Table of contents

Oscillatory structure has been observed in the intrinsic photoconductivity of bulk semi-insulating Fe- and Cr-doped InP and nominally pure n-type vapour-phase epitaxial InP at 2K. The oscillatory period corresponds to (1+(m_e/m_h))h(cross) omega _LO=48 meV in both cases, but in the semi-insulating material the structure is phase shifted to lower energies by 9 meV. The shift can be explained in terms of different electron relaxation mechanisms for the electrons in the two types of material.

With the matching Green function method and a simple surface potential, two sp-d hybridisation-gap surface states were found at Gamma on Mo (001), one very close to the Fermi energy and one at -3.3 eV. Another resonance appeared just under E_F on moving away from Gamma . The results are in agreement with directional photoemission experiments. The origin of the Gamma surface states is studied.

An exact calculation of all the thermodynamic properties of a linear chain of classical spins in the xy plane with random nearest-neighbour Heisenberg interactions and a Dzyaloshinski-Moriya interaction normal to the (xy) plane is presented. Only bond disorder is considered. The largest eigenvalue of the transfer kernel is real and other eigenvalues are complex; the thermodynamic quantities such as free energy, entropy, and specific heat depend on combinations of the absolute magnitudes of these eigenvalues. The phases appear in the spin-spin correlation functions of both the x-x and x-y type and hence also in the susceptibilities (wavevector-dependent as well as their long-wavelength limits). It is suggested that neutron scattering experiments may be able to determine these phases.

General expressions are given for the calculation of interlattice tensors and inner elastic constants by summation in real space and reciprocal space in the case of central forces and in reciprocal space for the case of volume-dependent band structure energies. Cauchy relations applicable to two of the tensors have been tabulated.

By inelastic coherent neutron scattering experiments performed in various Brillouin zones, it is shown that the lineshape of the TO phonon at, and in the vicinity of, q=0 has a double peak structure. The analysis of the data confirms and extends a previously proposed model in which this line shape is explained by a strong interaction between the harmonic TO phonon (the dispersion of which is here obtained) and a two-phonon density of states which is found to be independent of q.

The electronic band structures of the beta , epsilon and gamma polytypes of GaSe are studied by the tight-binding method. The calculation takes into account interlayer interactions and the generally neglected lowest excited atomic states. The energy levels of the three polytypes differ from each other by 0.3 eV at most. Inclusion of excited atomic states strongly modifies the conduction bands but has little effect on all valence states. The calculated valence band structure compares well with both angular-resolved and integrated photoemission data. The electronic charge-density distribution resulting from tight-binding Block states confirms the chemical bonding picture proposed in recent pseudopotential investigations.

Measurements of cyclotron resonance and the infrared absorption in the region of the direct band gap are reported for the complete range of alloys InAs_1-xP_x. The experimentally determined values of the effective mass are found to be almost linear functions of the alloy composition. This is in contrast to the predictions of k.p theory which, using a linear interpolation of the momentum matrix element (P²) between the values for the two alloy constituents, predicts a strong bowing. The discrepancy is thought to arise from a reduction in P² brought about by a mixing of the conduction and valence bands by random local disorder.

Magnetophonon oscillations have been observed in the alloy system InAs_1-xP_x. Two magnetophonon series were detected across the complete range of alloy compositions, resulting from the two LO phonon modes present in mixed crystals of InAs_1-xP_x. The fundamental fields have been used to calculate the LO phonon energies and to determine the temperature dependence of the effective mass. Magnetophonon oscillations were also observed in the Hall coefficient and these exhibited phaseshifts and reversals of sign with respect to the extrema observed in the transverse magnetoresistance.

From an exact calculation of the density of states for one electron in the Sherrington-von Molnar model of an s-f resonance coupled to a local optical phonon mode the authors find the conditions necessary for a polaronic reduction in the width of the resonance. With the notation nu for the relaxation shift, Delta ₀ for the resonance width in zero coupling, omega ₀ the phonon frequency and W the conduction band width, they find for nu in the range W> nu > Delta ₀ very little reduction in the resonance width, and obtain the polaronic reduction exp(- nu / omega ₀) only if nu >W. This condition is too strong to be satisfied in intermediate-valence systems. However, the model is not an adequate description of intermediate-valence materials as it is an essentially one-site model. The authors generalise the model to include two sites coupled to phonons such that the sites compete a favourable local lattice distortion. In this model they can examine the dependence of the intersite transfer on lattice coupling strength. For w> nu > Delta ₀ (the plausible physical region), this intersite transfer is reduced by a polaronic reduction factor leaving the widths of the virtual bound-state resonance almost unaffected.

