Table of contents

The authors calculate the critical exponent of percolation conductivity for 35*35*35 three-dimensional network by using Monte Carlo simulations. They obtain the critical exponent t to be equal to 1.90+or-0.10 and discuss the values of t in terms of present models of backbone of infinite cluster. They also obtain the critical exponents beta =0.38+or-0.02 and beta '=0.93+or-0.03.

DLTS studies are reported on certain defects introduced in boron-doped floating-zone silicon after 1.5 MeV electron irradiation at 80K. A defect which appeared after annealing out of the vacancy was seen to exist in two charge state related metastable configurations. Two levels at E_v+0.31 eV and E_v+0.37 eV are attributed to one of these configurations: a level at <or approximately=E_v+0.13 eV to the other. The evidence suggests that the defect is the boron substitutional-vacancy complex.

The authors have used their Green function method to study the electronic structure of an isolated mercury impurity at the substitutional cation sites of cubic CdTe. They find that the mercury potential is strong enough to lower a hyperdeep s-like state below the valence band of CdTe. This weakens the covalent bond and leads to changes in the valence density of states. They also find localised resonances at the bottom of the conduction band. It follows that HgCdTe alloys cannot be adequately described by the virtual crystal approximation. Their prediction is in good accord with the experimental and theoretical findings of W.E. Spicer et al. (1982).

Debye's idea of an 'a priori probability function' of interatomic distances in a liquid is generalised by the theory of paracrystals. In the case of a square lattice this generalised model is studied using a two-dimensional coin model. Between the nearest-neighbour separations the only correlation is with respect to their directions. The netplanes formed by nearest neighbours are 'bearing netplanes', so-called because of the bearing role they play in microparacrystals. In the expression for free enthalpy Delta G(N) a new term ³/₂NA₀g² takes into account the minimum given by the alpha *-law, where N defines the number of bearing netplanes in a spatial direction, g is the nearest-neighbour-distance fluctuation and A₀ the enthalpy density given by the tangential potentials. Delta G depends on the number of valence bonds within a lattice cell and therefore increases with the distance d₀ between the bearing netplanes. alpha * then decreases automatically and reaches values of alpha *=0.15+or-0.05 for d₀<or approximately=3 AA and alpha * approximately=0.05 for d₀>3 AA. Even larger alpha *-values have been observed in the DuPont Kevlar fibre; this can be partly explained in terms of a non-equilibrium state. The bearing netplanes build up the surface of a microparacrystal in an equilibrium state and thus replace Wulff's rule.

The energy, the elastic dipole tensor and the entropy of point defects in ionic crystals are usually calculated by the Mott-Littleton approach, which treats a single defect in an infinite crystal. The authors suggest that there may be advantages, particularly for the dipole tensor and the entropy, in performing the calculations in periodically repeating geometry. They examine how this 'supercell' method can be used for the energy and the dipole tensor, paying attention to the problem that for charged defects the repeating unit carries a net charge, so that the Coulomb energy is divergent. They test the supercell method on a number of both charged and uncharged defects, and show that the results for the energy and dipole tensor are in close agreement with those obtained by the conventional approach.

The correct expression for the heat capacity at constant volume (C_v) of anharmonic crystals has been given recently. Some authors make a volume expansion of the Helmholtz energy to calculate C_v, where the Helmholtz energy is not even a function of volume at constant volume. In the opinion of the author their works require correction.

The statistical properties of non-linear two-component ortho-para exciton gas are considered, employing the Bogoliubov variational principle and the mean-field approximation. The exciton concentrations are determined from a set of non-linear self-consistent equations. The exciton distribution functions depend essentially upon the exciton level shifts, which in turn depend on the exciton concentrations. There are three sets of solutions in any range of values of the chemical potential. These solutions mean the existence of three different exciton phases: the para phase, the ortho phase and a mixed ortho-para phase. The most stable of them is the para phase, as was shown by studying the pressure dependence of the chemical potential. In the framework of the non-linear statistics the Bose-Einstein condensation of not only para-excitons but also ortho-excitons may occur.

The authors consider a two-dimensional interacting electron gas in a strong perpendicular magnetic field with an integral number of fully occupied Landau levels. An expression for the spin-dependent response function of this system has been derived in a Hartree-Fock approximation. Numerical results for the static polarisability have been obtained for one, two and three full Landau levels. Dispersion relations for the collective excitations of the system have been determined by locating poles in the response functions. Some conclusions are made concerning the reliability of the approximation by referring to the low-field limit.

