Table of contents

The dynamic structure factor, S(q, omega ), in liquid rubidium has been determined on the basis of a number of generalized hydrodynamic schemes. The calculations are compared with both the quite extensive results obtained by neutron scattering experiments and also some obtained by molecular dynamics computations. In the wavenumber range <or=1 AA^-1, where both experiment and computer show the existence of a well defined collective mode, none of the generalized hydrodynamic models are entirely satisfactory, and it is emphasized that the lack of understanding of the damping of the modes is probably their weakest feature.

From the measurement of the frequency dependence of the ultrasonic attenuation coefficient for the longitudinal wave in KMnF₃, the width of the central peak is obtained by applying extended Schwabl's theory (1973). This central peak shows a critical slowing-down, and its width is about 20 MHz at T=T_c+1K and about 170 MHz at T=T_c+4K.

By using the Coulson et al (1962) valence bond theory, an expression is obtained for the ionicity of the zincblende compounds in terms of the lower valence band gap at the X point of the Brillouin zone (X₁-X₃). The extension of the theory to the rocksalt structure is discussed.

In the analysis of cyclotron resonance experiments with GaAs, the free-electron terms must not be neglected. When this term is correctly included in the equations, excellent agreement is found among data from different experiments.

Both thermally stimulated currents under high applied fields and photon-stimulated detrapping by monochromatic light under conditions of space charge limitation have been utilized to yield unequivocal estimates of the depths of separately identified electron and hole traps in single crystals of ultrahigh-purity anthracene. The traps are much more efficiently populated by appropriate injecting electrodes than by irradiation with light. Hole traps occur at 0.75, 0.85 and 0.95 eV above the valence band, and electron traps at 0.75 and 0.85 eV below the conduction band.

High-field expansions are given for the anisotropic Ising model on a face-centred-cubic lattice. The possible groupings of terms are discussed and some scaling predictions tested.

High-resolution specific heat measurements have shown repeatedly that real magnetic crystals lose their divergent character in the immediate neighbourhood of T_N. White et al (1974) and others have accounted for the observed rounding with a distribution of critical points, attributable to residual stresses that vary throughout the sample. Bartis has opposed this view with a model containing a distinct intermediate stage in the ordering process. New experimental evidence is presented in favour of the residual stress interpretation by means of a noise analysis of continuous heating measurements on CoBr₂.6H₂O. A large component of low-frequency noise is found near T_N that is attributed to the sample studied and not the apparatus or method of analysis. It is argued that the graininess of the transition temperatures of a stress-induced ensemble of domains has been detected.

Low-temperature, high-resolution measurements of the far-infrared absorption spectrum of CaF₂:Er³⁺ have revealed two sharp lines at 21.10 and 32.76+or-0.05 cm^-1. These lines correspond to transitions between the lowest ⁴I_15/2 Stark levels of the Er³⁺ ion in a tetragonal crystal field. The temperature dependence of the lines is also presented and successfully analysed in terms of thermal population effects.

Monte Carlo calculations have been made on the basis of the rigid-ion model for a number of alkali halides at zero applied pressure in the solid state at 298K and both solid and liquid phases near the melting temperatures. Two different forms of the short-range repulsive interaction were used: an exponential potential of the Huggins-Mayer form (1933) and an inverse-power repulsive potential in the form due to Pauling (1928). The potential parameters were in all cases taken from the work of Tosi and Fumi (1964). Some care has been taken to minimize errors in the calculation of the coulombic energy and the accuracy of the results is believed to be considerably greater than that previously achieved in the computer simulation of ionic systems. The Huggins-Mayer form of the potential is found to be superior to the Pauling form in virtually all respects.

Positron lifetime distribution measurements in Si crystals (using both float-zoned and Czochralski crystals), intrinsic and with different dopings, have been made using approximately 0.290 ns (FWHM) prompt time resolution with approximately 37 ps slope and 40000:1 peak-to-background ratio. The general trend of the time spectra essentially consists of a single exponential component tau ₁(0.223-0.245 ns). Because of the extra care taken in the source sample preparation the intensity, I₂, of the long component ( approximately 1 ns) was kept <0.2% in all the cases investigated and was assigned to source sample effects. Although no difference in lifetime was observed between various kinds of doped crystals, the lifetime values in the intrinsic Si crystals were found to be consistently lower than in the doped ones by approximately 6-9%, the experimental error being approximately 2%. This effect has been detected, possibly for the first time.