Generalisations of the two apparently rational eigenvalues found by Thorson and Moffitt for the Jahn-Teller system Gamma ₈(X) tau ₂ are studied. Equations of finite order are established for determining the strength D of the Jahn-Teller interaction of which an eigenvalue lies on a baseline. (A baseline is the line that constitutes the asymptote of a sequence of rotational levels that come into existence as D tends to infinity.) The analysis is extended to the system E(X) epsilon and is illustrated by a number of special examples.

The thermoelectric power of n-GaAs has been computed at different lattice temperatures and electron concentrations incorporating the band non-parabolicity, the p-function admixture and the effects of electron screening and degeneracy on scattering probabilities. All the relevant scattering mechanisms have been considered and the currently established material parameters are used in the computations. The calculated results agree very well with experiment.

The exciton and polariton mechanisms of superconductivity for a thin metal layer on a dielectric surface is discussed. It is shown that the polariton mechanism gives a weaker interaction among the electrons than the exciton one. The effective electron interaction due to excitons increases with the increase of the magnitude of the dipole as well with the decrease of the exciton gap of the dielectric.

The thermal properties of the XY model for spin-¹/₂ on a linear chain, in the presence of random fields h_iS_i^z, are evaluated using the fermion representation of the Hamiltonian and the coherent potential approximation (CPA). The specific heat and the thermal conductivity are obtained from the appropriate Green functions. The results are discussed for a model of praseodymium ethyl sulphate (PrES) where the hyperfine interaction is considered to give rise to random fields. This interaction is found to give only a small reduction in the spin specific heat, but to be very effective reducing the spin thermal conductivity. Other mechanisms must be invoked to give a full account of the experimental results.

Calculations are given for the spin-dependent response functions (or Green functions) of a finite-thickness ferromagnetic slab using magnetostatic theory. The results are employed to obtain expressions for the integrated intensities for Brillouin scattering from the surface (magnetostatic) modes and bulk spin-wave modes of a ferromagnetic slab. Numerical estimates are made to compare the theory with recent experimental measurements for Fe and EuO, and it is found that many of the experimental observations can be satisfactorily explained.

The results of two model calculations of the excitations of an amorphous spin-¹/₂ ferromagnet are presented and compared. The first calculation uses an analytical Green function method in which the disorder-ensemble averaging uses techniques borrowed from the theory of liquids. This analytical theory is expected to produce its best results for dilute systems. The second calculation involves a Monte Carlo simulation of a three-dimensional disordered hard-sphere liquid and uses linearised spin-wave theory to describe the dynamics of the resulting structure. This technique is expected to produce its best results for dense structures. The calculations yield dispersion and density-of-states curves for the spin waves of the system which are in reasonable agreement with each other. The magnetisation and critical temperatures are also obtained. All calculations are performed for several different degrees of disorder.

Spin-wave relaxation has been studied in a range of polycrystalline YIG samples, at 9.4 GHz and at room temperature, using the perpendicular pump technique. The samples were of YIG (a) in the pure state, (b) doped with rare-earth ions and (c) partly aluminium-substituted. Results are given in terms of the effective spin-wave linewidth Delta H_k. For pure YIG samples of average grain size a₀ between 2.1 and 12 mu m, Delta H_k=1.6+1.2/a₀, with Delta H_k in Oe and a₀ in mu m. This is not in agreement with data obtained by Patton using a parallel pump technique (results so obtained are identified as Delta H'_k). A linear relation between Delta H_k and 4 pi M was found for the aluminium-substituted series over a range of 4 pi M from 410 to 1740 G. Measurements by West, Blankenship and Huntt (1971) show little change in Delta H'_k over a comparable magnetisation range.

The effect of a double pulse applied to isolated pairs of spin-1 nuclei is calculated exactly. In contrast to the spin-/₂ case, the calculations are performed in the representation in which the truncated dipolar interaction is diagonal. A sequence of three solid echoes is predicted. The approach is compared with that of expansion in time.

The Faraday rotation and MCD spectrum about absorption bands in praseodymium- and holmium-doped soda glass has been studied at temperatures less than 100K. The MCD spectrum was analysed by the method of moments and the relative contributions of the A, B and C terms obtained, it being found that C terms dominate in rare-earth-doped glasses. The Faraday parameters are derived for all optical transitions and the rare-earth site in the glass matrix is shown to be trigonal by interpretation of the sign of the C terms. Brief comments on the g-values of the bound states are made in relation to crystals of similar symmetry. It is also suggested that these glasses may find application in magneto-optical devices.