Table of contents

The neutron scattering functions S(Q, nu ) for liquid nitrogen at 77.4K has been measured with a triple-axis spectrometer on the high flux reactor (HFR) at ILL Grenoble. Scattered neutron spectra were recorded at five Q values in the range between 4 AA^-1 and 19 AA^-1 using the constant-Q method. The scattering functions are compared to the predictions of a single-particle excitation model and are found to be in good agreement.

The valence electron band structure and density of states of an isolated polysulphur nitride chain are calculated by the semi-empirical Extended Huckel method. Metallic behaviour is shown to be associated with the symmetry of the chain structure.

The Coulomb interaction between localized electrons is shown to create a 'soft' gap in the density of states near the Fermi level. The new temperature dependence of the hopping DC conductivity is the most important manifestation of the gap. The form of the density of states within the gap is discussed.

The observed splitting of the direct exciton lines in cubic semiconductors is due to the short range, not the long range, electron-hole exchange interaction in the exciton. The value of the exchange integral can be deduced from the measurements of the splitting, if the dielectric constant and the exciton reduced mass are known.

By exploiting optical selection rules in a microwave-optical double-resonance experiment, it is shown that the fine-structure parameter D for the lowest triplet state of the self-trapped exciton changes sign from (+) to (-) along the sequence of crystals from alkali iodides to fluorides. This results from interplay between a positive spin-orbit-exchange interaction and a negative magnetic dipole-dipole interaction.

Irradiation of alkaline earth fluorides with X-rays produces a strong fluorescence band in the ultraviolet. This band shows pronounced polarization effects when the crystals are placed in a magnetic field. EPR experiments on excited states using optical detection show that perturbed excitons with the molecular configuration of an H centre contribute to the fluorescence.

Positron annihilation has been studied in samples of lithium disilicate glass heat treated to produce different degrees of internal crystallization. It has been found that the intensity of the longer positron lifetime component (I₂) decreases with percentage crystallinity. This decrease can be correlated linearly with the reduction in free volume on crystallization in this system. A decrease in the lifetime, tau ₂, with crystallinity has also been observed.

Detailed analytical and numerical calculations are undertaken to determine the ultrasonic attenuation of transverse waves propagating along the (1, 0, 0) axis in twelve crystals of cubic symmetry. The numerical calculation takes full account of crystal anisotropy and introduces finite thermal phonon lifetimes into the calculations. In many cases the main contribution to the absorption comes from interactions with slow transverse thermal phonons that are nonenergy conserving, near collinear interactions or in the case of strong anisotropy are energy-conserving interactions. The former processes may then give a temperature dependence Tⁿ where n>4 and a less than linear frequency dependence, whilst for the latter processes the presence of anisotropy and finite thermal phonon lifetimes may in some circumstances explain an experimentally observed value of n<4 and a frequency dependence greater than linear. It is shown for some crystals where both of these types of process exist that the nonenergy-conserving processes play a major role in or dominate for some ranges of temperature.

It is shown that the internal pressure responsible for the thermal expansion of crystals according to Brillouin can be calculated assuming a phonon momentum transfer at the crystal walls.

Data from two types of experiments, namely oscillating discs and second sound, have been analysed. It was found that the temperature dependence of the superfluid density in the ³He-⁴He mixtures is the same as that in pure ⁴He and that its concentration dependence can be expressed by an empirical equation: rho _s(X,T*)= phi ² rho _s^o(T*) where T* is the dimensionless temperature T/T_lambda (X) and phi is the volume fraction of the ⁴He component in the superfluid ³He-⁴He mixtures. The molar concentration of the ³He component is denoted by X. At very low temperatures and in the dilute limit this equation gives as the value of the hydrodynamic effective mass of ³HeM₃*=2.7M₃.

The chemisorption energy and charge transfer of a transition-metal adatom of the 5d series on a tungsten substrate are calculated using a degenerate three-atom Hubbard Hamiltonian which is solved by making an ansatz for the wavefunction. The observed experimental trend in the chemisorption energy is reproduced for large values of the intra-atomic Coulomb interaction U, thus emphasizing the importance of correlation in this problem.

