a wide spectrum of applications in such various fields as microwave applications, electronics, infrared detection, magnetic sensors (SQUIDs) and energy technology.

The understanding of the nature of quantum interference in disordered conductors gave the strong impulse for investigation of disordered systems. There were explained a lot of old puzzles in physics of conductors (e.g. anomalous temperature dependence on resistivity and different kinetic effects, the negative magnetoresistance etc.) and discovered new attractive phenomena. Such effects as oscillating effects in multiconnected systems, universal conductance fluctuations, local fluctuating currents in conductors have been converted in new branches of physics of disordered metals. The development of this new physics during the last years will be discussed.

It is a great pleasure for me to talk today on behalf of my friends and colleagues. I would like to show the schematic picture where I tried to sketch the basis and development of electron coherence and interference phenomena in disordered conductors. The main basis of all this field of metal physics is the Anderson–Mott transition in disordered systems. This transition separates all substances on two groups – metals and dielectrics. Three trees grow on this basis now on the metal side: the weak localization, interaction effects and mesoscopics.

Heavy fermion systems are metallic rare-earth or actinide compounds with extremely strong electronic correlations. The latter are reflected in the fact that the f-states exhibit both localized character at high temperatures while they appear as delocalized yet strongly renormalized quasiparticles when low-energy excitations are considered. The central result of the present paper is the confirmation of this general picture. The key to the problem is a realistic, i.e., material-specific description of the low-energy excitations in heavy fermion systems. Of particular interest is the question whether Luttinger's theorem is valid. The strongly renormalized quasiparticle bands are successfully described by the renormalized band method. This ansatz allows one to integrate correlation effects into the conventional band structure calculations in close analogy to Landau's theory of Fermi liquids.

Recent developments in Si/Ge heteroepitaxy led to spectacular improvements of electrical and optical properties of Si based heterostructures and superlattices. We discuss the realization of high mobility electron and hole channels in Si or Ge quantum wells, the band gap related luminescence in SiGe quantum wells and strain symmetrized Si/Ge short period superlattices and far-infrared intersubband transitions in narrow quantum wells. Possible device applications are pointed out.

The present status of the field-theoretic renormalization-group (RG) approach to finite-size effects in ⁴He near the superfluid transition is reviewed. The perturbation theory is discussed for periodic and Dirichlet boundary conditions. Results of RG calculations of the specific heat, the superfluid density and the thermal resistance near T_λ are presented and compared with experimental data.

The excitations in liquid ⁴He are phonons and rotons. They have been extensively studied both theoretically and experimentally over a long period and we now have a considerable understanding of them. This work is briefly reviewed here. There are still important outstanding questions such as how phonons and rotons are created by hot surfaces, the scattering of high energy excitations and the nature of the roton.

Classical nucleation theory relies on the capillarity approximation: the assumption that the free energy of a small cluster of a new phase can be described by a bulk free energy difference (from thermodynamics) plus a surface term (with effects of curvature neglected). We have used density functional theory to develop a nonclassical theory, one which allows the order parameters to vary through the cluster in order to give the lowest possible barrier to nucleation. This approach has been applied to crystallization of liquids from the melt and to condensation of liquids from the vapor, and shows significant deviations from classical nucleation theory. A simple, exactly soluble model will be introduced and used to explore the limits of validity of classical nucleation theory.

The theoretical ideal of hard sphere particles can be approached by sterically stabilized colloids, which are index matched in nonpolar solvents. Indeed such systems show the expected fluid to solid phase transition at volume fractions close to 0.5. The kinetics of this transition were investigated by time resolved small angle light scattering. Typical scattering patterns are contracting rings, which follow different scaling laws through the initial period of nucleation and growth and the late time period of coarsening or ripening.

This talk deals with probability density function (pdf) of longitudinal velocity differences, namely

δv = v_r(x + r) − v_r(x)

where v_r is the component of the velocity v along r. These pdfs evolve from a gaussian shape for large scales r, to a strongly non gaussian one with reinforced tails (intermittency) for r close to the dissipative length. Recently some light was put on this evolution by a theory which takes into account the finite value of the Reynolds number. In some sense, intermittency could be a finite Reynolds effect.

We highlight recent results gained by a method to approximately solve the Navier–Stokes equation for high Reynolds numbers. Within this approximation, intermittency corrections are found to be scale dependent, large in the viscous subrange, moderate in the nonuniversal stirring subrange but, surprisingly enough, extremely small if not zero in the inertial subrange.

We discuss a shell-model for the 3D fully developed turbulence. By calculating the structure functions we find corrections to the Kolmogorov theory due to an intermittent structure of energy dissipation. The intermittency can be described in terms of a multifractal scaling and it is related to the Lyapunov exponents and eigenvectors. Moreover we obtain that the maximum Lyapunov exponent λ and the variance of the effective Lyapunov exponent diverge as a power of the Reynolds number with scaling exponents which can be calculated in terms of a multifractal spectrum.

