Table of contents

A simple tight-binding Hamiltonian is used in the band model to calculate binding energies of crystal structures in silicon using the recursion method. The use of matrix orthogonal polynomials is described for efficient computation of the symmetry-preserving matrix Green functions. This provides the basis for a proposed rotationally invariant interatomic force algorithm.

An ab initio pseudopotential local density functional calculation is performed on Si₅H₁₂ to determine its equilibrium structure and the energy for a number of slightly distorted structures. The parameters of a Keating potential are then found by a least-squares fitting to these energies. The photon frequencies of the bulk solid are derived from this potential and found to be in reasonably good agreement with the experimental results.

The authors use real-space renormalisation group techniques to study the kinetic Ising model (with Glauber dynamics) on bond-diluted d-dimensional hypercubic lattices, at the percolation threshold. The results confirm and generalise the recently proposed breakdown of dynamic scaling and they predict a dependence on dimensionality of the factors A and B on the temperature-dependent dynamic exponent z=A log xi +B (where xi is the thermal correlation length). These factors also depend on the lattice scaling factor (b); the authors suggest that the limit b=1 may be taken as the best choice for the important coefficient A, leading to A=d(d-1)/2 nu _p.

A new trial wavefunction for the fractional quantised Hall effect is shown to produce a lower ground-state energy than Laughlin's trial function (1983).

Sum rules for non-linear susceptibilities are given for the case of difference-frequency generation. The derivation is based on the classical anharmonic Lorentz oscillator model and the theory of several complex variables is made use of.

Using the finite-cluster approximation, phase diagrams of bond- and site-diluted two-dimensional square-lattice Ising models with competitive nearest-neighbour (NN) and exhibit tricritical points for -0.238< alpha <0( alpha being the ratio of NNN to NN interactions) and re-entrant ferromagnetism for -0.173< alpha <0 in the case of bond dilution. For site dilution, tricritical points are found for -0.238< alpha <0 and re-entrant ferromagnetism for -0.182< alpha <0. These results indicate that re-entrant behaviour seems to be a characteristic feature of systems in which frustration and disorder are present.

The thermal expansion of V₃Ga has been studied using the X-ray method in the temperature range 299-691 K. The thermal expansion coefficient increases linearly with temperature over the range studied, within experimental error. The room-temperature value of alpha is compared with that obtained from specific heat data. The Gruneisen constant ( gamma _G) for vanadium-based A15 compounds is found to be 1.61.

The authors have measured the low-temperature acoustic attenuation in a range of vanadium-doped GaAs samples between 200 MHz and 2 GHz. In n-type material three distinct loss phenomena are found, distinguishable by their symmetry properties. The authors attribute the relation peak associated with the elastic constant C₄₄ to V²⁺centres. Analysis of the peak shape indicates the presence of levels 34 K above the ground state. The behaviour of a loss component which becomes stronger towards lower temperatures is attributed to the presence of the vanadium-related complex (V⁺-X), while an intense light-induced low-temperature loss component is attributed to (V²⁺-X).

A theory for Eu²⁺ ground S-state splitting in low-symmetric crystals analogous to that developed earlier for the iso-electronic Gd³⁺ ion is proposed. The characteristic parameters of the contributions to splitting influenced by the ligand point charges are calculated. The phenomenological values of the characteristic parameters of the contributions to the splitting dependent on the metal-ligand exchange effects are found. The Eu²⁺ S-state splitting in CaFCl, SrFCl, BaFCl crystals is interpreted.

The authors show under what conditions a pseudo-Jahn-Teller effect, for which odd distortion modes are active, can couple with the E(X) epsilon Jahn-Teller effect, which arises for certain transition-metal ions in octahedral and tetrahedral arrangements of ligands. The geometries of CuCl₆ octahedra as found in CsCuCl₃ and (CH₃)₄NCuCl₃ at room temperature, which show this pseudo-Jahn-Teller effect, are discussed.

The expressions for spin-spin and spin-other orbit correlated crystal fields are obtained by the conventional method of expanding the crystal-field potential in terms of irreducible spherical tensors and using second-order perturbation theory. The explicit expressions for their contributions to the ground-state splitting of the S-state ion, the ⁸S_7/2 state of Gd³⁺, are also obtained. A formulation is developed for parametrisation of the general correlation crystal field.

