Table of contents

The advantages of a mathematical technique involving a quadratic map and Pade approximants is discussed in the context of the calculation of spectral functions of solids based on moment expansions. The method is expected to be of great value, particularly in calculations dealing with the band-edges with square-root-type behaviours. The practical relevance of the approach is illustrated in several examples.

The metal-insulator transition in compensated n-type indium phosphide, with an impurity concentration just above the critical concentration obtained from the Mott criterion has been induced by a magnetic field. First-order phenomena are considered, corrections to the conductivity due to electron-electron interaction or quantum interferences in the metallic regime will be described elsewhere. Two aspects of the MI transition (Anderson and Mott transitions) are observed by increasing the magnetic field at temperatures down to 50 mK. The Mott minimum metallic conductivity, whose existence has been shown earlier at higher temperatures, is still found down to the lowest attainable temperatures, and its value remains in agreement with the first Mott estimation. For a sample near the MI transition, arguments are given for conduction mechanisms occurring in the impurity band rather than in a tail of the conduction band.

The dynamic density response function is expressed in terms of a polarisation potential using Mori-Zwanzig theory. The polarisation potential consists of a static part determined by the direct correlation function and a dynamic part explicitly given by Mori's relaxation kernel. A renormalised free-particle model is used for the dynamic part and the dynamical structure factor S(q, omega ) of dense krypton gas is calculated. The full width at half maximum of S(q, omega ) has been compared with results obtained from recent neutron inelastic scattering experiments. It has found that the authors' results are in good qualitative agreement with experimental data.

The results of theoretical and experimental study of vibrational relaxation processes in impurity centres with a strong electron-phonon interaction are presented. General expressions are obtained which describe the temporal evolution of the configurational-coordinate distribution function and the envelope of the emission spectrum consisting of ordinary and hot luminescence. The problem is solved exactly in the harmonic approximation with allowance made for both linear and quadratic vibronic coupling providing a small number of actual configurational coordinates. The hot luminescence bands in KI-Tl positioned at 3.1 and 2.25 eV are detected. The computations show that in Tl⁺ centres the main portion of the excess vibrational energy relaxes in a time shorter than the mean period of vibrations ( approximately=0.5 ps) and, therefore, the observed spectral features are caused by the first classical turning point in the configurational-coordinate relaxation law. Thus, the centres under study serve as an example of ultrafast relaxing systems. Other weak spectral features are related with the complicated structure of adiabatic potential surfaces in the excited state and are interpreted as a barrier and post-tunnel hot luminescence.

The structures of two non-cubic phases of KCdF₃ are deduced from a study of superlattice reflections and lattice parameter measurements. On cooling from the cubic phase two modes, unrelated by symmetry, condense out simultaneously. This can be explained using Landau's theory of phase transitions by including a coupling term in the free-energy expression.

The X-ray photoemission spectra of Pr, PrH_2.1 and PrH₃ have been measured. The core levels of PrH₃ show satellites due to H 1s to Pr 4f transitions, which become weaker in PrH_2.1. PrH_2.1 exhibits satellites due to Pr 5d to Pr 4f transitions and those due to the empty localised 5d level. Taking these satellite structures into account, chemical shifts between PrH_2.1 and PrH₃ are ambiguous while there are large implying that a charge transfer occurs from Pr to tetrahedral hydrogen but not to octahedral hydrogen. A chemical shifts between Pr and PrH_2.1, hydrogen-induced band appears at about 6 eV below the Fermi energy, and comparison with theoretical densities of state suggests the importance of hybridisation between Pr 6sp and H 1s in the metal-hydrogen bonding.

Angle-resolved photoemission spectra have been recorded on 2H-SnSe₂ crystals using synchrotron radiation facilities. The experimental band structure obtained from these measurements is compared with that obtained by Williams and Parke (1978) and with the theoretical predictions. The relatively weak dispersion of the bands with respect to the photon energy in normal emission shows that the two-dimensional model is a satisfying way of approaching the three-dimensional band structure. Taking advantage of the strong polarisation factor of the synchrotron radiation, information has been obtained on the symmetry of the valence states. In the uppermost part of the valence band the hybridisation of the states is found to be relatively strong and only the predominant character of the bands may be proposed.

Lanthanum strontium vanadate, La_1-xSr_xVO₃, has been studied by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) and high-resolution electron energy-loss spectroscopy (HREELS) over a composition range 0<x<0.3 embracing the metal-nonmetal transition (MNMT) at x approximately=0.22. XPS confirmed that the surface stoichiometry of samples cleaned by prolonged annealing in ultra-high vacuum mirrored that of the bulk. Owing to Mott-Hubbard-splitting of the V:3d band, LaVO₃ is nonmetallic with a localised t_2g² configuration. However, the observed width of the V:3d band in UPS reflects both electronic (band-structure) effects and final-state phonon broadening. Strontium doping leads to broadening of both primary and secondary electron-bands in UPS, introduction of holes into the V:3d band, and to the appearance of a Fermi edge in photoemission above x=0.2. The observed value of the density of states at the Fermi energy is consistent with the value predicted from Mott's estimate of the minimum metallic conductivity in a disordered system. However, the MNMT does not appear to take place in an impurity band separated from the V:3d band. The low frequency of the surface plasmon loss in HREELS emphasises the importance of electron correlation even in the metallic regime.

