Table of contents

The dependence of the electron effective mass m* on the band gap E₀ has been studied in the quaternary alloy Ga_xIn_1-xAs_yP_1-y (y=2.2x) for the two compositions y=0.55 and y=1.0. As hydrostatic pressure was applied m* was measured by magnetophonon resonance while E₀ was determined by photoconductivity techniques. It was observed that the rate of increase of m* was much larger than predicted by the three-band or multiband formulations of the k.p theory based on the authors experience of III-V compound semiconductors. It has been tentatively concluded that alloy disorder profoundly affects the k.p interaction in semiconductors.

Zeeman measurements on the 1.03 eV system of lines in the absorption and photoluminescence spectra of GaP(Cr) are interpreted in terms of internal transitions of Cr³⁺.

The calculation of the thermoelectric power of superlattices, previously carried out in a tight-binding approximation, is here extended to a general energy-momentum relation. For the Kronig-Penney potential, corrections to the thermoelectric power are obtained and are shown as a function of the Fermi level within the miniband. The corrections are shown to be appreciable for narrow barriers and/or small conduction band offsets.

It is shown that, due to the temperature dependence of screening, the low temperature-dependent conductivity is given by sigma (T)= sigma (0) (1-1.39C(n)(T/ epsilon _F)-0.81C(n)²(T/ epsilon _F)³²+O (T²)) if elastic impurity scattering is considered. 0<C(n)<1 is a density-dependent form factor. With weak localisation and interaction corrections to the conductivity a temperature T_m is found, where the conductivity of the two-dimensional electron gas is maximal.

The conductance of a loop shorter than the inelastic diffusion length, connected to leads with many quantum channels, can be asymmetric under magnetic field reversal.

ODMR investigations of e-irradiated InP:Zn show that the P_In antisite resonance is associated with emission at 0.89 eV. The authors assign this recombination process to the capture of the second electron at the P_In⁴⁺ centre. In analogy with GaAs where the EL2 centre pins the Fermi level at mid-gap, they propose that the P_In³⁺ (D₁⁰) level at (E_c-0.89 eV) should allow as grown semi-insulating InP to be prepared by off-stoichiometric growth.

Lucky drift theory is used to derive an alternative expression for the impact ionisation coefficient in a semiconductor to that given by B.K. Ridley (1983). This alternative expression has a simple physical interpretation. It also agrees well with a Monte Carlo simulation for a model semiconductor.

Liquid metals and plasmas are treated as binary mixtures of nuclei and electrons. The thermodynamic potential for the inhomogeneous system caused by fixing one of the nuclei in the mixture is constructed by associating it with a non-interacting ion-electron mixture as a reference system to describe this nucleus-electron mixture. With the use of the hypernetted chain (HNC) approximation, the Kohn-Sham-Mermin formalism based on this leads to a set of integral equations for the correlation functions: this set of integral equations also determines the average nuclear charge of the ion, the electron-ion potential and the electronic bound energy levels in a self-consistent manner, and thus becomes identical to the HNC equation for the ion-electron mixture. The electron-ion direct correlation function determined by these equations is shown to be equivalent to a pseudopotential introduced by Dagens et al. (1975) to take account of the non-linear effects. The conditions for the validity of the various conventional plasma theories, including the problem of estimating the average ionisation, are discussed in view of the present theory.

The authors discuss the result of taking the classical limit on the ions of a metal while retaining a quantum-mechanical description of the electrons. One finds that when this limit is possible the ionic motion is determined by a classical Hamiltonian, but in general this gives rise to velocity-dependent forces. These forces are non-adiabatic in origin and the motion of the ions can be fully classical only if the non-adiabatic terms in the Hamiltonian are small enough for second-order terms to be negligible, although a quasiclassical description may still be possible if this condition is not fulfilled. The existence of the Hamiltonian is of fundamental importance to the theory of electromigration as it clarifies the origin of the driving forces and enables the authors to discuss the response of the ions to the driving forces in terms of the usual formulation. The formal solution of the linearised Liouville equation is obtained to first order and is discussed for the case where the driving forces are obtained to second order in the electron-ion scattering.

A microdynamic theory for the self-diffusion coefficient, which involves a wave-number-dependent viscosity, eta (q), is successfully tested on a hard-sphere system. The models the authors introduce for eta (q) are adapted to the Lennard-Jones fluid. New molecular dynamics data for the relative diffusion coefficient, in which the initial separation of two particles appears as an additional degree of freedom, are reported for this fluid. A systematic investigation of near-neighbour pairs shows (i) clear evidence of effects that slow their relative motion and (ii) that the relative diffusion coefficient varies remarkably smoothly with separation. The theory is extended to interpret the results and it is shown that the expressions for both types of diffusion coefficient take a form that is strongly reminiscent of a hydrodynamic treatment. This, the authors suggest, is why hydrodynamic theories (e.g. the Stokes-Einstein equation for the self-diffusion coefficient, and the Oseen approach to relative diffusion) appear to be applicable even at an atomic level. They emphasise, instead, the essentially microscopic nature of the diffusion process in monatomic liquids.

Uncoupled Hartree-Fock perturbation theory calculations of the Sternheimer antishielding factor gamma _{varies as} are reported for the experimentally important ions of N^3-, Cl^- and Cd²⁺ imbedded in ionic solids. Whereas the gamma _{varies as} value for Cd²⁺ is found to be similar to that for Cl^-, the corresponding antishielding function gamma (r) shows significant differences. The core-only estimates of the antishielding factors for the ions considered are given.