An effective mixture method, which allows the treatment of polydispersed fluids, is presented. Within this method, correlation and thermodynamic functions are evaluated solving the multicomponent Ornstein–Zernike equation with the Rogers–Young closure. The method is tested by making comparisons with simulations and with experimental data on colloidal suspensions.

The rheological behavior of a suspension of Brownian spheres at low Reynold numbers can be predicted with the help of Stokesian dynamics. In this approach the stress on each particle is calculated for a given configuration of the particles, then the configuration is changed according to the equations of motion based on Stokesian dynamics. The average stress follows from the time average over the configurations. The shear viscosity obtained by this method compares very well with the experimental results, both as function of the volume fraction and of the Peclet number. The origin of shear thinning and of shear thickening behaviors is respectively due to the saturation of the Brownian stress with the shear rate on one hand and to the formation of large aggregates on the other hand. At intermediate Peclet numbers a shear induced ordering is observed which is not directly linked to the behavior of the viscosity. Other kinds of transitions induced both in the presence of a shear flow and of an electric field will be presented.

The partially crystalline structure of fluorinated latex particles makes them optically anisotropic and has relevant consequences on the average scattered intensity and on the time correlation function of the scattered light. The temporal fluctuations of the depolarized component of the light scattered by optically anisotropic colloids allow to determine the particle self-translational and rotational diffusion coefficients. Static depolarized intensity is moreover generally insensitive to interparticle interaction effects and may then be used as an accurate local probe of particle concentration. We show that the peculiar scattering properties of anisotropic particles can be profitably exploited in experimental studies of the dynamics of strongly interacting particles, gravity sedimentation profiles, irreversible colloidal aggregation and particle motion in polymeric gels.

A mode-coupling theory of entangled polymer liquids has been developed and extensively applied to dense melts of long chains. The theory predicts the emergence of a plateau shear modulus due to dynamical correlations induced by chain connectivity and excluded volume interactions. Mode-coupling predictions for the dependence of transport coefficients on degree of polymerization agree with the phenomenological reptation/tube theory. Anomalous early and intermediate time diffusion also occurs due to power-law decay of the fluctuating force memory functions. The mode-coupling renormalizations are found to depend sensitively on spatial and polymer fractal dimensions in analogy with dynamic critical phenomena. An alternative, self-consistent approximation for the memory functions results in the prediction of a nonreptation viscosity scaling law for chains, and an ideal "entanglement-induced" glass transition at very large molecular weights.

Viscoelastic surfactant phases often contain long, flexible aggregates that behave as reversibly breakable polymers. A model that couples the reptation of the micelles to their reversible breakdown explains many recent experimental observations. Recent progress in theory and experiment is reviewed, with an emphasis on extracting quantitative structural and kinetic data from viscoelastic spectra. A rheological theory of nonlinear response in steady shear is described. The microscopic constitutive equation displays a flow instability, which leads to a plateau in the shear stress under increasing shear rate. This is quantitatively in accord with experiment.

We discuss the shape formation and shape transitions of simple bilayer vesicles in context with their role in biology. In the first part several classes of shape changes of vesicles of one lipid component are described and it is shown that these can be explained in terms of the bending energy concept in particular augmented by the bilayer coupling hypothesis. In the second part shape changes and vesicle fission of vesicles composed of membranes of lipid mixtures are reported. These are explained in terms of coupling between local curvature and phase separation.

Photoexcitation experiments performed in oxygen deficient but metallic YBa₂Cu₃O_xc-axis oriented films revealed that the persistent photoinduced enhancement of conductivity and superconductivity are not restricted to the insulating side of the metal-insulator transition. A clear increase of the critical temperature T_c, which is accompanied by a decrease of the resistivity in the normal state ρ_ab, is observed for all studied oxygen contents (6.4 ⩽ x ⩽ 6.6). In this oxygen content range, the persistent photoinduced changes in T_c and ρ_ab have a strong dependence on the oxygen stoichiometry of the film. Several mechanisms which may explain the persistent photoinduced phenomena are discussed.

Phase separation into metallic and antiferromagnetic regions in cuprate superconductors was proposed from theoretical models recently. In this contribution we want to provide some experimental evidence for phase separation from nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR) and microwave absorption (mw) experiments. We distinguish "chemical" and "electronic" phase separation. Furthermore a distinction between "static" and "dynamic" phase separation is made.

