Table of contents

Various explanations proposed for the low temperature deviations from Matthiesen's rule in the electrical resistance of dilute alloys are examined and found to be either deficient or unsuccessful.

Using Sawada's Green function technique, which separates various orders of correlations, the author solves the Hubbard model and shows that the single particle Green function has the same structure as Hubbard's solution using the double time Green function technique. The demonstration helps to clarify the uncertainty with regard to the decoupling procedure.

In the analysis of electrical measurements on ion implanted layers in semi-conductors it is important to take account of the possibility of incomplete donor or acceptor ionization, and the fact that the material is likely to degenerate. Methods for computing these effects and the effect of mobility variation with depth are presented.

It is shown that electron scattering from spin correlations leads to a characteristic resistive anomaly for antiferromagnets above their Neel point and that a recent calculation, supporting the contrary conclusion, is not correct.

The angular dependence of resonances broadened by random internal strains or electric fields in trigonal crystals is derived for magnetic fields applied in directions close to the c axis. Two cases arise, in the first the line shows the expected 2 pi /3 periodicity, in the second the line divides into two components, each having 4 pi /3 periodicity.

The critical dependence of electron scattering factors, calculated in the free atom formulation, on the range of the potential is examined. With particular reference to carbon (Z=6) and gold (Z=79) calculations are made for the real electron scattering factor, f^B (first Born approximation) and for the complex electron scattering factor, f exp (i eta ). The termination of the atomic potential at distances corresponding to half the typical atom-atom separation in the solid state demonstrates that f(0) varies by a factor of 1.6 for carbon (Hartree-Byatt potential) and 1.2 for gold (Thomas-Fermi potential).

Dilution experiments show that the ethylene molecules in solid ethylene undergo 180 degrees flip reorientations about their figure (C-C) axes. The intermolecular contribution to the spin lattice relaxation of the protons is a factor of 2 stronger than that suggested by Page and Rushworth (1969).

The structure factors and Compton profile for LiH are calculated using the selfconsistent local orbital wave functions of Kunz (1969) and are shown to be in marked disagreement with reported experimental results.

The author uses a variational form of the Kohn and Sham (1965) general effective exchange energy to derive an upper bound for the effective exchange interaction. Exact bounds (within the Hartree-Fock scheme) are deduced for the usual (test particle) static dielectric constant and the electron-dielectric constant.

The ground state energy, pressure and compressibility of solid ³He were calculated in the molar volume range 12 to 18 cm³ using the improved selfconsistent phonon theory of Goldman et al (1968). A soft core potential due to Beck (1968) was used and relatively good agreement was obtained with experiment for the pressure and compressibility.

A synthetic iron sulphide of spinel type became anisotropic when immersed in an electrostatic field. Analysis of the electron diffraction patterns observed from the Fe₃S₄ specimens showed that a preferred induction took place at about 313 K along the (111) axis in the crystal. This anisotropy became negligibly small over about 373 K.

The Auger spectrum of Si, in which primary ionization occurs in the L shell with subsequent emission of an Auger electron from the valence band, is shown to be a consequence of the band structure. Energy levels within the valence band from which such emission takes place may be identified with peaks in the theoretical density of states calculations. The effects of oxidation on the spectrum which result from a change in the density of states is discussed.

Low temperature crystallographic phase transitions in DyVO₄ and TbVO are attributed to a co-operative Jahn-Teller effect. A preliminary report of the theory is given, based on a harmonic oscillator Hamiltonian for the crystal and an electron lattice coupling which is small compared to typical phonon zero point energies. An Ising Hamiltonian is shown to describe the co-operative ordering.

The entropy of formation of divacancies has been calculated assuming a more realistic elastic displacement and including the anharmonic effects. The agreement with the experiment is good.

A general analysis is made of uniaxial stress, Stark and Zeeman spectra of sharp line optical transitions in colour centres by using the symmetry properties of the centres, crystal lattice and perturbing field. Various general prescriptions are derived for calculating features of the spectra such as the number of components in a pattern and the number of independent polarized spectra which exist for a particular direction of the perturbation. Two results of special interest are the isotropic and unpolarized nature of emission (for nonselective excitation) and absorption spectra of unperturbed centres in cubic crystals, and the inability of perturbative experiments using a uniaxial stress or a magnetic field to completely specify the point group of the centre in the absence of detectable hyperfine electron-nuclear interactions.

Phonon frequencies in RbBr at 80 K have been obtained by inelastic scattering of thermal neutrons. The measurements were performed in the symmetry directions Delta , Sigma and Lambda and along the line Z. Some zone boundary phonons were also measured at 300 K. Four versions of the shell model were fitted to the experimental frequencies, and the elastic and dielectric constants were calculated from the model parameters. From an eleven parameter model the authors have also calculated the phonon frequency spectrum and the temperature variation of the Debye temperature.

Measurements are reported of acoustic attenuation in zinc oxide, cadmium sulphide and cadmium selenide. The measurements were made in the temperature range 15-120 K, at frequencies of 1.0, 3.6 and 8.8 GHz. The results are found to be in good agreement with the predictions of three phonon theories of acoustic attenuation.

