Table of contents

The authors present large computer simulations of the sequential random close packing of binary disc mixtures. The growth mechanism is representative of gravity deposition in two dimensions. The details of the composition dependence of the close-packing fraction are related to the form of the structural phase diagram and to the network formed from disc contacts. Limited are placed on the validity of random close-packing models for binary disc mixtures.

The misfit layer compound (SnS)_1.17NbS₂ belongs to the class of intergrowth compounds. In the simplest approximation, the structure can be described as that of two interpenetrating sublattices. The true structure consists of two interpenetrating, incommensurately modulated structures. In this paper the theory of superspace groups is applied to (SnS)_1.17NbS₂. It is shown that the symmetry is fully characterised by the (3+1)-dimensional superspace group P_11s^Cm2m( alpha 00). Definitions of the component structure space groups and superspace groups are given. For the NbS₂ part they are found to be G₁=Cm2m and G_s¹=P_11s^Cm2m. For the SnS part the authors obtain G₂=Cm2a and G_s²=P₁₁₁^Cm2a. It is shown that this unified approach gives more information about the symmetry than when the subsystems are considered independently. One result is that in a structure refinement fewer possibilities have to be considered. Finally, the superspace group is used to derive the symmetry restrictions on the modulation functions of both subsystems.

Second-neighbour defect clusters, described in the Hartree-Fock approximation with Kunz-Klein localising potentials out to fourth neighbours, embedded in an infinite shell model lattice are analysed with consistent distortion and polarisation for F⁺, F (H^-)⁺, and (H^2-)⁰ defects in MgO. Basis sets for the defects are optimised, and are improved by recontraction for the first and second neighbours. Questions of total energy, nearest-neighbour displacements and localisation are addressed for each defect. For the F⁺ centre, its ground state in relation to the valence band is discussed, as is the localisation of its unrelaxed excited state. For the F⁺ and (H^2-)⁰ centres, spin densities are evaluated at each step of the calculation, and compared with experiment. Calculations that completely neglect the ion-size effect of all ions except at the defect centre give some qualitatively plausible results. The full-cluster results are used to derive short-range shell model parameters for H^-and H^2- in MgO. The method and results are critically reviewed.

A non-conventional theory for superconductivity that is not based on a Fermi liquid description is presented. Using a functional integral method the authors show that the two-dimensional Hubbard model coupled weakly in the third dimension has a superconducting solution for the non-half-filled band case. The superconducting critical temperature is determined by the Bose-Einstein condensation temperature, which increases with the coupling to the third dimension and with the hole concentration. The critical behaviour is similar to that of a three-dimensional type II superconductor with a neutral Higgs field induced by spinon fluctuations. In the language of the lattice gauge theory they find that the spontaneous breaking of the U(1) local gauge symmetry in the quantum paramagnet confining phase leads to superconductivity.

In a recent comment, Kaveh and Wiser (1986) defend their theory on deviations of Matthiessen's rule (DMRs) by questioning related experiments of the author's on aluminium, because of inconsistencies in experimental data as well as fatal influences of an extended size effect. The present reply points out essential errors with both criticisms and thus strongly insists on the validity of the experiments originally reported. The DMRS are far smaller than predicted by Kaveh and Wiser. Nevertheless, owing to some correlation of data in literature with deformation mode and annealing treatment, the DMRs appear to arise from long-range strain fields close to the dislocations. This suggests that the Kaveh-Wiser theory could apply for particularly strain-intensive dislocation arrays and help to quantify the dilatation-specific part of dislocation resistivity.

The conductivity of an extrinsic semiconductor sample subjected to a magnetic field can be reduced by illumination. This unusual situation occurs when the importance of the emerging mixed magnetoresistance balances the increase in free carriers. An analytical model is proposed together with a discussion of the optimum conditions to obtain a negative photomagnetoconductivity. The model is experimentally verified using n-type GaAs and InSb epitaxial films.

The effect of energy bands on the amplification coefficient of Rayleigh waves in thin bismuth films is investigated quantum mechanically in the gigahertz frequency region. It is shown that the amplification coefficient for the modified non-ellipsoidal non-parabolic model is much closer than that for the non-ellipsoidal non-parabolic (NENP) model to that for the ellipsoidal parabolic model or the ellipsoidal non-parabolic model in the low-temperature limit. Therefore the NENP model in the thin bismuth films does not explain very well the electronic transport properties in the gigahertz region at very low temperatures.