We review the NMR results in the normal state (T > T_c) of superconducting YBa₂Cu₃O_6+x single crystals as a function of oxygen hole doping. We show that the characteristic energy Γ = πωχ'/χ'' for low-energy excitations can be deduced both from NMR (for antiferromagnetic Q_AF and zero wave vector), and from inelastic neutron scattering for Q_AF only. In underdoped (dT_c/dx > 0) samples Γ(0) is found to be constant down to T_c, while Γ(Q_AF) starts to increase below ∼ 150 K, i.e., well above T_c, reflecting the opening of the pseudogap for magnetic excitations at Q_AF. In overdoped samples (dT_c/dx < 0) there is only the superconducting gap opening at T_c, supporting the phase diagram based on the spin-charge separation.

NMR of ¹⁶⁹Tm and NQR/NMR of ^63/65Cu was applied to investigate the order of oxygen in the HT_c superconductors of the type RBa₂Cu₃O_x. From the spectra of three different lattice sites, one for Tm and two for Cu, it is evident, that three ordered structures exist: tetra for x = 6.0, ortho-II for x = 6.5 and ortho-I for x = 7.0. No other ordered structures could be detected. A coexistence of ortho-I and -II is evident in a wide region between 6.6 and 6.8. The existence of extended domains of ortho-II is essential for the onset of superconductivity. In the tetragonal region below 6.3 we find mainly insulated oxygen ions in the Cu(1) plane; in superconducting samples of the 60 K plateau oxygen is ordered in chains of a mean length of about 12 oxygen ions. In the region of the 90 K plateau the chains become very long and the oxygen vacancies are clustered in empty fragments of chains.

We describe the recent uses of the technique of small-angle neutron diffraction to investigate flux-line structures within the bulk of superconductors in the mixed state. Despite the small signal in superconductors with a long penetration depth, useful results have been obtained in both High-T_c and heavy-fermion superconductors. These can give information about the perfection of the flux lattice, the values of characteristic lengths, the influence of crystal anisotropy and defects on the flux lattice structure and orientation, and on temperature and flux lattice melting effects.

By means of the magneto-optical Faraday effect the spatial magnetic flux density distribution is investigated at the surface of High-T_c-superconductors possessing different microstructure and geometry. In external fields a homogeneous flux front is observed in films and in single crystals whereas in sintered samples the properties of the grain boundary phase and the single grains determine the flux distribution. In particular calculations of the field distribution in dependence of the sample thickness and geometry explain the observed flux distribution and yield the critical current density.

In layered superconductors as high-T_c materials but also organic superconductors, chalcogenides and superlattices, the simple concept of an distorted hexagonal lattice of straight vortices becomes unsufficient. Due to anisotropy and short coherence lengths, quite new vortex structures may arise. Some of them, as staircase vortices, simply add a modulation in the direction of vortex lines. This phenomenon is reviewed, together with the resulting lock-in transition, especially when the effects of the layered structure are weak. More exotic structures like a decomposed vortex lattice can also occur in specific situations: they involve two perpendicular sublattices, one parallel and one normal to the layers. We propose that extended defects as twin boundaries or irradiation tracks can trigger such a structure even in moderately anisotropic compounds as Y:123.

We formulate a local picture of strongly correlated systems as a Feynman sum over atomic configurations. The hopping amplitudes between these atomic configurations are identified as the renormalization group charges, which describe the local physics at different energy scales. For a metallic system away from half-filling, the fixed point local Hamiltonian is a generalized Anderson impurity model in the mixed valence regime. There are three types of fixed points: a coherent Fermi liquid (FL) and two classes of self-similar (scale invariant) phases which we denote incoherent metallic states (IMS). When the transitions between the atomic configurations proceed coherently at low energies, the system is a Fermi liquid. Incoherent transitions between the low energy atomic configurations characterize the incoherent metallic states. The initial conditions for the renormalization group flow are determined by the physics at rather high energy scales. This is the domain of local quantum chemistry. We use simple quantum chemistry estimates to specify the basin of attraction of the IMS fixed points.

The non-linear dynamics of Josephson junctions is well described by the perturbed sine-Gordon equation together with the appropriate boundary conditions. A possible solution of the pure sine-Gordon equation are solitons. In the junction the (quasi) solitons manifest themselves as Josephson vortices, each of them carrying one magnetic flux quantum. We used the technique of Low Temperature Scanning Electron Microscopy to study the soliton dynamics in superconducting tunnel junctions of various geometries. The electron beam power can be adjusted to work in different regimes. For high power, the beam acts as an active probe and we were able to introduce individual solitons in annular junctions one by one and studied multisoliton behavior without the influence of boundaries. For low power, the beam acts as a passive probe influencing the soliton dynamics in a well controlled way. Here, we investigated details of the dynamic soliton behavior.

