Table of contents

The defect molecule method has been used to determine the ordering of the energy levels associated with the various charge states of the vacancy in diamond and silicon. The calculations are based upon the assumption that only the one-electron molecular orbitals of t₂ symmetry are localized within the band gap. The results derived from a single t₂ⁿ configuration are in poor agreement with experiment even when symmetric relaxation and Jahn-Teller distortion effects are included.

In this letter results are presented of a new study of the electronic structure of semiconductor surfaces. Optical data were used to obtain the Slater-Koster parameters of the tight-binding bandstructure for silicon, and these parameters have been used to study the bulk and (111) surface region of silicon. The main conclusion is the existence of only one surface state per atom concentrated just below the bottom of the absolute bandgap, and with a localization of two atomic layers (a half-length of 2.1 AA), while the competition between the termination of bulk states and the decay of surface states gives rise to lesser features throughout the valence band and lowest conduction band.

One-magnon light scattering in antiferromagnets is analysed theoretically in the spin-flop phase at low temperatures. The results show that in low-anisotropy crystals the scattering intensity varies rapidly at the spin-flop transition, making the technique of light scattering potentially useful for its investigation.

Energy spectra of electrons reflected at a very smooth surface of liquid indium show both energy losses due to the excitation of surface plasmons (8.4 eV) and multiple peaks which correspond to a gain of one surface plasmon.

To compute the electrostatic Coulomb potential at a point inside an ionic crystal by the method of direct summation, one simply adds all the potentials due to the point charges of the crystal structure. The shape of the crystal piece, used for such computations, must be chosen such that it represents the actual infinite crystal: neutral and having the general shape of the structure. In order to compute the potential inside all cubic crystals, it suffices to compute the potential inside a simple cubic lattice, since all cubic crystals are superpositions of a SC lattice. Since the point charges of this SC lattice are all of one sign, the potential inside it was computed by the use of Hund's identity. A table is given for the potential at a grid of points.

It is shown how Wilson's theory of critical phenomena (including the incomplete integration theory) may be derived entirely within the context of graphical perturbation theory.

The total energy of the uniform electron gas is calculated in the alternant molecular orbital (AMO) approximation for a cubic Fermi surface. This energy is lower than the restricted Hartree-Fock energy for the cubic Fermi surface for r_s>or=9.27 au. This transition point is discussed from the point of view of Adams' instability criterion.

A technique for calculating intervalley electron-phonon coupling constants is developed. The numerical values obtained for germanium, silicon and gallium arsenide are in acceptable agreement with published experimental values, but the results do not support the recently proposed three-level oscillator structure for indium phosphide. It is suggested that under conditions of high electric field strength and low electron occupation probability, such as commonly occur in hot electron phenomena, the scattering rates may be increased by an order of magnitude over the corresponding low field values. This field-induced enhancement requires experimental verification, but has particularly interesting implications for high field transport in indium phosphide.

The force on a moving ion in an electron gas to which an external electric field may be applied, is investigated in a semiclassical approximation, using a simple collision-time model for the mechanism which keeps the electron gas in equilibrium. It is found that in this simple model the screening of the ion has no effect on the force, so that the only correction to the force acting is that due to the 'wind effect', the momentum transfer due to the scattering of the electrons which are streaming past the ion. A complete solution of the equations is given for a weakly charged ion, to first order in the charge.

The author presents a new formalism for the study of low temperature properties of a dilute Heisenberg ferromagnet. By simplifying the diagonal term in the equation of motion for the spin Green function, he was able to write single-site selfconsistent relations for the effective exchange constant that defines the configurational averaged medium. The author calculated the density of states and the results obtained for the critical concentration are in very good agreement with previous theories.