Hartree-Fock equations are solved for a large unit cell (LUC) periodic system using the linear combination of atomic orbitals (LCAO) approach and within the k=0 semiempirical complete neglect of differential overlap (CNDO) approximation. Convergence difficulties in previous approaches are resolved by careful analysis of the density matrix dependent terms in the Fock matrix. Detailed studies of the density matrices of analytically solvable one-dimensional tight-binding models give insight into the properties of distance-dependent modulating functions that weight certain Coulomb interaction terms. It is shown that the k=0 approximation cannot be fully self-consistent, but it is nevertheless possible to develop satisfactory total energy algorithms that converge rapidly with interaction distance.

Angle-resolved photoemission spectroscopy (ARPES) measurements of the E(k/sub ///) band structure are reported for 1T-TaS₂ and 2H-TaSe₂ at temperatures where commensurate CDWS are well developed. Empirical tight-binding calculations of the band structure in the presence of the experimentally known CDW superlattice are also reported. In the case of the square root 13* square root 13 CDW in 1T-TaS₂, experiment and theory both reveal a gross distortion of the band structure, in which the Ta-derived d band collapses into three sub-band manifolds separated by gaps. The thirteenth electron is predicted to reside in a conduction band only 50 meV wide, where it is susceptible to a Mott-Anderson transition. In the case of 2H-TaSe₂, additional peaks are seen in the ARPES data in the presence of the CDW, but the effects are too weak to distinguish between a band-structure distortion and weak Umklapp associated with the 3*3 superlattice. Calculated densities of states, decomposed according to the inequivalent atomic sites, are presented.

For pt.I see ibid., vol.17, p.5337 (1984). The recently developed theory for the differential conductivity in a disordered system including the dynamical aspects of the electron-phonon interaction, is applied to various models of band tails in amorphous semiconductors. The results indicate, that this model of a spatially homogeneous system accounts for a large body of experimental data of DC transport properties of undoped and weakly doped samples, provided the slope parameter, T₀, of the band tail is small and the electron-phonon coupling is not negligible.

For an electron interacting with harmonic dispersive phonons in a periodic lattice, the possibility of wave (soliton)-like solutions of the generalised master equations (GME) for the electron site occupation probabilities is investigated. For the current form of the small-polaron Hamiltonian, the long-time asymptotics of the memory functions (kernels of the GME) in the small-polaron basis are discussed to all orders in the electron hopping integrals.Using this result, one obtains (for the initially localised electron) the t²-behaviour of the mean-square electron displacement at zero temperature. This corresponds to existing soliton solutions of the Schrodinger equation. At non-zero temperatures, the wave solution is damped.

First-principles calculations of the electronic structure for the crystals KMgF₃:Fe²⁺ and KMgF₃:V²⁺ have been performed. The one-electron Hamiltonian of the impurity crystal is written as the perfect-crystal Hamiltonian plus a local potential which accounts for the ion substitution and the distortion of the host crystal. The statistical exchange approximation is centred at atomic sites through the eleventh nearest neighbours to the impurity ion. The zeroth-order approximation to the impurity crystal potential is constructed from the self-consistent-field (SCF) solutions of the pure KMgF₃ crystal and of the free impurity ions. An iterative scheme is devised to make the solution for the wavefunctions self-consistent. The calculated 3d to 4s and 3d to 4p transition energies for KMgF₃:V²⁺ agree well with the optical absorption experiment. In the case of KMgF₃:Fe²⁺, the authors calculated value of the 3d to 4s transition energy is close to the experimental value and suggests that the observed absorption at 68000 cm^-1 or near 70000 cm^-1 may be attributed to the 3d to 4p transition. The calculated oscillator strengths for the 3d to 4p transitions are in reasonable agreement with experiment.

Resonant phonon scattering by the stress-split acceptor ground state has been investigated in Si:B and Ge:Ga. Superconducting tunnel junctions were used as monochromatic phonon sources both in the single-particle tunnelling regime and in the AC Josephson regime. The 'static' deformation potential constant D_u'^a was determined from the linear dependence of the splitting energy on uniaxial stress and was found to be in good agreement with values obtained in previous work. The strength and the width of the resonance line were used for a comparison with existing scattering theories and from this a 'dynamic' value for D_u'^a was obtained; this is in good agreement with the 'static' one. The high-frequency cut-off of resonant phonon scattering in Si:B can be described reasonably well within the effective-mass approximation by using the acceptor wavefunctions of Suzuki and co-workers (1971-2). However, this approach is not satisfactory for Ge:Ga.

The eighth-order perturbation formula for the EPR cubic field splitting parameter alpha has been obtained by taking a cubic crystal field and the spin-orbit interaction as the perturbation Hamiltonian. In the calculation, not only spin quartet states but also spin doublet states have been considered. The calculated result shows that the contributions of the double states and eighth-order perturbation cannot be neglected. For MgO:Mn²⁺ and MnCl₂.2H₂O, the d-d transitions and the cubic zero-field splitting parameter have been calculated by using Zhao's self-consistent-field (SCF) orbital for the Mn²⁺ ion and the high-order perturbation formula for the cubic zero-field splitting parameter have been obtained. The results of applications agree well with experiments.

Mossbauer spectra of K₂FeF₅ near the spin-flop phase transition indicate that domains of antiferromagnetic and spin-flopped spins coexist over a range of applied fields Delta B_qpp=0.4T centred at B_sf=3.65T. This is discussed with reference to the first-order character of the spin-flop transition.

Following the oxidation of a terraced beta Sn(001) surface between 135K and 415K, an amorphous Sn(II) oxide was identified by ELS and LEED. The thickest oxide layer was formed at 135K; at this temperature shifts of between 3 and 5 eV in the major tin Auger transitions were recorded. Between 293 and 415K there were no shifts in the Auger peaks on oxidation.