Experiments probing vortex dynamics in square and triangular Superconducting-Normal-Superconducting Josephson junction arrays are reviewed. A low frequency two coil mutual inductance technique allows to investigate phase coherence in these samples. In a perpendicular magnetic field the interaction of the field induced vortices with the periodic pinning structure provided by the array leads to frustration effects which, interestingly enough, differ in the two types of arrays. This difference in behavior is mainly due to the smaller pinning energy in the triangular array.

It is shown that all the universal properties of glasses can be explained in the framework of the Soft Potential Model (SPM). At low temperatures (below a few kelvin) this model is equivalent to the well-known model of two-level systems of Anderson, Halperin, Varma and Phillips, which explains quite well the universal low-temperature properties of glasses. The SPM predicts that in addition to two-level systems there are soft harmonic oscillators in glasses which are responsible for their universal behavior, especially at higher temperatures.

Both the two-level systems and harmonic oscillators are described by anharmonic soft atomic potentials by the uniform way. They interact with phonons (or electric field) by means of bilinear terms with the same coupling constant for both types of low-energy excitations. The elastic interaction between soft atomic potentials leads to a density of the low-energy excitations, which does not depend on the concentration of defects in the glass structure.

Superconducting tunnel junctions may be used in detector applications in which an energy resolution comparable to that of crystal spectrometers is desired, but combined with the counting capability of semiconductor detectors. We have developed a detector consisting of superconducting aluminium tunnel junctions and tin absorbers. Using these superconductors with different energy gaps, potential wells for quasiparticles are established. Quasiparticles created after energy deposition in the absorber are thus concentrated in the attached superconducting tunnel junctions. A model for the propagation of quasiparticles is developed and the influence of the electrical parameters of tunnel junctions on the read-out electronics is discussed. For detectors with large active area, a long mean free path for quasiparticles in the absorber layer is essential. We fabricated vanadium films grown epitaxially for this purpose.

We have realized a thorough quantum mechanical analysis of an extensive number of low lying levels of the Heisenberg Hamiltonian on small periodic samples. Striking qualitative differences appear between the spectra of the triangular lattice and those of the Kagomé lattice. Symmetries and finite size scaling of the low lying levels of the triangular antiferromagnetic point to an à la Néel symmetry breaking, and the dynamical structure factors corroborate this conclusion. The Kagomé antiferromagnet spectra are qualitatively different: no signature of a symmetry breaking has been exhibited and the dynamical structure factors of the low lying levels display broad bands. The Heisenberg antiferromagnet on a Kagomé lattice is a serious candidate for a spin liquid.

A 122-channel d.c. SQUID magnetometer with a helmet-shaped detector array covering the subject's head has been operational in the Low Temperature Laboratory of the Helsinki University of Technology since June 1992. The new system allows simultaneous recording of magnetic activity all over the head. The probe employs 122 planar first-order thin-film gradiometers in dual units with two exactly orthogonal channels at 61 measurement sites. The performance of the device is analyzed and compared with more conventional axial gradiometer arrays by considering signal-to-noise ratios, spatial sampling theory, confidence intervals for the estimated equivalent current dipole positions, and information-theoretical channel capacity. The signal-to-noise ratio and the resolution of the planar and axial arrays with the same number of channels are found practically equal. The number of channels and their spacing in our new Neuromag-122 system are found fully adequate for neuromagnetic measurements. An example of whole cortex recordings of auditory evoked brain activity is presented and analyzed.

We summarize recent theoretical results concerning strongly correlated fermion system at nonzero temperature within a mean-field picture. In case of narrow-band systems (Part 1) we describe the Mott-Hubbard (localization) boundary, the presence of the T³ ln T term in the specific heat in almost localized systems, as well as the metamagnetism and spin-split heavy masses. We also mention the concept of a statistical spin liquid state and the stability of an extended s-wave superconducting state in two dimensions for this case. In Part 2 (hybridized systems) we introduce an interobital pairing mediated by a Kondo-type interaction and propose an universal scaling of superconducting properties for heavy fermion systems.

Atomic site tunneling spectroscopy has been applied to bulk single crystals of BSCCO and laser-ablated thin films of YBCO. The tunneling spectrum on the cleaved surface of BSCCO showed both semiconducting and superconducting behaviors, depending on the tip-sample distance. This observation is consistent with the quasi two-dimensional layered model. In both systems, clear superconducting gap structures with quite low zero bias conductances were observed, suggesting the s-wave like symmetry of the order parameter.

A mechanically controllable break junction is used to obtain superconducting point contacts of atomic dimensions. The interaction of these point contacts with a microwave field is studied. In the current voltage characteristics of point contacts with a normal resistance much smaller than the impedance of vacuum, Shapiro steps occur as a result of the interaction with a microwave field. For normal resistances larger than the impedance of vacuum we observe a different structure in the I-V curves, which is attributed to a different interaction process of these high impedance point contacts with the radiation.

