Table of contents

Short-wave sound absorption propagating along the axis of a semiconductor superlattice by electrons of the lowest miniband in a nonquantised electric field is investigated theoretically. The mechanism of the attenuation of phonons is due to Landau damping. The relationship discovered for the absorption coefficient Gamma = Gamma (E) is essentially different from that for homogeneous materials, and, additionally the threshold field at which the absorption switches over to amplification depends on the superlattice parameters and the length of the sound wave. The more important difference between short-wave sound amplification by a superlattice and amplification in the homogeneous materials is the possibility of finding a field E* such that - Gamma (E*)>or= Gamma (-E*). This situation allows in principle the use of a superlattice as a hypersound generator similar to a generator of long-wave sound. The above generator is impossible to construct in the case of homogeneous semiconductors.

A new method enables a satisfactory treatment of the Schrodinger problem for two-dimensional electron systems with finite widths and in a perpendicular magnetic field. The density of states is determined numerically. Under the spikes representing discrete levels, there are states that grow parabolically as the electron energy increases.

The authors consider various possible hole-coupling mechanisms in the high-T_c superconductor, doped La₂CuO₄. Negative-U processes are energetically unfavourable. It is possible that bipolaronic species may be stabilised when antiferromagnetic energies and coupling terms arising from the distortions of the CuO₆ octahedra are taken into account.

The interplay between magnetic properties and chemical ordering in the Co_1-xPt_x system is investigated theoretically and experimentally. Experimental results for the Curie temperature, magnetisation and paramagnetic susceptibility in ordered and disordered states are discussed within the framework of statistical models. The authors show that the magnetic moments on Pt atoms depend sensitively on the local chemical environment. A complete equilibrium phase diagram is also calculated and compared with the experimental data.

The authors investigate the thermodynamics of a lattice gas model for intercalation, employing a nearest-neighbour intra-plane attraction and both attractive and repulsive forces which decrease with increasing distance between the planes. The free energy F is calculated within the mean-field approximation. The phase diagram is then found analytically by minimisation of F with respect to the number of intercalant atoms in each plane. The authors show how specific choices of the balance between the repulsive and attractive forces deduced from the properties of the material under consideration can qualitatively account for the observed staging behaviour of the different classes of intercalation compounds: graphite intercalation compounds, transition-metal dichalcogenides and clays.

At low temperatures, ZnS:V crystals display three structured photoluminescence bands. Their individual fine structures, their behaviour under various conditions of preparation and excitation and their different excitation spectra demonstrate the presence of the charge states V³⁺(d²), represented by the ³T₂(F) to ³A₂(F) emission band centred around 5600 cm^-1; V²⁺(d³), represented by the ⁴T₂(F) to ⁴T₁(F) emission band around 4800 cm^-1; and possibly V⁺(d⁴), tentatively connected with an emission near 3800 cm^-1. A model of one-electron states is presented in which donor-type as well as acceptor-type changes of the vanadium oxidation states become comprehensible, commencing from V²⁺, the state with neutral effective charge. The excitation spectra for the V³⁺ and V²⁺ emission bands are interpreted on the basis of both a conventional and a modified Tanabe-Sugano scheme. Modifications considered concern mainly the inclusion of (i) a fit to the free-ion terms and (ii) different radial extensions of t₂-and e-type wavefunctions. Values are derived for the crystal-field splitting parameter Dq and the Racah parameters B and C. Evidence is presented for a Jahn-Teller splitting of the ³T₁(P) V³⁺ level and the ⁴T₁(P) V²⁺ level.

The authors derive a highly vectorising Green function recursion algorithm and a finite-size scaling description for diffusion in two-dimensional disordered systems. Extensive numerical calculations are presented for two different types of distribution function for the transfer rates between nearest-neighbour sites. The data are in good agreement with the dimensional crossover and scaling behaviour analytically predicted. Additionally, in contrast with one-dimensional systems with a box-shaped distribution of transition rates, they find no evidence for quasi-localisation.

The authors study the dispersion of collective excitations for fractional quantum Hall states using Feynman-Bijl ansatz wavefunctions. They take the ground-state pair correlation function from direct diagonalisations of the Hamiltonian for a small number of electrons confined to the surface of a sphere, instead of from the simulation of large systems described by a Laughlin-like wavefunction. The results seem remarkably well converged considering the small systems used. They also treat systems with partially filled N=1 Landau levels.

The magnetic structures of the orthorhombic rear-earth manganates ErMn₂O₅ and TbMn₂O₅ have been studied by single-crystal neutron diffraction at temperatures between 1.5 and 25 K. Both materials exhibit amplitude-modulated magnetic structures with propagation vector (0.5, 0, 0.247) (Er) or (0.5, 0, 0.308) (Tb) and moment directions in the ac plane (Er) or the ab plane (Tb). The measurements indicate that the Mn⁴⁺ and Mn³⁺ ionic moments are almost fully magnetically ordered at all the observed temperatures but that the rare-earth ions show an increasing degree of magnetic order at the lower temperatures. The detailed magnetic structures are significantly different from those already in the literature, which are based on powder neutron diffraction data.

Presents a generalisation of the Slonczewski and Thiele approaches to the description of domain-wall dynamics for a medium with relaxation spatial dispersion. A theoretical analysis is given for the peculiarities of the relaxation spatial dispersion in dynamics of domain walls with complex internal structure. The possibility of experimental determination of the phenomenological nonuniform relaxation parameter is discussed.

A theoretical crystal-field model for the X^- (X=O, S) centres with sigma _c-symmetry leads to three equivalent operators for the np⁵²P ground state, from which the g tensor of the defects can be calculated analytically. Three equivalent operator coefficients p, q, r are fitted to four a priori independent experimental quantities g_xx, g_yy, g_zz and phi . This leads to an excellent agreement for NaCl:S^-, RbCl:O^- and KCl:O^- and to a fair agreement for KCl:S^-.

The photon energy spectra arising from positrons annihilating in the presence of oxygen films condensed on exfoliated graphite surfaces have been analysed. Parapositronium is formed in linear proportion to the monolayer coverage up to about 50% and remains steady at higher coverages.