Table of contents

Shubnikov-de Haas (SdH) measurements of the two-dimensional organic conductor (BEDT-TTF)₂TIHg(SCN)₄ have been performed in order to clarify the Fermi surface. At 0.05 K, in addition to the closed orbit and the magnetic breakdown orbit in the original Fermi surface,eight different SdH oscillations are found. All the frequencies follow the same angular dependence. The results are well explained in terms of the reconstruction of the Fermi surface due to spin density wave formation.

A preliminary investigation of the effects of doping arsenic triselenide with indium was carried out. Measurements of DC conductivity, photomobility and optical absorption were performed. We find a significant improvement in the main transport properties. The findings are explained in terms of a shift of the valence band edge towards the centre of the mobility gap as a function of doping rather than a shift of the Fermi level towards the band edge.

In Ba₂Sr₂YCu₂O₈ single crystals at compositions below that for metallic conduction, the electrical resistivity decreases with increasing temperature, while the Hall coefficient is temperature independent and of positive sign. These features are explained in terms of hopping of hole-like bipolarons in the CuO₂ planes.

The result of DC magnetization (M) measurements on the compounds RCu₂Si₂ (R=Gd, Tb, Dy, Ho, Er and Tm) in the paramagnetic state up to 55 kOe are reported. In all cases, except that of Gd alloy, M is found to be a non-linear function of magnetic field tending to saturate at higher fields over a wide temperature range in the paramagnetic state. This magnetization behaviour is found to be essentially a single-ionic effect, presumably arising from strong 4f quadrupolar effects.

Results on the diffusion of Cl into CdTe are described. Diffusion anneals were carried out at selected temperatures in the range between 200 degrees C and 700 degrees C in evacuated SiO₂ ampoules using a diffusion source of CdCl₂ under saturated vapour pressure conditions. The concentration profiles were measured using a radiotracer sectioning technique. The profiles were found to be composed of four parts, to which a computer package consisting of the sum of four complementary error functions (erfc) gave satisfactory fits to the data. The fastest-diffusing component gave values of the diffusivity that agreed with previously published results. Proposals explaining how this type of diffusion may occur are given.

The resonating-valence-bond (RVB) State with one localized state per site is a state in which correlations are important and therefore for its determination it is necessary to go beyond the one-determinant approximation. Although the number of one-electron space orbitals is equal to the number of electrons, N, the number of necessary determinants increases factorially with N, thus making a rigorous treatment of the correlation effects impracticable even for systems containing relatively small number of electrons. On the other hand, by introducing a many-electron spin function a coupled system of equations for one-electron localized non-orthogonal space orbitals and N-electron spin function can be obtained which can be solved in practice. In this procedure the main difficulty resides in the equation for the N-electron spin function. In this paper it is shown that a perturbation theory based on the decomposition of the spin system into clusters, when taken beyond the first order can, in general, offer a suitable method of mapping the results of numerical calculations of spin clusters onto the total system.

The strongly coupled Jahn-Teller (JT) system is studied in which an ion in an orbital T₁ triplet state is coupled to both e and t₂ modes of vibrations of its neighbours. Such a system is usually considered to be either a T(X)(e+t₂)JT system, in which orthorhombic minima in the five-dimensional Q-space are lowest in energy, or a T(X)d system, which has a trough of lowest energy. However, it is possible also for the tetragonal and trigonal minima, usually associated with the T(X)e and T(X)tJT effects respectively, to coexist with very similar energies to each other (and be of overall lowest energy) when the bilinear term of the vibronic interaction is present. This situation is described in this paper. A set of vibronic ground states is obtained by mixing the symmetry-adapted vibronic T₁ ground states of the T₁(X)e and T₁(X)t₂JT systems. This is different to the set of states associated with the orthorhombic minima. Analytical expressions for the first- and second-order JT reduction factors are also derived for the coexisting system. As a consequence of this analysis, an improved version of the theory of second-order reduction factors is obtained. The reduction factors are compared to those of existing numerical calculations for the T₁(X)dJT system and it is shown that very good agreement is obtained between the two in the strong-coupling limit.

The mechanism of spin orientation under the interband tunnelling transport of carriers due to the longitudinal current has been considered. The spin-dependent interband tunnelling probability has been calculated for Kane and Dirac-like models. For lead chalcogenides and InAs/AlGaSb/GaSb heterostructures the degree of spin orientation can be greater than 10% under weak current-induced longitudinal anisotropy of the distribution function.

We compute the current-voltage (I-V) characteristic of a mesoscopic Josephson junction, formed from two weakly linked, superconducting dots. A key feature of such structures is the presence of quasi-particle resonances in the current-phase relation. We show that these lead to measurable changes in the I-V characteristic compared with conventional macroscopic junctions. For an overdamped junction, the I-V characteristic is predicted to shift to higher values of the current, but the critical current is unchanged. For an underdamped junction exhibiting hysteresis, this shift is accompanied by an increase in the critical current associated with the lower branch of the I-V curve.

Results are provided for the spin-spin response function of a three-dimensional, antiferromagnetically coupled Heisenberg magnet covering a range of temperatures from the critical temperature to deep in the paramagnetic phase. The wave vectors considered span the Brillouin zone. Damping rates are given at the zone centre and the antiferromagnetic-ordering wave vector, w, together with copious numerical results for the full response function. The calculations are based on the non-linear, integral-differential equations obtained from so-called coupled-mode theory. In a confrontation between experimental and theoretical findings for RbMnF₃ nearly all aspects have a positive outcome. The main exception is found at T, for wave vectors close to w. Here, the measured response comprises three distinct components, reasonably ascribed to diffusive and oscillatory collective processes. In the corresponding predictions, the diffusive component is conspicuously missing. A less pronounced discrepancy if found at the antiferromagnetic zone boundary where, once again, there is more structure in the observed spectrum than in the calculated one.

Results are presented of a numerical calculation of the tunnelling gap for a domain wall moving in the double-well potential of a pair of voids in a magnetic insulator. Both symmetric and asymmetric double-well potentials are considered. It is found that the prospect for observing domain wall quantum coherence on a mesoscopic or macroscopic scale appears highly unlikely.

For ²⁷Al nuclei in Al₂O₃ crystals, both nominally pure and doped with Cr³⁺, suppression of the impurity nuclear spin-lattice relaxation under the condition of additional stationary magnetic saturation of the NMR line has been observed for the first time. Measurements of the lattice and impurity components of the spin-lattice relaxation time have been performed. The study was carried out at 77 K for different crystal orientations. Possible mechanisms of the impurity NMR relaxation suppression are discussed.

The cooling of a hot electron-hole plasma in In_0.53Ga_0.47As is studied by means of an analytical model. It is assumed that the carriers are thermalized at different effective temperatures and their distributions are of a Fermi-Dirac form from the shortest times. The energy-loss rates from the plasma due to coupling to transverse and longitudinal optical and acoustic phonon modes are included. Non-equilibrium phonon mode occupancy occurs, which decreases the cooling rate, but it proves necessary to incorporate Gamma to L intervalley scattering into the model in order to provide a realistic description of recent experimental results. From this study we infer a Gamma to L scattering time, at a lattice temperature of 4 K, of 500+or-100 fs, and a return time of 9+or-2 ps.