Antiferromagnetism on the face-centred cubic lattice is frustrated. The different competing spin arrangements are degenerate in the classical limit of the Heisenberg model. The author considers quantum-mechanical spin fluctuations, using the Holstein-Primakoff transformation, as a perturbation to lift this degeneracy and find that the collinear spin arrangement is stabilised. When he considers the inclusion of paramagnetic impurities, however, he finds that non-collinear arrangements are locally preferred. He suggests this scenario as a possible explanation for the fact that manganese, when quenched in a face-centred cubic lattice, can be driven from a collinear to non-collinear spin arrangement by doping with either nickel, iron or iridium.

The single-crystal elastic moduli for the monoclinic (ferroelastic) phases of BiVO₄ and LaNbO₄ at 300 K are analysed with respect to variations of elastic properties in directions normal to the axes of twofold symmetry. In both crystals the directions of minimum shear modulus, G=1/S₅₅', are within 4 degrees of rotation from the directions of the ferroelastic domain walls. The maxima in G occur in directions rotated 45 degrees to the domain-wall directions and are therefore parallel to the principal axes of the spontaneous strain at the phase transition from the high-temperature paraelastic tetragonal II phase. In contrast, the Young's moduli have maximum values parallel to the domain-wall directions but minimum values parallel to the axes of spontaneous strain. However, the cross-coupling compliance moduli, S₁₅' and S₃₅', both have zero values for these principal axes of the spontaneous strain. The elastic symmetry in domain-wall directions within the plane normal to the twofold symmetry axes is almost identical to that for the tetragonal II prototype phase.

The analytic solution of the set of site-site Ornstein-Zernike equations within the Chandler-Andersen approximation for a mixture of hard spheres and symmetric rigid molecules is presented. Using the zero-pole approximation, the site-site radial distribution functions are calculated.

A one-dimensional particle system with arbitrary nearest-neighbour interaction and arbitrary external field is investigated. The system is considered as an inverse problem, that of determining the internal and external interactions needed to produce given singlet and nearest-neighbour pair densities. The profile equations, the entropy functional which generates them, and the redefined direct correlation function are found exactly. Equivalent expressions are derived in which the full pair distribution and potential-range truncation are the controlling functions.

The authors have measured the electrical resistivity, rho , and the thermopower, S, for the liquid alloy MgZn across the entire concentration range. New data for rho and S for pure liquid Zn are also reported. Data for the liquid alloy are shown to be consistent with the Faber-Ziman theory, provided the pseudopotential for Zn is assumed to be energy dependent. A comparison of the data for the liquid alloy with data previously reported for the solid amorphous alloy indicate that this energy dependence should also be considered in the solid.

The authors have reported on self-consistent second-order perturbation theory (SC) calculations of one-particle spectra of metals described by a simple model (rectangular and semielliptic) uncorrelated band local density of states. They have found that any additional (satellite) structure in one-particle spectra appearing in the non-self-consistent approach is smoothed out by the SC process. However, real metals reveal rather complicated structure of the density of states so they feel it to be necessary to verify again to what extent a more complex and d-band nickel realistic band LDOS is modified by correlation effects. Firstly they present a short recapitulation of the SOPT approach developed by Treglia et al. (1980) and describe the SC procedure; secondly report on numerical results and give a short discussion.

The superconducting transition temperatures of BaPb_1-xBi_xO₃ (BPB) and Ba_xK_1-xBiO₃ (BKB) are evaluated on the basis of the strong-coupling theory. The electron-lattice interaction is calculated microscopically by using the realistic electronic band structure. It is shown that the superconducting properties of BKB, such as high transition temperatures ( approximately=30 K) and rather small isotope effects, can be understood within the phonon mechanism.

Experimental results for NaCl:Pb²⁺ crystals on the luminescence polarisation of lightly doped samples indicate mainly trimer centres with C₃ symmetry and a low concentration of D_4h dimer centres. Calculations of the cluster stability performed by Bannon and co-workers (1985) for the same systems are in agreement with the aggregation pathway proposed by the authors.