Table of contents

The structure of a non-stoichiometric copper compound Cu_1.8S (digenite) has been studied by X-ray diffraction. Although digenite has a cubic anti-fluorite structure, satellite reflections were seen along the (111)_c axis. Single-crystal X-ray diffraction data, including first- and second-order satellites, were collected using a four-circle diffractometer. A rhombohedral twinning model was used to determine the modulation densities of the copper ions. The final R-factor is about 10% including the main and satellite reflections. The present analysis shows that the crystal is composed of domain-like structures: in each domain, the dimensions of the regular lattice, formed from copper ions, are slightly larger than those of the basic sulphur lattice. The origin of this domain-like structure, based on the Frenkel-Kontorova model, is attributed to the non-stoichiometry and apparently smaller cation-anion interaction.

The implicit approximations made in the derivation of the kinetic rate laws proposed by Salje, Glauber and Kawasaki are discussed. The Glauber and Kawasaki rate processes are relevant for discrete Ising systems with non-conserved or conserved order parameters, respectively. The Salje rate equation describes continuous rate processes. Molecular dynamics simulations of the phi ⁴-Hamiltonians are in close agreement with the predictions of the Salje rate law.

When a quasi-particle current passes into a disordered superconductor, from ideal normal leads connected to external reservoirs, a finite electrical resistance R_S arises from scattering processes within the superconductor. A new formula for R_S is obtained, which reduces to the well-known Landauer formula in the absence of superconductivity. If R₀ ,R_a(T₀, T_a) are reflection (transmission) coefficients associated with normal and Andreev scattering respectively, one finds, in one dimension at zero temperature, R_S=(h/2e²)(R₀+T_a+ delta )/(R_a+T₀- delta ) where delta is a small parameter arising from the absence of inversion symmetry. Generalizations of this result to finite temperatures and higher dimensions are also obtained.

The critical resistivity required for the temperature coefficient of electrical resistivity to change sign, for weakly disordered transition metals, may be larger than the Mooij universal boundary. A theoretical modification of the conduction electron wavefunction that involves including the d-state hybridization for the transition metals has a screening effect on the electron-phonon interaction and enhances the effect of localization on the inelastic scattering,.

Films of a-Si:H have been prepared in a vacuum deposition system both with and without the help of low energy ion beam bombardment. Water vapour was introduced into the system in order to determine how it is incorporated into the Si film and to observe the effects of the ion beam on this process. Silane was used as the feed gas for the ion source and the ion beam enabled the oxygen to form only the bridging configuration between Si atoms. Without the ion beam, three oxygen related absorption peaks are obtained in the infrared spectrum. Water-free films of a-Si:H were prepared to investigate the effects of ion beam voltage, ion beam intensity and substrate temperature on the resulting film photoconductivity. These parameters are interrelated and a maximum in the photosensitivity is found for rather narrow ranges of these parameters.

The authors report magnetotransport measurements and the quantum Hall effect in lateral surface superlattices, realized in GaAs/AlGaAs high mobility heterojunctions with grid gates. They observe perturbation or destruction of the integer quantum Hall effect by a sufficiently strong periodic potential.

The effects of a random field on the transition temperature in a quenched site-diluted Ising ferromagnet with coordination number, z=3 are investigated by the use of the effective-field theory with correlations. The authors find that the critical concentration of magnetic atoms at which the transition temperature reduces to zero depends on a random field. The influence of the dilution on the re-entrant phenomenon is also studied.

The authors consider an integrable SU(2)-invariant model consisting of the Heisenberg chain of arbitrary spin S (Takhtajan-Babujian model) interacting with an impurity of spin S'. The impurity is assumed to be located on the mth link of the chain and interacts only with both neighbouring sites. The starting point is a set of commuting transfer matrices, whose local weights satisfy triangular Yang-Baxter relations. The diagonalization of the transfer matrices leads to the Bethe ansatz equations for the model. The thermodynamics of the system is studied. Three situations have to be distinguished: (i) If S'=S the impurity just corresponds to one more site in the chain. (ii) If S'>S the impurity spin is only partially compensated at T=0, leaving an effective spin of (S'-S). (iii) S'<S the entropy has an essential singularity at T=H=0, giving rise to critical behaviour as H and T tend to zero. These properties are in close analogy to those of the n-channel Kondo problem.

The EPR spectra of the AsO₄^4- centre in CsH₂AsO₄ are overwhelmed by inhomogeneous broadening due to superhyperfine (SHF) interactions predominantly with one caesium and two protons but with small contributions from many other nuclei. The temperature dependence of these spectra shows marked changes in the temperature range 250-320 K and thus far has not been explained. Using a model which involves SHF coupling of the AsO₄^4- centre with two caesiums, two close protons and two far protons we have performed calculations based on the modified Bloch equations to simulate the EPR spectra. The ENDOR of ¹³³Cs and ¹H was also performed to obtain the necessary ingredients for computer simulations. Motional correlation times for the fluctuations involving relative motions of ¹³³Cs, ⁷⁵As and ¹H nuclei have thus been obtained in the temperature range 260-320 K. The near equality of these motional times which bears strong similarity to the soft-mode motions but on a different time scale is suggested and discussed.

The electronic structure and local magnetic properties of disordered Fe-rich Fe-Si alloys were calculated using the first-principles self-consistent cluster method. The calculated local magnetic moment, isomer shift and hyperfine field are compared with the existing experimental results. The Fe 3d magnetic moment was found to decrease with the number of Si atoms appearing in the first and second neighbouring shells. The isomer shift increases and the hyperfine field decreases in magnitude with an increasing number of Si atoms in the cluster.

The author makes a contribution to the understanding of the phenomenon of the electron distribution under the circumstances of a beam-induced charge build-up in a target. Such phenomena are encountered in electron-beam-assisted microanalysis or in Auger electron analysis. The theoretical problem of the motion of a beam of charged particles in a solid is resolved by interpreting the transport equation in terms of an integro-differential equation. A treatment of the Boltzmann equation is developed for the energy dependence in order to predict the electron penetration in targets bombarded under the simultaneous influence of scattered and decelerated incident particles. The exact values of the specific moments of the electron distribution in a semi-infinite medium are calculated. The author suggests a procedure applied to the calculation of an analytic expression characterizing the asymptotic trend of the electron spatial distribution at very small residual energies. The perturbing effects of insulator irradiation are treated and charge deposition profiles near the surface are calculated for mixed alkali silica glasses. Results are compared with previous data; agreement is in most cases fairly good.

The authors have measured the optical absorption coefficient of liquid As₂S₃ and As₂Se₃ up to 900 degrees C in a wide absorption range up to about 3*10⁵ cm^-1. They have developed an optical cell of their own design with a sample thickness of about 2000 AA. The optical gap obtained from the spectra decreases rapidly with increasing temperature, eventually becoming zero at about 950 degrees C for liquid As₂Se₃, where the semiconductor-to-metal transition starts to occur.