Rigorous upper and lower bounds to the integrated density of states of systems described within the tight-binding scheme are calculated using a Lagrange multipliers formalism which is valid for both equality and inequality constraints. Upper and lower bounds to the density of states are calculated as well; in this case, the constraints are the first few moments of the density of states and a bounded-below derivative of the density of states function. The bounds have been calculated for an oversimplified model of a binary alloy and for disordered magnetic insulators described within the Hubbard-model framework. Since the bounds are rather narrow, a great deal of information can be obtained from them. Upper and lower bounds are obtained to the density of states at the band tail energies in the antiferromagnetic arrangement of spins case. In the other cases studied, only upper bounds are obtained at these energies. Rigorous averages of the density of states and of the integrated density of states have also been obtained in terms of the moments of the density of states.

The high resolution electron energy loss spectra of solid xenon, solid krypton and solid argon have been measured at 10K with primary beam energies in the range 100 to 1000 eV. The spectra are interpreted in terms of excitons, plasmons, interband transitions and ionizations of inner shells. The classifications of the lower resolution differential spectra of Farrell and Strongin (1972) have been clarified and extended.

The semiconductor-metal transition in VO₂ has been investigated by means of XPS measurements. The band gap in the semiconducting region is found to be temperature-dependent with a room temperature value of E_g=0.3 eV, in agreement with resistivity measurements. The core lines, which are asymmetric below T_nu broaden and become symmetric above T_t. A model is discussed to explain the broadening in the metallic region, which requires that the core hole-valence electron interaction is comparable to the bandwidth. The relative shift of the O 1s and V 2p peak positions below and above T_t indicates that the charges on the ions are highest in the metallic phase, suggesting a more ionic bond in this region.

Experimental investigation has been made of the variation of electrical conductivity and thermoelectric power with composition and temperature in the As-Te and As_40-xTe_60-yIn_x+y molten systems. For the As-Te system, the electrical conductivity, sigma , is a continuous and decreasing function of the arsenic content over the range 5-55 atomic per cent arsenic; while the thermoelectric power is a continuous and increasing function of the arsenic content over the range of 15-55 atomic per cent arsenic. The temperature coefficient of conductivity, d sigma /dT, is positive for all investigated alloys. Just above the melting points of these alloys, in a temperature range which increases with increasing As content, the conductivity activation energy is constant and independent of composition. Thereafter, the activation energy decreases with temperature. The behaviour of the thermoelectric power, S, is consistent with the behaviour of the electrical conductivity discussed above. Additions of small quantities of indium to replace either arsenic or tellurium in As₂Te₃ lead to a decrease in conductivity.

A microscopic theory of the AC conductivity and optical absorption on localized states in the mobility gap of amorphous semiconductors is developed. The resulting formulae are exact to all orders in the electron-phonon coupling constant and to second order in overlaps of electronic localized orbitals. In the DC limit, previous results for the phonon-assisted hopping conductivity which are in agreement with assumptions of Mott's theory are obtained. The role of higher-order effects, including both the extended and possible localized phonon modes is discussed.

The natural spectral linewidths of individual waves in a stationary (nonlinear) acoustoelectric many-wave system are investigated theoretically within the Fokker-Planck approach to fluctuation phenomena. Using methods of turbulence theory it is found that the wave-wave coupling provides the dominant contribution to the linewidths, thus exceeding the influence of spontaneous emissions and thermal fluctuations. The final result is given in the form Delta omega = alpha /g+ Delta alpha , where the first term is the spectral linewidth of a free Gaussian wave with thermal loss constant alpha and amplification factor g above thermal level. The second term, Delta alpha , is the wave-wave coupling contribution which equals the differential loss per mode at the operating point. Numerical results for the linewidths of modes in the acoustoelectric oscillator operating in several modes, in quasi-stationary fluxes and in stabilized acoustic domains are also given.

A series of compounds of the type Cu_xCd_1-xMn₂O₄(0<or=x<or=1) have been synthesized and characterized using X-ray diffraction analysis. Low-temperature magnetic susceptibility and preliminary electrical and ESR measurements have been made. It appears that BB interactions are the dominant exchange forces that are responsible for the observed ferrimagnetic behaviour in all the compositions excepting CdMn₂O₄. A probable cationic distribution for solid solutions is proposed on the basis of structural, electrical and magnetic behaviour.