Results on the optical characterisation of copper deep acceptors throughout the alloy composition range (0<or=x<or=1) of GaAs_1-xP_x are reported. A comparison of the hole photoionisation spectra for different compositions, measured by the transient photocapacitance technique, reveals a broadening effect near the threshold which is interpreted in terms of random alloy effects on the deep level. An approximately linear variation of the hole photoionisation threshold with composition is found.

Cyclotron resonance (CR) in inversion layers on p-InSb has been measured at laser energies h(cross) omega =10.4 and 26.6 meV as a function of temperature (T approximately=4-200K) and inversion electron density (n₅ approximately=0.2-2*10¹² cm^-2) to investigate the CR lineshape. In particular, the authors study quantum oscillations and their influence on sub-band CR. They find that quantum oscillations show the same periodicity and temperature dependence as the corresponding Shubnikov-de Haas oscillations of the static conductivity. The experimental peak position of sub-band CR is not significantly (<or approximately=5%) affected by quantum oscillations. Cyclotron masses can most reliably be extracted at elevated temperatures (T approximately=50K) when the oscillations are smeared out.

The quantum inverse transform method is applied to an anisotropic Heisenberg ferromagnet with a domain-wall-like ground state. The quantum Gelfand-Levitan-Marchenko equations are obtained and the profile of the domain wall is calculated.

Inelastic neutron scattering measurements have been performed on monocrystalline samples of antiferromagnetic iron monoxide (Fe_1-xO) for several compositions (x=0.056, 0.070 and 0.072). The magnetic excitations up to 8 THz have been investigated in order to find with precision the stoichiometry dependence of the spin dynamics. The experimental results have been analysed using a Green function formalism taking into account the spin-orbit interaction, the magnetoelastic coupling and the exchange interactions extending to the second neighbours. The main effect of the stoichiometry on the spin wave branch and the first excited level is a broadening of the magnon lineshapes. Nevertheless, in the case of x=0.056, several peculiar excitations have been found which are probably related to the defect structure and long-range ordering. This ordering has been corroborated by the observation of elastic scattering corresponding to a modulated magnetic and nuclear structure.

Using the Monte Carlo technique, the authors study a simple cubic fully frustrated Ising crystal. They find a sharp second-order phase transition, contrary to what is predicted by mean-field theories. By finite-size scaling, the authors find k_BT_c/J=1.355 for an infinite lattice. Various physical properties are studied in detail. The behaviour of the system at low temperatures is particularly interesting: due to excitations of linear chains, some sublattices become disordered far below T_c. This results in the appearance of a shoulder of the specific heat below the transition peak. They derive the critical exponents alpha and nu above T_c from the specific heat and spatial correlation functions. These results suggests a crossover to another class at higher temperatures. The critical exponent delta is also given.

Raman spectra showing features associated with the internal and external modes of vibration of the NO₃^- ion isolated within a KI crystal have been recorded for a sample temperature of 80K under hydrostatic pressures of up to 5 kbar. Over this pressure range the frequencies of all the observed modes appear to increase linearly with the applied pressure at rates between 0.4 and 1.05 cm^-1 kbar^-1. Contrary to the conclusion reached in a recent paper by Khatri and Verma (1982) it is shown that the volume dependence of the frequency of the 72.3 cm^-1 gap mode is the dominant effect in the temperature dependence of this frequency.

The L₃ X-ray absorption near-edge structure of R in RM₂X₂ (R=Nd, Sm, Gd, Tm, Yb; M=Mn, Co, Ni, Cu, Pd; X=Si or Ge) intermetallic compounds exhibit a prominent absorption maximum called a white line. The position, halfwidth and height ratio of the white line are interpreted in the light of recent reports.

On the basis of an effective Dirac-Hamiltonian, which describes the interaction of a quasiparticle or quasihole with a semi-infinite crystalline ferromagnet well below the Curie temperature, a one-step-model Green function theory of energy-, angle- and spin-resolved ultraviolet photoemission and bremsstrahlung is formulated. The elastic photocurrent spin density matrix is evaluated in a manner analogous to a Schrodinger-equation-based layer-KKR method (due to Pendry). Since the authors spin-dependent formalism includes both spin-orbit coupling and magnetic exchange, its applicability extends from ferromagnets a fortiori to non-magnetic materials, in particular those with strong spin-orbital coupling.