The experimental information about the characteristic length of the dynamic glass transition and its temperature dependence is collected. Extrapolation to the splitting region (several 10 K above T_g) gives a length of very few nanometers. Using arguments from thermodynamic fluctuation theory it is concluded that a sharp kinetic transition temperature T_c > T_g is not possible for such small lengths.

The dynamics of the segmental α-relaxation in three different polymeric systems, poly(vinyl methy ether) (PVME), poly(vinyl chloride) (PVC) and poly(bisphenol A, 2-hydroxypropylether) (PH) has been studied by means of relaxation techniques and quasielastic neutron scattering (backscattering spectrometers IN10 and IN13 at the ILL-Grenoble). By using these techniques we have covered a wide timescale ranging from mesoscopic to macroscopic times (10⁻¹⁰–10¹s). For analyzing the experimental data we have developed a phenomenological procedure in the frequency domain based on the Havriliak–Negami relaxation function which in fact implies a Kohlrausch–Williams–Watts relaxation function in the time domain. The results obtained indicate that the dynamics of the α-relaxation in a wide timescale shows a clear non-Debye behaviour. The shape of the relaxation function is found to be similar for the different techniques used and independent of temperature and momentum transfer (Q). Moreover the characteristic relaxation times deduced from the fitting of the experimental data can also be described using only one Vogel-Fulcher functional form. Besides we found that the Q-dependence of the relaxation times obtained by QENS is given by a power law, τ(Q) ∝ Q⁻ⁿ (n > 2) n being dependent on the system, and that the Q-behaviour and the non-Debye behaviour are directly correlated. We discuss this correlation taking into account several data of the dynamics of the α-relaxation previously reported in the literature. We also outline a possible scenario for explaining this empirical correlation.

Broadband dielectric spectroscopy in the frequency range from 10⁻⁵ Hz to 10⁶ Hz is employed to study the relation between α-relaxation and normal mode process for oligomeric polyisoprene and polypropylene glycol samples of different molecular weight. It is shown that relaxation time of the α-relaxation has a much stronger temperature dependence than that of the normal mode process. This means that applicability of the Rouse model to the normal mode relaxation breaks down near the glass transition. This result is further discussed in terms of the different temperature dependence of the two relevant length scales, the correlation length for the α-relaxation ξ and the end-to-end vector of the polymer chain. A quantitative evaluation gives more evidence that there exists a characteristic length scale for the α-relaxation which is in the order of magnitude of 2 to 3 nm at the glass transition temperature.

Photon correlation spectroscopy in the depolarized mode and dielectric relaxation spectroscopy were employed to investigate local and global chain motions in poly(styrene-b-1,4-isoprene) diblock copolymer melts near the order-to-disorder transition (ODT). The presence of two distinctily different segmental orientation processes and the temperature dependence of the width of the slow relaxation function in the disordered state imply strong composition fluctuations in calorimetrically and rheologically homogeneous diblocks. the estimated amplitudes of composition fluctuations increase with the proximity to the ODT. The polyisoprene subchain orientation is characterized by a single broad distribution of relaxation times above the ODT, that apparently senses an average friction. This is due to the fact that the end-to-end vector orientations sample fluctuations at a different length scale than segmental orientations do.

The plateau zone in the dynamic modulus of large chain polymer melts characterizes most strikingly their viscoelasticity reflecting the rubber-like properties of entangled chains. In this paper we shall summarize neutron spin-echo experiments which unravel the underlying microscopic large scale chain motions. NSE thereby facilitates a space-time analysis of the density fluctuations in a labeled chain among equals. The experiments show the microscopic existence of an intermediate dynamic length scale, related to the rubbery plateau. Scaling models of entanglement formation are scrutinized by systematically varying the two length scales considered to be important. Diluting with oligomers changes the contour length density. A temperature variation changes the Kuhn length. The NSE results favour binary contact models and exclude packing models. Finally, the build-up of entanglements at the crossover from Rouse to entanglement behavior is addressed by varying the chain length.

Copolyesters of poly(ethyleneterephthalate) (PET), poly(ethylene-2,6-naphthalenedicarboxylate) (PEN) and poly(p-hydroxybenzoate) (PHB) form liquid crystalline (LC) phases if the fraction of PHB is larger than about 30%. In some of these copolyesters the liquid crystalline state as well as the isotropic state can be frozen in by quenching the material from different temperatures. This makes it possible to compare both states with each other. By means of dynamic mechanical analysis it is demonstrated that the glass transition temperature T_g of the LC-state is about 30°C lower than that of the isotropic state. Neutron scattering measurements show that the radius of gyration of the molecules in the LC-state is larger than for molecules in the isotropic state. The molecular mobility below and above T_g in both the LC and the isotropic state is investigated by means of deuteron NMR.

