Table of contents

A dynamical theory of low-energy electron diffraction (LEED) is applied to the determination of the geometry of the 2*1-reconstructed (111) surface of silicon from experimental LEED intensity versus energy profiles. The top layer is 'buckled' by 0.30 AA, while the second layer shows a slight pairing between adjacent rows. The two topmost interlayer spacings are contracted by 10 and 3.5%, respectively, compared with the bulk values.

The molecular crystal s-triazine, C₃N₃H₃, is known to undergo a transition from a trigonal to a monoclinic structure when cooled below T_c=200K. Inelastic neutron scattering measurements have been carried out on a single crystal of s-triazine at various temperatures. A pronounced softening of transverse modes governed by the elastic constant c₄₄ is observed in the vicinity of T_c. The phonon response remains underdamped at energies down to at least 0.07 meV and no 'central peak' is observed. The transition seems to be slightly first order ('quasi-continuous') in accordance with recent observations, while it has been reported to be of second order at higher pressures.

Real-space renormalisation group techniques were used to transform a site-percolation problem on a lattice with randomly distributed voids into a classical site-percolation problem. This allows the influence of voids on the percolation threshold to be determined.

A second, optically detected magnetic resonance spectrum of triplet excitons has been observed in GaSe, showing that there are two types of GaSe crystals. The bound excitons have the same g_ex// in both types of crystals but in type I GaSe the zero-field splitting is D=0.110+or-0.004 cm^-1 whereas in type II crystals D=0.288+or-0.004 cm^-1. It is suggested that the observed resonance spectrum is related to the predominant polytype in a crystal.

A comparison of the electrical transport properties of amorphous and crystalline Ge films codeposited with Al and Cu is presented. The properties of amorphous Ge-metal films are consistent with the formation of homogeneous alloys and the impurity states introduced on alloying. On the other hand, in crystalline Ge-metal films, formation of alloys and/or heterogeneous Ge-segregated metal/compound phases at high metal concentrations explains the observed electrical properties.

The shift of the Fermi energy with substitutional doping has been determined for glow-discharge amorphous silicon films by photoelectron spectroscopy. The values so obtained for n- and p-type specimens are in good agreement with the shift expected from the activation energy of the electrical conductivity and confirm that the Fermi level can be moved throughout most of the mobility gap of amorphous silicon. The data also show that the surface-state density at the Si-SiO₂ interface is extremely low, so that information from field-effect experiments should be representative of the bulk states.

The derivation of generalised integrity bases for finite orthogonal transformation groups is discussed. In physical applications, to avoid introducing superfluous coefficients in polynomial expressions for physical quantities, it is necessary to define an integrity basis in such a way that any polynomial invariant may be expressed uniquely as a polynomial of the integrity basis members. On this criterion it is shown that numerator invariants which are products of other numerator invariants must be included in the integrity basis. The analogous requirement for generalised integrity bases for polynomial covariants is discussed. Methods of deriving numerator invariants or covariants are considered in relation to the above criterion. Two counting rules are derived giving the number of numerator invariants or numerator covariants and the sum of the powers of these numerator quantities.

High-resolution inelastic neutron scattering is used to study the tunnelling rotation spectrum of the 4-methyl group in MDBP (4-methyl-2, 6-ditertiarybutylphenol) over a temperature range 4-30K. The spectrum shows the approximately Lorentzian inelastic and quasi-elastic peaks and the measured parameters are the two widths and the energy transfer of the inelastic peak. At low temperatures the latter is equal to the tunnelling splitting of the ground torsional state, but at higher temperatures the energy of the inelastic peak also depends on the tunnelling splitting of excited torsional states of the methyl group. The observed temperature dependence is used to test a stochastic model for the dynamic averaging of the splittings of the thermally occupied states. The results show two different rotational processes are dominant above and below 15K. These are assigned to a torsional excitation process and a Raman process involving only a virtual torsional excitation.

The third-order elastic constants of the rare gas crystals are evaluated at the absolute zero. The calculation takes into account two-body static lattice terms, two-body zero-point energy terms calculated from the harmonic approximation and the lowest-order three-body long-range interaction static lattice terms. Deviations from the Cauchy relations are calculated and predictions made concerning the signs of these deviations which may be tested experimentally.

The equation of motion of the orbital axis l of ³He-A has been solved numerically for a situation in which the liquid is contained in the channel between two parallel planes when there is a flow of heat between the planes and a magnetic field applied perpendicular to them. For zero field, the texture with l uniform and perpendicular to the planes is stable, and there are time-dependent solutions satisfying the same boundary conditions. Precessing solitary wave solutions are found when l is constrained to have a direction on one plane opposite to that on the other. A magnetic field destabilises the texture of uniform l and with increasing field a time-independent state in which l spirals, a state with time-dependent l and a state with l perpendicular to the field over most of the channel are found successively. Three-dimensional geometries and persistent current decay through orbital motion is discussed.

Quasi-self-consistent energy band structures are reported for HfC, TaC, and UC using the relativistic KKR method. The mechanism of chemical bonding for the compounds under investigation is discussed on the basis of kappa -like partial charges for one-electron states and kappa -like total valence charges.

The simple model of a polyene chain, presented in a previous paper (Chem. Phys. Lett., vol.53, p.45, 1978), is modified to include longer-range forces. A new state, the spin-polarised state, is found to be the ground state. Based on this ground state, the various collective excitations are investigated by calculating the response function. States corresponding to both singlet and triplet excitons are found as well as the continuum and more exotic states. The optical absorption due to these states is investigated and a typical exciton wavefunction is calculated.

