Table of contents

Recent claims that for the Lloyd model in arbitrarily many dimensions electrons are always localised are shown to be ill founded.

The spin-orbit degenerate Anderson model is investigated in the limit of large spin-orbit degeneracy (N). Low-temperature properties are discussed, yielding the full range of intermediate to local moment behaviour. The relation between ground state susceptibility and valence is examined to leading and next-leading orders in 1/N.

The authors have studied the temperature dependence of the susceptibility of insulating Fe_xMg_1-xCl₂ in various uniform magnetic fields up to 8000 Oe. For x=0.286 spin glass behaviour is observed and the reversibility temperature moves down with field. For x=0.552 results give evidence for two transitions: paramagnetic-antiferromagnetic phase and antiferromagnetic-spin glass phases; the uniform magnetic field enlarges the domain of existence of the spin glass phase (H<5000 Oe), giving an experimental illustration of random field effects.

Incoherent neutron scattering results on the layered hexagonal phase of CsH₃O₂ are presented. The elastic incoherent structure factor (EISF) measured at 402K indicates that the protons are dynamically disordered in a double well potential between adjacent oxygen atoms. The dynamics are well approximated by a two-site random jump model with a mean residence time of 1.25*10^-11 s per site and a jump distance of 1.03 AA. The large value of the latter suggests a non-linear hydrogen bond.

The structure of solitary waves in a quantum molecular chain, with the contribution of phonons with different wavenumbers taken into account, is investigated. The problem is solved on the basis of a non-linear non-local wave equation. It is shown that there exist two types of localised collective excitations which depend on the excitation velocity nu and the carrier wave phase velocity nu _c. The criteria are considered of continuity and stability of excited bound multiparticle states and the estimation of their validity for the typical parameter values of a molecular chain is presented.

For pt.II see ibid., vol.16, p.2455 (1983). The temperature and pressure dependences of the elastic and strain-related properties of the 'pseudo-proper' ferroelectric material BiVO₄ have been determined using mean-field theory. In the tetragonal paraelastic phase the elastic constants c₁₁-c₁₂, c₆₆ and c₁₆ are all predicted to behave anomalously near T_c although none are expected to vanish. From analysis of the acoustic properties of BiVO₄ zero-velocity pure transverse acoustic modes are expected to propagate along the ferroelastic domain wall directions at T_c. Predicted relationships between pressure and temperature-dependent properties agree well with experimental observations. The theory also accurately predicts the ratio of secondary and primary spontaneous strains in the ferroelastic phase.

For pt.III see ibid., vol.16, p.5093 (1983). The spontaneous strain properties of the ferroelastic material BiVO₄ are shown to be related to its acoustic properties. At the ferroelastic phase transition temperature, it is proved that a mutually perpendicular pair of pure transverse acoustic modes possess theoretically zero velocity. The propagation and displacement directions of these modes are collinear with the ferroelastic domain walls. The monoclinic ferroelastic strain in BiVO₄ results from the freezing-in of the pure transverse displacements of these soft acoustic modes.

For pt.IV see ibid., vol.16, p.5119 (1983). Measurements of the Bi³⁺ displacement and the primary and secondary spontaneous strains in the ferroelastic material BiVO₄ have been made as a function of temperature by single-crystal X-ray diffraction. The results are discussed using both macroscopic and microscopic mean-field theories of the ferroelastic phase transition. Measurements of the temperature dependence of the optical absorption edge have also been made, which show that the band-gap in BiVO₄ varies markedly with temperature. A method for the analysis of structural distortions in ferroelastics based on bond-length/bond-strength relationships is described. A discussion of the structural mechanism of the phase transition in BiVO₄ is presented, indicating that the lone-electron pair on the Bi³⁺ ion plays a prominent role.

For pt.V see ibid., vol.16, p.5127 (1983). The relationship between the domain wall orientation and the spontaneous strain in ferroelastics is discussed, with particular reference to the W' walls in BiVO₄. The microscopic, wall-defining property is shown to be the coherence of the lattices of the twin components. This concept, commonly used in metallurgy, is valid for both finite and infinitesimal strains and is more fundamental than the macroscopic strain compatibility condition derived by previous workers in the field of ferroelasticity. Observations of domain walls in BiVO₄ as a function of temperature are presented which suggest that they may be of importance in determining the properties of the material close to the ferroelastic phase transition.

A computer simulation on a cluster model of n-type doped semiconductors is performed at the impurity concentration corresponding to 1.7*10¹⁸ cm^-3 of Si:P, with the aim of testing the general physical picture and theoretical treatment in the 'transfer-diagonal' formulation. A simple Gaussian function is used as the basis of one-electron states, in order to calculate and take into account exactly all the matrix elements including those of the electron-electron interaction. The energy levels and wavefunctions of the ground and excited states of the clusters consisting of six randomly distributed donors are obtained. It is found that there are a large number of states which are occupied by single electrons and that the spins of these singly occupied states interact either by ferromagnetic or antiferromagnetic exchange interactions. The numerical results on specific heat and spin susceptibility averaged over 76 clusters reproduce fairly well the characteristic features observed in experiments on Si:P. The microscopic mechanism of these remarkable features is clarified by examining the many-electron wavefunctions obtained and the treatment in the 'transfer-diagonal' formulation is confirmed.

The electric field dependence of the photocarrier generation rate has been measured for crystalline polydiacetylenes (PTS and DCH) in the range 10<E<10⁵ V cm^-1. For E<10⁴ V cm^-1 it follows the predictions of the one-dimensional Onsager theory, above 10⁴ V cm^-1 a superlinear increase with no indication of saturation is noted. The high-field behaviour is interpreted in terms of field-assisted ionisation of Frenkel excitation states that are degenerate with conduction band states. Applicability of the 1D Onsager theory to treat geminate pair dissociation is consistent with the recently developed concept of strong electron-phonon interaction leading to short carrier mean free paths. The microscopic anisotropy of the carrier mobility parallel and perpendicular to the polymer chain direction is found to be greater than 10⁴, larger than inferred from macroscopic conduction experiments.

Local internal fields h_i⁰ in vector spin glasses with random uniaxial local anisotropy are shown to be positive on all non-isolated spins whenever the spin state corresponds to any minimum of configurational energy within the classical theory. Moreover it is shown that there exists a lower positive bound on the values of the local internal fields. This bound depends on the exchange interaction constants and on the strength of the local anisotropy.

It is now well established that the intrinsic bonding defects in amorphous SiO₂ are normally charged (C₁^- with negative correlation energy and C₃⁺ with positive correlation energy) and that they are different from those for amorphous chalcogenides. Intrinsic defects similar to those in amorphous SiO₂ are expected to exist and contribute to a.c. conduction in amorphous SiO films. It is proposed here that the a.c. conduction in evaporated SiO films can be interpreted, using this model based on intrinsic bonding defects, in terms of correlated barrier hopping by single polarons (i.e. one-hole hopping between neutral and negatively charged defects). The expression obtained for the a.c. conductivity agrees with the experimental results. The densities of the defects are estimated from the model, and the results are supported by results from studies using ESR signals.