Table of contents

It is shown that Clebsch-Gordan coefficients for the superalgebra osp(1/2) and its quantum deformation U_q(osp(1/2)) satisfy a conditional Regge symmetry. For U_q(osp(1/2)) the symmetry derives in a very simple way from an analytical formula of the Clebsch-Gordan coefficients obtained by application of the projection operator method. For osp(1/2) the symmetry is proven by taking the limit q → 1 and by analysing the structure of the coefficients defined by Scheunert, Nahm and Rittenberg and by Berezin and Tolstoy.

On the multidimensional sine-Gordon equation ∂_α ∂_α φ = sin φ, some new soliton solutions are constructed. The application of Jacobi's decision method shows that these solitons are stable under general boundary conditions. A particular z-soliton which seems an array of concrete particle is studied in detail.

The size dependence of the ionization potentials of cadmium clusters is studied by a localized-orbital theory based on the tight-binding approximation. The total density of state (DOS) of the Cd_n clusters is obtained by means of the moment expansion method in order to obtain the Fermi energy of the system. The Fermi energy E_F combined with the correction of the classical conducting sphere droplet (CSD) model are used to calculate the ionization potential (IP) of the cluster. The theoretical values reproduce the experimental results quite well. The size dependence of the ionization potential shows a rapid convergence towards the bulk limit and obey the 1/R law provided by the CSD model in general, indicating that the small Cd clusters have been already metallic-like.

A two-dimensional two-colour, nonlinear, Galilean-invariant, Boltzmann cellular automaton is developed and used to study two-fluid hydrodynamics and to model fluid interfaces. The interfacial tension and interface width are flexibly controlled by the parameters defining collision rules for the colored particles. Results of studies of both the static and dynamical relaxation of the surface tension and wetting properties are presented.

We present first experimental evidence for period doubling as an alternative mode of coarsening during surface-directed spinodal decomposition in a binary mixture. The system under consideration is a symmetric blend of deuterated polystyrene (dPS) and partially brominated polystyrene (PBr_xS). We compare the surface domain coarsening of a thin film of a critical mixture with that of a multilayered structure of near-binodal compositions. While the former is found to coarsen monotonically via diffusion, leading to self-similar composition vs. depth profiles, the latter grows by dissolution of one of the layers feeding the adjacent domains of the respective phase. The results compare well with cell-dynamical simulations.

A perturbative study of the localization length in the Anderson model on strips is presented. It is shown that in the neighbourhood of those energies corresponding to the opening of a new channel, a certain rearrangement of the perturbation expansion significantly improves the accuracy of the prediction. Moreover, the localization length scales in a way which is reminiscent of the strictly 1D behaviour, but is nevertheless different.

Extensive LDA and quasi-particle calculations have been performed on boron nitride (BN) single-wall and multi-wall nanotubes. Strain energies are found to be smaller for BN nanotubes than for carbon nanotubes of the same radius, owing to a buckling effect which stabilizes the BN tubular structure. For tubes larger than 9.5 Å in diameter, the lowest conduction band is predicted to be free-electron-like with electronic charge density localized inside the tube. For these tubes, this band is at constant energy above the top of the valence band. Consequently, contrarily to carbon nanotubes, single- and multi-wall BN nanotubes are constant-band-gap materials, independent of their radius and helicity. In addition, we expect them to exhibit remarkable properties under n-type doping.

We have investigated the electrodynamic response of single-crystalline FeSi over a frequency range from d.c. to 10⁵ cm^-1. At low frequencies, we found evidence for an Anderson-Mott localization behaviour of the low-temperature optical conductivity, while at high frequencies the excitation spectrum resembles that of a conventional semiconductor. The missing spectral weight below the gap energy is redistributed around the gap edge, in disagreement with previous claims based on optical measurements.

The c-axis transport of impure layered metals is discussed in the cases of clean and dirty limits. The crossover from Drude to hopping regime in transverse conductivity is treated qualitatively as the change from anisotropic diffusion to Zener oscillations of electrons.

Many properties of the superconducting state of high-T_c materials have been interpreted within the Eliashberg framework with a very large coupling (λ ⩾ 3). Here we point out that the inclusion of non-adiabatic corrections acts as an effective coupling for various properties that can, therefore, be reproduced with a much smaller coupling (λ ≈ 1) within the generalized theory. This can provide a new perspective for the analysis of superconductivity data with moderate couplings but within a theory that goes beyond Migdal's theorem.

We consider the Josephson-vortex motion in a long one-dimensional Josephson junction in a thin film. We show that this Josephson vortex is similar to a mesoscopic capacitor. We demonstrate that a single-Cooper-pair tunnelling results in non-linear Bloch-type oscillations of a Josephson vortex in a current-biased Josephson junction. We find the frequency and the amplitude of this motion.

We present a class of exactly solvable quantum spin models which consist of two Heisenberg subsystems coupled via a long-range Lieb-Mattis interaction. The total system is exactly solvable whenever the individual subsystems are solvable and allows to study the effects of frustration. We consider i) the antiferromagnetic linear chain and ii) the Lieb-Mattis antiferromagnet for the subsystem Hamiltonians and present i) the complete ground-state phase diagram and ii) the full thermodynamic phase diagram. We find a novel phase which exhibits order-from-disorder phenomena.

Valence level photoemission measurements of the normal-state electronic structure of the new boro-carbide intermetallic superconductor YNi₂B₂C are presented. The behaviour of the Ni-dominated valence band peak at 1.4 eV binding energy and the states at E_F across the Ni 3p absorption threshold clearly indicate significant Ni3d contributions at these energies. In addition, the transfer of spectral weight from the main Ni3d valence band to a two-hole bound-state satellite at ∼ 8 eV signals the role played by electron correlation in these materials. The peak in the DOS predicted to lie at E_F in band structure calculations is not observed in the experiment, possibly also due to the effects of electron correlation.

Erratum for Europhys. Lett.28 (2) 129-134 (1994).