The modification of chemical structure may modulate the properties of polyconjugated system in order to optimize the applications in different fields. Many properties can be tuned ranging from the solubility to the band gap of the polymer. Moreover, the presence of a polar group in the side chain open new possibilities in the preparation of ordered molecular architecture such as Langmuir Blodgett films. We will discuss these parameters focusing our attention mainly on the preparation of Langmuir Blodgett films of poly(3-alkylthiophene) and of low band gap structure.

No abstract supplied.

Scanning tunneling microscopy has been employed to investigate chain molecules, which have been self-assembled at the interface between their melts or organic solutions, and a solid flat surface of a semimetal or a semiconductor. Also it has been used to observe chain molecules in a precursor wetting layer in the vicinity of the edge of a solidified alkane melt drop. With a resolution in space and time of 10 pm and 10 μs, respectively, it is possible to investigate (i) commensurability between substrate and adsorbate lattices, (ii) the packing of molecules and molecular mixtures with various symmetries on several structurally and chemically different substrates, (iii) temperature and composition induced order-disorder transitions between 2-dimensional crystals and liquid crystals, and (iv) the current through individual molecular segments at different bias voltages. We summarize some of our work in this area, and report on new results on an alkoxylated poly(p-phenylene) on the basal plane of highly oriented pyrolytic graphite (HOPG).

The influence of spin-dependent electron and impurity potentials on transport properties of magnetic layered structures is analysed theoretically. In the current-in-plane geometry the conductivity is calculated within a quantum-mechanical formalism. For currents flowing perpendicularly to layered structures the spin-dependent effective electron potential plays an important role and can generate giant magnetoresistance also when the impurity scattering is spin-independent. Interface resistance for perpendicular transport is calculated in the limit of incoherent scattering by adjacent interfaces.

The time delayed reversal of the magnetization of magnetic system with distributions of energy barriers, is in general strongly dependent on temperature at high temperature. At low temperature it becomes independent of the temperature. This result is interpreted in terms of a crossover between a thermal regime, in which the irreversible rotation of the magnetization is thermally activated, and a quantum regime in which it proceeds by quantum tunnelling. In this paper we show some results obtained in nanoparticles, layers and multilayers. In particular an analogy with Josephson junction will be made for the case of the sandwiches (domain wall junction). The role of dissipation will be discussed.

We study some effects due to frustration in itinerant systems: we show that new ordered magnetic phases, in which magnetic and non-magnetic sites are present, can be stabilized in frustrated systems close to the magnetic-nonmagnetic instability; simple examples of such phases on the triangular lattice have been studied within the Hubbard model. In the paramagnetic phase we show that large values of the specific heat coefficient are obtained due to enhancement of the spin fluctuations. Comparison with experimental results on the RMn₂ compounds is presented.

Some new results on the recently discovered UPd₂Al₃ and UNi₂Al₃ heavy fermion superconductors will be discussed, particularly tunneling spectroscopy and the properties of the Ce counterpart. The properties of uranium intermetallic boron compounds will be reviewed. Some new results on mixed compounds tetragonal compounds will be shown.

Magnetic properties of the ternary rare earth intermetallics are discussed on example of three groups of compounds RNi₂X₂, RNiX₂ and RNiX (R-rare earth metal, TX = Si, Ge). RNi₂X₂ compounds crystallize in a tetragonal where RNiX₂ and RNiX in orthorhombic structures. The magnetic phase diagrams (H, T) determined for some of these compounds. The magnetic properties are analyzed in terms of the RKKY exchange interactions and crystal electric field effects.

Recent magneto-optical investigations of exotic rare-earth compounds, such as reduced-moment magnets and magnetic superconductors, are critically reviewed. Representatives of the first class are cerium compounds like CeSb and CeRh₃B₂. The former material has attracted particular theoretical attention due to its record Kerr rotation of 14°. CeRh₃B₂ raised the question, whether a Curie temperature of 115 K can originate from reduced Ce moments. A line shape analysis of the magneto-optical spectra shows that Ce, indeed, is responsible for the moment in CeRh₃B₂ and it allows to specify the nature of its ground state.

Representatives of the second class are Chevrel phases containing Eu²⁺ ions. The characteristic magneto-optical spectrum of this ion has been identified in various Eu_1-xPb_xMo₆S₈ and Eu_1-xSn_xMo₆S_8-y Se_y Chevrel phases. The temperature and field dependence of the Kerr rotation allow a study of the evolution of the spin polarization of the 4f⁷ state in the normal, superconducting and field-induced superconducting state.