The authors report measurements of non-linear current-voltage characteristics and harmonic distortions in the conductance of several morpholinium (TCNQ)₂ salts. The harmonic distortions show that there is no threshold field for the onset of non-linearities in the conductivity. Furthermore they show that the I-V characteristics can be expanded in Taylor series around E=0. A simple expression is proposed to describe the non-linearities at low-fields. At high fields the relative change in conductivity is approximately the same for most materials investigated. This suggests that the transport mechanisms at high fields and at low fields are related.

The absorption spectrum of the shallow acceptor in diamond is shown to contain Fano resonances involving the one-, two- and three-phonon optic mode sidebands of bound-to-bound transitions, in contrast to silicon, where only one-phonon resonances have been observed. The authors report strongly temperature-dependent oscillations in the photoconductivity spectrum of the acceptor in diamond, in addition to the essentially temperature-independent Fano-induced minima.

The authors study a two-dimensional Ising model on a triangular lattice which is divided into two sublattices. One consists of a honeycomb of sites and the other of the remaining interstitial sites. The interaction energies within the honeycomb and between the honeycomb and the interstitial sites differ from each other. The method used consists of a combination of transfer matrix techniques and the phenomenological renormalisation procedure. Results are presented for the critical temperature and exponents in zero field and the phase diagram in the temperature-field plane is obtained.

AC susceptibility ( chi ) measurements have been carried out in disordered spinel ferrites Co₂TiO₄, Co₂SnO₄ and Co_1.2Zn_0.8TiO₄ between 1.7 and 100 K. The last two systems are studied in order to understand the puzzling result for Co₂TiO₄. The measurements show as many as five peaks in the chi against T curve of this system. In Co₂SnO₄ only three chi peaks, and in Co_1.2Zn_0.8TiO₄ only one chi peak, are seen. DC magnetisation and neutron diffraction studies by earlier workers have shown Co₂TiO₄ and Co₂SnO₄ to be re-entrant systems and Co_1.2Zn_0.8TiO₄ to be a spin glass below about 16.5 K. Furthermore, whereas A site canting is present in Co₂TiO₄, Co₂SnO₄ has collinear magnetic structure indicating strong A-B coupling. On the basis of all these results it is concluded that the A and B site spins in Co₂TiO₄ freeze separately, which is a consequence of the weak A-B coupling in this system. The nature of the variation in the position of the chi peaks with the applied DC field, H, supports this pictures. H is applied collinearly to the AC field.

The cubic to tetragonal phase transition in CsCaCl₃(T_c=95 K) is investigated by monitoring the electron paramagnetic resonance lines of the Gd³⁺-S^2- pair substituted for a Ca²⁺-Cl^- bond, from 40 to 300 K. A slight discontinuity in the temperature dependence of the local rotation of the pair axis and the existence of tetragonal domains in the cubic phase give evidence for a first-order transition. Lineshapes and critical line broadening are tentatively discussed and compared to results obtained in SrTiO₃:Fe³⁺-V₀ and RbCaF₃:Gd³⁺-O^2-.

The Mossbauer spectra of paramagnetic Fe(II) ions in natural orthopyroxenes eulite and bronzite have been taken over 1.3 to 295 K. At 4.2 K they show the effect of magnetic relaxation which arises from the spin-spin interaction, as observed for hypersthene. The spectra of bronzite are sensitive to small applied fields approximately a few kOe and they confirm the dominance of the spin-spin relaxation mechanism at low temperatures, while the spin-lattice relaxation remains effectively suppressed. These orthopyroxenes appear to provide a range of systems, depending upon their Fe(II) content, where magnetic spectra with slow spin-spin relaxation rates are observed with or without the presence of an external magnetic field.

The authors develop the theory of magneto-optical transitions between any of the valence bands and the conduction band for semiconductors described by the Kane model, or its anisotropic generalisation, the Bodnar model (1977). Formulae are presented for all allowed interband matrix elements, for materials with both positive and negative (inverted) gap. Special attention is given to transitions, which are only allowed by virtue of the anisotropy or of a magnetic field tilted with respect to an asymmetry axis.

Oxygen-related centres in silicon containing the acceptors boron, aluminium, gallium or indium, were studied by photoluminescence. Only the O band showed a dependence on the kind of acceptor. In acceptor-implanted material the formation of the P luminescence was enhanced and a new photoluminescence line was observed at 1.117 eV, which was attributed to a boron-oxygen complex.