The results of photo-ESR experiments have allowed the authors to interpret the absorption band beneath the fundamental absorption edge of ZnS as being due to the Fe²⁺+h nu to Fe³⁺+e_CB photoionisation transition. Analysis of the temperature dependence of the shape of the photoionisation absorption band enabled them to establish the position of the Fe²⁺ energy level below the edge of the ZnS conduction band (E_th=16200+or-700 cm^-1).

The low-frequency behaviour of the dynamical conductivity sigma ( omega ) for electrons on very long (10⁶ sites) one-dimensional disordered chains is investigated by computer simulation. Berezinskii's result (1974) which is asymptotically exact in the limit of weak disorder and shows the behaviour sigma ( omega )/ sigma ₀ to 32( omega tau )²(ln²(4 omega tau )- pi ²/4+(2C-3)ln(4 omega tau )+ ...) at low frequencies ( sigma ₀, tau and C denote the Drude value of the conductivity, the relaxation time of the electrons and Euler's constant, respectively), is established for the first time by a direct numerical method. A discussion is given of a simple derivation of the coefficient 32. It is also found that this low-frequency behaviour remains almost when disorder is introduced, unchanged even when the disorder is strong.

A percolation model is formulated for dispersive transport in systems with both site and bond disorder. The thermalisation of injected electrons is considered and the time dependence of the current is found for time-of-flight experiments. The results are compared with those of the CTRWA model.

Results of Monte Carlo studies of thermodynamical quantities in a generalisation of the Edwards-Anderson model are presented. The model contains a fraction x of antiferromagnetic bonds and (1-x) of ferromagnetic ones. The magnitudes of the exchange constants are given by a gaussian distribution and the Heisenberg spins are coupled by nearest-neighbour interactions. The samples with x=0 and x=0.2 are ferromagnetic, below a critical temperature. The samples with x=0.4 and x=0.5 are spin glasses. In these two cases the specific heat shows a broad maximum whereas the susceptibility seems to follow a mixture of cusp-like and 1/T-behaviours. T=0 studies of the magnetisation as a function of concentration suggest that the spin-glass-ferromagnet transition occurs at x=0.31+or-0.04 and is continuous.

The well known 2.8 eV luminescence band in quartz has been studied using optically detected magnetic resonance (ODMR). A triplet state with a very large fine-structure splitting is found to contribute to the emission. The principle axes of the triplet are identified. The possibility that the luminescence originates in the decay of a self-trapped exciton involving a transient oxygen Frenkel pair is discussed.

The magnesium ferrite-aluminate series MgAl_xFe_2-xO₄ (x=0.2, 0.4 and 0.7) has been synthesised, the lattice constants determined and the applicability of Vegard's law has been tested. The Mossbauer spectra have been recorded from 77K to a temperature well above the Neel point. The Neel temperature decreases linearly with the increase of aluminium concentration. The centre shifts are found to be independent of the aluminium concentration. The Mossbauer spectra have also been recorded at 4.2K in an external magnetic field of 4.2 T applied parallel to the direction of the gamma rays. For x=0.2 the canting of Fe spins is observed only on the A site and the B-site Fe spins have a collinear spin structure. This is the only spinel ferrite where the spin canting has been clearly observed on the A site alone when the B-site Fe spins have a collinear spin structure. For x=0.4 and x=0.7 both A- and B-site Fe ions have a noncollinear spin structure. The distribution of hyperfine fields at the A and B sites has been analysed in terms of supertransferred hyperfine fields from neighbouring ions.

The optical spectra of triply ionised thulium in thulium, ammonium, magnesium, potassium and sodium nitrate solutions has been recorded for the first time. The second-derivative spectrum reveals interesting results. It is very interesting to note that the broad band of the normal spectrum of thulium nitrate, consisting of closely lying electronic levels above 30000 cm^-1, could be well resolved in the second-derivative spectrum. The splittings observed for certain levels in all the complexes in the second-derivative spectra have been successfully explained as arising from the superposition of vibronic transitions on electronic spectra. Radiative lifetimes have been calculated theoretically for the fluorescent levels of the ion in all five complexes.

The photocapacitance spectrum of n-type GaP:Ni shows a peak near 0.74 eV and a rising edge at higher energies. The properties are consistent with the peak being due to photothermal ionisation to the conduction band via the excited state of the d⁹ configuration, while the rising edge is due to photoionisation directly from the d⁹ ground state to the conduction band. Experiment and theory agree that the effective capture rate of electrons is the same whether excitation is via the excited state or direct to the conduction band. A previously reported absorption process with threshold energy 0.78 eV is shown to be different from that giving the photocapacitance spectrum, and it is explained how one centre may dominate the absorption spectrum and a different one dominate the photocapacitance spectrum.