Magnetic X-ray Dichroism (MXD) is related to the dependence of the absorption cross-section of a given atom (with well-defined angular momentum) with the state of polarization of light. Synchrotron Radiation (SR) sources, because of their well-defined states of polarization, their highflux and high brilliance, are thus ideal to perform MXD experiments even in those cases where the total number of absorbing atoms does not exceed 10¹⁵ which is interesting in the case of surface magnetism. We consider how the magnetic absorption cross-section may be directly related to the local magnetic properties of the absorbing atom via spin-orbit coupling and exchange interactions. Taking benefit of selection rules, the ability to separate the contributions to the total angular momentum (i.e. the magnetic moment) of individual electronic shells (s, p, d, f) is shown. Moreover, the possibility to separate the orbital and spin contribution is addressed. The high selectivity explains why MXD appears to be a unique tool for the study of magnetism.

Several examples are given, especially those where the knowledge of the individual contributions to the total magnetic moment is of prime importance (highly-correlated materials for instance). We shall also briefly discuss some unclear points related with this rather new experimental technique, like the validity of a one electron picture to describe the magnetic absorption cross-section.

The study of magnetic XANES of circularly polarized x-rays at the K-edges and the L-edges in 3d-, 4f- and 5d-elements in ferro(i)magnetic materials is a powerful method to investigate in an element-selective manner magnetic aspects of the electronic structure of solids. It is demonstrated on the basis of simplified single-particle band-structure pictures that for 3d- and 5d-elements, which are polarized by 3d-neighbors, information on the local spin and orbital moments can be directly deduced from the magnetic L_2,3-spectra similar to the application of the sum rules recently developed by T. Thole and P. Carra. Spin-polarized EXAFS studies of various systems are presented demonstrating the sensitivity of the SPEXAFS to the magnetic neighborhood only and their direct correlation to magnetic moments of neighboring atoms.

Nanocrystalline structures offer a new opportunity for tailoring soft magnetic materials. The most prominent example are devitrified glassy FeCuNbSiB alloys which reveal a homogeneous ultrafine grain structure of bcc-FeSi with grain sizes of typically 10-15 nm and random texture. Owing to the small grain size the local magneto-crystalline anisotropy is randomly averaged out by exchange interaction so that there is only a small anisotropy net-effect on the magnetization process. Moreover the structural phases present lead to low or vanishing saturation magnetostriction which minimizes magneto-elastic anisotropies. Both the suppressed magnetocrystalline anisotropy and the low magnetostriction provide the basis for the superior soft magnetic properties comparable to those of permalloys or near zero-magnetostrictive Co-base amorphous alloys but at a higher saturation induction. Like in other soft magnetic material the hysteresis loop can be tailored by uniaxial anisotropies induced by magnetic field annealing.

The outlook for improving the energy product of permanent magnets beyond the theoretical limit of 516 kJ/m³ for Nd₂Fe₁₄B is discussed. Most promising are nanostructured two-phase magnets consisting of an aligned hard-magnetic skeleton phase possessing exceptionally high uniaxial anisotropy and a soft-magnetic phase which assures a high magnetization. Brown's paradox is discussed for inhomogeneous magnets, and micromagnetic calculations are used to predict energy products as high as 1090 kJ/m³ in suitable nanostructured Sm₂Fe₁₇N₃/Fe₆₅Co₃₅ composites, such as multilayers or disordered two-phase magnets with common c axis.

We report measurements of nuclear magnetism of degenerate liquid ³He below 0.1 K. Nuclear polarization up to 60% was transiently obtained by rapid melting of a 80% polarized ³He solid. From simultaneous measurements of the magnetization and of the heat released during the relaxation towards magnetic equilibrium, we deduce the magnetization dependence of the free energy of the liquid, and hence the magnetization curve up to 200 T. At 26 Bar and 80 mK, we find that the susceptibility decreases with increasing magnetization. This rules out any significant metamagnetism of the liquid, in contradiction with former expectations. In order to confirm this behaviour by a static experiment, we have developed a torque magnetometer to carry out precision measurements of the equilibrium magnetization up to 30 T (corresponding to a 12% polarization) and at 50 mK. Preliminary results of this experiment are reported.

Recent experimental results on antiferromagnetic and ferromagnetic ordering in silver at nanokelvin temperatures are reviewed. The dominant interaction between nuclear spins is the Ruderman–Kittel exchange which leads to spontaneous antiferromagnetic ordering below the record-low Néel temperature T_N = 560 pK. Above T_c = −1.9 nK, ferromagnetic order has been found, which is a consequence of the fact that the spin system tries to maximize its Helmholtz free energy. The ferromagnetic ordering is observed as a saturation of the susceptibility to −1, which is caused by the formation of plate-shaped domains. Except for the re-entrant form of the antiferromagnetic phase diagram at T > 0, the overall agreement between experimental and theoretical results is good.