Various approaches to the theory of electrical conduction in disordered media are examined to make a better assessment of the possibility of improving the simple effective approximation. A cluster calculation is reported for the bond percolation model in three dimensions and the results are compared with the effective medium theory and Monte Carlo calculations. A comparison is made between the bond percolation model and the generally accepted equivalent model for a continuum. A breakdown is found in this assumed equivalence, which stems from correlations present in the continuum model but absent in lattices. The cumulant theory is studied in the lattice context. Although it gives good results for the simple cubic lattice when compared with Monte Carlo calculations, the reasons offered in the continuum model study do not apply here. The effect of cluster shape on site percolation calculations is studied.

A variational calculation of the thermodynamics of a system of two-level ions interacting with the phonons of a crystal is shown to lead to a nonlinear equation in the variational parameter which has a structure similar to the BCS integral equation of super conductivity theory. Solutions corresponding to helical displacements of the Jahn-Teller ions can be obtained close to the critical temperature and an expression for the transition temperature is obtained. The conditions for the existence of helical ordering are indicated.

The electron paramagnetic resonance of Mn²⁺ in single crystals of CoM₂'(SO₄)₂.6H₂O(M'=Rb, Cs, Tl), M"Tl₂(SO₄)₂.6H₂O(M"=Mg, Zn), ZnNa₂(SO₄)₂.4H₂O and CdNa₂(SO₄)₂.2H₂O has been studied in X-band at room temperature (300K). Two magnetically inequivalent, but otherwise identical, complexes of Mn²⁺ have been found in all the systems. The complexes are related to each other by two-fold symmetry about the b axis. The resonance line positions are fitted to the spin-Hamiltonian of orthorhombic symmetry. The hyperfine splittings of Mn²⁺, surrounded by an octahedron of water molecules and by an octahedron of oxygens, were used to calculate the covalency in crystals containing Mn²⁺ surrounded both by water and by oxygen molecules.

The room-temperature ⁵⁷Fe Mossbauer spectra of the two iron-tellurium defect structure compounds FeTe and Fe₂Te₃ are analysed, and shown to be superpositions of (respectively) three and four quadrupole-split pairs of lines. Each pair of lines in the total spectrum of each compound can be correlated with the different nearest-neighbour surroundings of the ⁵⁷Fe atoms.

By extrapolating to 0 and 30 eV experimental data in the energy range from approximately 1-1.3 to 14 eV a Kramers-Kronig analysis of the reflectivity spectrum at room temperature from the basal plane (E perpendicular to c) of single crystals of 2H-MoS₂, 2H-MoSe₂ and 2H-MoTe₂ has been performed. The optical data so obtained have been discussed and interpreted in terms of the proposed band model for these materials.

The coupling to lattice vibrations affects the photoionisation spectra of defects in semiconductors. This is especially important for deep defects. The effects are characterised mainly by a Huang-Rhys factor S₀ and by a spectral moment. These are calculated for a variety of electron-photon coupling mechanisms as a function of the observable ionisation energy E_I rather than the unobservable effective radius used by previous workers. For Frohlich coupling a good approximation for the Huang-Rhys factor is S₀(x)/S₀(0)=X/ square root ((5+x)/6) with x=E_I/(effective Ryd for purely hydrogenic centre).

Using the synchrotron radiation of the Deutsches Elektronen-Synchrotron DESY, the extended X-ray absorption fine structure at the L-edges of Au has been investigated. The magnitudes of the Fourier transforms of the L_I and the L_III fine structures yield maxima in real space showing a splitting which is the same for both edges. This splitting is attributed to the oscillatory behaviour of the back-scattering amplitude of the photoelectron wave, which is characteristic for heavy scattering atoms. Only transitions to d-symmetric final states contribute significantly to the L_II and L_III absorption.

The self-energy of an external moving charged particle near a surface is studied within the fast-charge approximation. For a reflecting trajectory, a formula for the complex self-energy, valid for an arbitrary dielectric function of the medium and angle of incidence, is first derived by means of a semiclassical analysis in which the imaginary part is obtained from a classical evaluation of the work done by the moving charge. This work is discussed in terms of conservative and dissipative contributions. The formula for the complex self-energy is then recovered from a quantum mechanical formulation in which the passage to the semiclassical limit is readily made. The quantum approach is used later to obtain a semiclassical formula for the complex self-energy when the charged particle penetrates the medium.

The reflection electron energy-loss spectra of clean (0001), (0001) and (1100) surfaces of ZnO have been studied with primary energies in the range 60-2000 eV. The spectra are interpreted in terms of interband transitions, longitudinal bulk surface excitation modes. At low primary energies, the general shape of the spectra differs from that at higher energies and differences between the features of the polar surfaces and the neutral face are observed. Exposure to oxygen (0.08-0.13 Pa s) influences the loss spectra. Electron beam and thermal desorption effects on the loss spectra are observed and discussed in relation to physisorbed and chemisorbed states of the adsorbed oxygen. Measurements of Auger electron spectra showed a very broad oxygen KL_2,3L_2,3 peak, the lineshape of which is discussed in relation to the quasi-atomic model.

The L_2,3M_4,5M_4,5 and L_2,3M-MM_4,5M_4,5 Auger spectra of Cu have been measured using CuL, MgK, and AlK X-ray exciting radiation. L₂M_4,5M_4,5 and L_2,3M-MM_4,5M_4,5 transitions are of lower intensity relative to L₃M_4,5M_4,5 transitions when the spectra are excited by CuL X-rays. The relative intensities of these transitions in the spectra excited by CuL and MgK X-rays are compared with calculated transition rates and assumed distributions of the initial L-shell hole population. A component of the spectrum excited by MgK X-rays agrees in intensity and position with theoretical predictions that a band-like feature should occur to high energy of the quasi-atomic L₃M_4,5M_4,5 group.