The electrical response of a cw shear mode acoustoelectric CdS oscillator has been investigated as a function of mechanical loading. The loading was produced by evaporating either copper or indium onto the oscillator. Copper films essentially produced a reactive loading, while there was considerable scattering associated with reflection from indium films. The frequency shift of the oscillator produced by copper loading was measured and compared with theory. The results suggest that the frequency spectrum was primarily controlled by the lowest observed frequency, even when subharmonic generation occurred. This lowest frequency was a normal mode of the platelet and its electrical output was generally not the strongest in the spectrum. The influence of loading upon the electrical amplitudes in the rf spectrum, the threshold and cut-off voltages and the shape of the dc current-voltage characteristic were examined. It was found that indium layers a few micrometres thick could prevent oscillation with corresponding reversion to ohmic of the V-I characteristic. For thinner layers the oscillator output spectrum is confined to smaller numbers of components but with increased amplitudes.

The quasiharmonic and lowest order anharmonic contributions to the Helmholtz free energy of sodium chloride have been calculated. A breathing shell model was used in the quasiharmonic calculation, and Coulomb and polarization contributions to the anharmonic matrix elements were included more completely than has been done before. The thermodynamic properties derived from the free energy are in good agreement with experimental results. The inclusion of Coulomb anharmonicity was particularly important, reducing the magnitude of the second order anharmonic contribution by 40%. The quasiharmonic calculation gives good agreement with experiment up to one quarter of the melting temperature, and the lowest order anharmonic theory appears to be adequate up to at least two thirds of the melting temperature.

The transversal electromagnetic oscillations and waves (excitons) in a homogeneous electronic solid state plasma, interacting nonlinearly with strong external electric and magnetic fields are investigated. Both the conductivity and complex dielectric constant tensors are determined. Investigation of the dispersion equation, describing the excitons in the electronic solid state plasma, shows the existence of both damping and nondamping oscillations. The spectra and the damping (amplification) of the electromagnetic waves are also determined. It is shown that the nondamping branch of oscillations is related to the occurrence of a negative differential conductivity, resulting from the nonlinear interaction of the nonequilibrium electronic solid state plasma with the external fields.

Metallic perturbation theory is inadequate to explain the B-32 crystal structure of NaTl, LiAl, consisting of two interpenetrating diamond structures. However these compounds are probably semiconductors or semimetals, and an extra term must be included in the energy calculation to allow for the full Jones' zone. The structure can then be understood in solid state terms, which are related to the chemical picture of diamond-type bonds between the Tl atoms.

Magnetic susceptibility measurements have been made between 4.2 K and 1.1 K on single crystals of Sm, Pr and Nd magnesium nitrate and the measurements are found to obey a Curie law plus a Van Vleck temperature independent term. The results are discussed in terms of wavefunctions obtained with the approximation that the crystal field is of icosahedral symmetry. There is good agreement between the measured and theoretical values of the Van Vleck temperature independent susceptibilities in the Pr and Sm salts but for the Nd salt we were unable to interpret the measured values of the temperature independent susceptibilities in terms of these icosahedral wavefunctions.

Abrikosov's form of a Wick theorem for spin operators is used to construct a diagrammatic expansion of the line shape in terms of the ion-phonon interaction. The Weisskopf-Wigner relation (the width of a spectral line is related to the sum of the inverse lifetimes of the states before and after the photon interaction) is valid provided all terms apart from the first in the Bethe- Saltpetre equation for the relevant two particle Green function are neglected. The neglected terms, which violate the relation, correspond to linewidth contributions from ion-phonon interaction such that there is no net transfer on energy between the ion and the lattice. This is proved for all orders of perturbation theory and for anharmonic as well as harmonic ion-phonon coupling.

Some multiphonon selection rules for infrared absorption and Raman scattering have been evaluated for ferroelectric and paraelectric sodium nitrite at symmetry points in the Brillouin zone. As the results for the paraelectric phase depend on the mechanism for the re-orientation of the nitrite ion, the normal modes are discussed in some detail for each case.

The authors present a comparison of the photoluminescent and electroluminescent spectra of Er³⁺ doped ZnSe and establishes that the emission sites in each case are different in origin. Two types of rare earth sites have been observed in electroluminescence and six sites in photoluminescence. The defect structure of each site is discussed.

Thermal bleaching of colour centres in irradiated LiF crystals at room temperature was studied under the same thermal conditions as for the measurement of thermoluminescence. When the irradiated crystals were annealed and rapidly cooled, the energy of formation of the trapping centres of carriers, which contributed to thermoluminescence, was estimated to be 0.13 eV.

The luminescence, absorption and excitation spectra of KCl.KI-Sn single crystal phosphors are investigated. By introduction of I^- in KCl-Sn a strong shift of the emission band to longer waves is observed, which is ascribed to an association of Sn²⁺ and I^-. The absorption and excitation spectra are affected too. The poorer structure of KCl. KI-Sn in comparison with KCl-Sn is assumed to be due to an increase of the absorption band number by incorporation of I^-. In the course of these investigations, a dependence of the tin band position on the activator concentration in KCl-Sn is established.

It is shown that the observable second moment of the nuclear magnetic resonance line due to a powder containing methyl groups executing tunnelling rotation at low temperatures may approach one half of the value for stationary methyl groups or twice the high temperature value for classically rotating groups. This prediction is discussed in the light of some previously unexplained published experimental data.