The effect of the requirement of local charge neutrality on persistent currents in normal mesoscopic rings is considered. The argument used in the case of global charge neutrality (canonical ensemble) is generalized to apply to local charge neutrality. The same result is then derived by treating the Coulomb interaction in the Kohn-Sham approach of density functional theory, using the local density approximation for the exchange-correlation term.

The effects of spin-flip scattering, spin-orbit interaction, and Zeeman splitting on the persistent currents in mesoscopic metallic rings and related correlation functions are investigated, with emphasis on the non-interacting electron case. Interaction effects are briefly reviewed.

Basic geometric and crystallographic concepts for ordered quasiperiodic structures are described. The embedding into N D lattices and the resulting geometry of atomic hypersurfaces and windows are analyzed and illustrated. The triangle pattern with 5-fold symmetry, its vertex configurations, and their windows are given. Two standard icosahedral tilings are discussed with emphasis on projections from the face-centered 6D lattice F. The icosahedral tilings due to Mosseri and Sadoc, to Socolar and Steinhardt, and to Danzer, all constructed without use of projections, can be derived with their projections and windows from the lattice F and the two standard tilings.

The phenomena of reactive diffusion developing the icosahedral quasicrystalline structure (i-QC) has been analysed in the Al-Mn thin film system prepared by sequential deposition of Al and Mn at the reaction temperatures (HTSD). The development of Kirkendall voids in the Al film indicates that Al is the dominant diffusion component and that growth takes place on the top surface of the QC layer were Mn deposited. Al has a high diffusion rate across the QC layer and depends on the local structure of the QC phase. The reaction rate of Al and Mn is large if Mn is present in vapour phase while it is extremely small at the interface of QC-Mn layers, during the heat treatment of Al-QC-Mn layer system. A concentration profile of Al has been proposed for the reactive diffusion taking place in the case of HTSD.

We present and analyze briefly partial valence and conduction band states distributions of Al₆Mn and both crystalline and quasicrystalline AlCuFe phases of close nominal composition. Low densities of states and a pseudo-gap are found at the Fermi energy. We discuss the formation of this pseudo-gap as due to the diffraction of electronic conduction states by Bragg planes and emphasize its consequences for the densities of d states of the transition element. We discuss also briefly the transport properties of some systems like AlCuFe which show the proximity of a metal-insulator transition.

The experimental observations on dislocations in quasicrystals are reviewed with particular emphasis on contrast behaviour in transmission electron microscopy. Quantitative contrast experiments indicate that the phason-strain field of dislocations in icosahedral Al-Cu-Fe is by about an order of magnitude stronger than the phonon-strain field. The data on room-temperature plastic deformation of quasi-crystals are reviewed. Depending on the alloy, Vickers hardness values range from about 400 to 1000. Microcracks are observed under the Vickers indentations. The hardness decreases steeply with increasing temperature. Controlled single-quasicrystal experiments indicated that high-temperature deformation is carried by a dislocation mechanism.

Local and average electronic and magnetic properties of transition metal alloys are strongly correlated to the distribution of atoms on the lattice sites. The ability of some systems to form long range ordered structures at low temperature allows to discuss their properties in term of well identified occupation operators as those related to long range order (LRO) parameters. We show that using theoretical determinations of these LRO parameters through statistical models like the cluster variation method (CVM) developed to simulate the experimental phase diagrams, we are able to reproduce a lot of physical properties. In this paper we focus on two points: (i) a comparison between CVM results and an experimental determination of the LRO parameter by NMR at ⁵⁹Co in a CoPt₃ compound, and (ii) the modelling of the resistivity of ferromagnetic and paramagnetic intermetallic compounds belonging to Co-Pt, Ni-Pt and Fe-Al systems. All experiments were performed on samples in identified thermodynamic states, implying that kinetic effects are thoroughly taken into account.

The modern first-principles theory of the electronic structure in ordered and random substitutional alloys will be reviewed with special reference to magnetic fluctuations on total energy calculations. Then, a variety of approaches to predicting alloy phase diagrams will be considered and critically compared. Finally, an attempt will be made to assess the extent to which such calculations can be useful in designing new alloys for specific applications.

Information about elementary atomic jump in intermetallic compounds can be obtained by quasielastic Mössbauer spectroscopy, a microscopic method. High temperature ⁵⁷Fe Mössbauer spectroscopy permits to separate jumps between various sublattices in non-Bravais lattice through measuring the angular dependence of diffusional line broadening. Results obtained from diffusion in B2 (FeAl) and DO₃ (Fe₃Si) structures and the effect of atomic disorder in non-stoichiometric iron-silicon intermetallic compound on diffusion are discussed.