A high-pressure cell for Mossbauer absorption experiments on liquids is described and results are presented for studies of a supercooled solution of ferrous chloride in glycerol carried out in the pressure range 1-3450 bar at temperatures between 241K and 273K, and at atmospheric pressure over the temperature range 100-273K. Line broadening was observed and analysed according to various models of ionic motion. Neither the pressure nor the temperature dependence of the broadening showed a proportionality to inverse viscosity and it was concluded that over distances of the order of 0.3*10^-10m the ferrous-ion motion was not even approximately described by a diffusion constant derived from the Einstein-Stokes relation. The area under the spectrum was analysed as a function of pressure and temperature and it was found that on warming at constant pressure the recoilless fraction fell effectively to zero (<or=0.005) at about 10K below the crystal melting temperature of glycerol at the pressure concerned.

The Faraday spectra between 450 nm and 800 nm are reported for the series of garnets Bi_xY_3-xFe₅O₁₂, Bi_xY_3-xFe_5-yGa_y- O₁₂(y=1.0) and Bi_xY_3-xFe_5-zSc_zO₁₂(z=0.4, 0.8) and a similar series without bismuth. The component of Faraday rotation induced by the bismuth in these series has a diamagnetic wavelength dependence and indicates that a transition/transitions around 24000 cm^-1 is a major contributor to the induced component. The behaviour of the induced component with octahedral Fe³⁺ and tetragedral Fe³⁺ concentration shows that both sublattices are involved and is consistent with the observations that the band at 23200 cm^-1 is an Fe³⁺ pair transition. The strength of the induced Faraday component, in the above series, normalized to bismuth, octahedral Fe³⁺ and tetrahedral Fe³⁺ concentrations and octahedral/tetrahedral magnetic moments is a constant. The magnitude of the induced Faraday component cannot be accounted for assuming the Bi³⁺ ions split only the 23200 cm^-1 pair band in YIG.

Spectroscopic studies of the shallow donor impurities in compensated samples of n-GaAs, InP and CdTe are made using the technique of photothermal excitation of photoconductivity. In addition to the usual 1s-2p line of the isolated donors, other features are observed which connot be explained in terms of the hydrogenic model of isolated impurities. The most prominent line consists of a relatively sharp peak observed at 60 per cent of the impurity Rydberg in all three materials. The position of this line is independent of impurity concentration but the line increases in intensity with increasing doping. Several models suggesting an explanation involving interacting donor impurities are considered. The most plausible explanation appears to be an analogue of the ionized hydrogen molecule in which the line arises from electronic transitions between the 2p sigma _u and 3p sigma _g states where the transition energy is almost independent of internuclear separation for a range from six to thirteen Bohr radii.

The feasibility of measuring by the PAX technique (photoelectron analysis of X-rays) continuous absorption profiles in the ultrasoft X-ray region was explored. Bremsstrahlung from a tungsten anode bombarded with 1 to 5 keV electrons served as a radiation source. Photoelectrons ejected by X-rays from the neon converter atom were analysed with a spherical electrostatic electron spectrometer having an instrumental resolution of Delta E/E=0.15%. As an example, absorption structures at the Sm N_4,5 and Al L_2,3 edges were measured and compared with results obtained at the DESY synchrotron. A brief evaluation of the technique is presented and some possible applications are discussed.

The complete L₂MM and L₂MM Auger spectra of selenium have been observed using an X-ray photoelectron spectrometer. The relative energies of all components are shown to be in agreement with calculations which use the binding energies of core levels and tables of atomic integrals, intermediate coupling being assumed for the two-hole final states. The total relaxation accompanying LMM Auger processes in solid Se is independent of the two-hole final state and has been found to be 23.8+or-2.0 eV. This includes a total atomic contribution estimated as 14.6+or-1.0 eV of which 7.4+or-0.2 eV arises from relaxation in the outer shell. The contribution from intra shell relaxation is deduced to be 7.2+or-1.2 eV and from extra-atomic processes as 9.2+or-3.0 eV. This analysis of the relaxation is supported by measurement of an Auger line from the L₃MN series.