The Ni_1+xTe₂ phase has been investigated primarily by powder neutron diffraction. Several compositions in the range Ni_1.82Te₂ to NiTe₂ have been studied at room temperature and all except Ni_1.82Te₂ showed complete occupation by nickel atoms of alternate layers normal to the c direction. No c axis long-range order could be detected in Ni_1.82Te₂, and in two samples of lower nickel content (Ni_1.67Te₂ and Ni_1.54Te₂) disordering occurs at high temperature. The order-disorder process transforms the CdI₂ superstructure containing interstitial nickel atoms into a cation deficient NiAs structure. Critical temperatures have been determined (T_c=1080+or-20K and 750+or-50K for Ni_1.54Te₂ and Ni_1.67Te₂, respectively) and interaction energies have been estimated from the Bragg-Williams model.

A microscopic theory of the quantum diffusion of light interstitials in a perfect crystal is developed. The basic aspects and features of the quantum diffusion and the role of the polaron effects are revealed. The expressions of the coefficients for under-barrier coherent and noncoherent tunnelling diffusion and for over-barrier diffusion, valid at the appropriate temperatures, are derived in a unified way. The basic role of the intra-well scattering for the coherent diffusion in limiting narrow bands is elucidated. The theory permits one to consider the transition from quantum to classical diffusion. It is found that coherent diffusion predominates at the low temperatures, at least in the perfect crystal. The criteria for the observation of the coherent diffusion mechanism are given.

The thermodynamics of a liquid-metal surface are investigated using a pseudo-atom model for the ions and electrons. By treating the energy of the electrons as an effective potential for the pseudo-ions, the statistical mechanics of the latter can be evaluated. The surface energy and tension are then derived following the classical Kirkwood-Buff procedure. In the metallic case, the surface energy and tension include indirect effects due to the variation of electron density at the surface as well as the usual direct contribution which arises from the change in the pairwise distribution of atoms caused by the presence of the surface. Some numerical results are presented and compared with experiment. These indicate that, for simple liquid metals, the observed increase in surface tension as a function of increasing bulk electron density is mainly due to an indirect term, that is, the change in self-energy of a pseudo-atom as a function of electron density.

To overcome the limitation of conventional quantum chemistry computer programs using the Hartree-Fock approximation which can only handle small clusters of atoms, Dyson's equation is used to embed an adsorbate/absorbent cluster in the surface of the semi-infinite adsorbent. The embedding problem is formulated correctly, but to have a practical scheme, an approximate version is needed. This version uses a tight-binding electron Green function in the adsorbent beyond the cluster, but the correct self-consistent Green function in the adsorbate/adsorbent cluster. Calculations have been made for hydrogen on the (100) surfaces of two simple, and two face-centred cubic s band solids.

A method for calculating the density of states and the effective mass of a model of an electron in a completely random system containing dense and weak scatterers, is presented. The scattering potential employed in the model is assumed to be a gaussian function. The technique used in the calculation is the path-integral method of Feynman applied to the polaron problem (1955). Special emphasis is devoted to the consideration of the asymptotic behaviours of the density of states m(E) and the effective mass m*, for two limiting values of the correlation length L. Some improvements upon the method as well as an extension to a real physical system are discussed.

The cyclotron resonance energy at zero temperature and vanishing concentration of carriers in a band with ellipsoidal constant-energy surfaces is studied under the assumption that the carriers interact with long-wavelength LO phonons. The problem is treated variationally, and the electron-LO phonon interaction is not assumed very small. Two special cases are considered: (i) the applied magnetic field is pointing in an arbitrary direction but is weak; (ii) the applied field has arbitrary strength but points along an axis of cylindrical symmetry of the energy ellipsoid. Numerical results are given for conduction band and on-axis valence band cyclotron resonance in AgBr.

For pt.I see ibid., vol.7, no.16, 2877 of 1974. The theory presented in part I is generalized for calculating cyclotron resonance energies of polarons in arbitrarily strong magnetic fields making an arbitrary angle with an axis of cylindrical symmetry of the energy ellipsoid. Numerical results are given for the variation of the cyclotron resonance energy of holes in AgBr with the strength and orientation of the applied magnetic field.

The effects of spin-phonon interactions in a uniaxial Heisenberg antiferromagnet are studied using a field-theoretical perturbation technique and an expansion in powers of the high-density parameter 1/z, where z is the number of spins interacting with any given spin. The renormalized energy and damping of the spin-wave excitations are evaluated to second order in the spin-phonon coupling, and the dependence of these quantities on temperature, wavevector and anisotropy factors is obtained. The calculations are valid over a wide range of temperature below the Neel temperature T_N and detailed discussions are given for the cases of (i) low temperatures and (ii) temperatures close to T_N. Some numerical estimates are made for MnF₂.

For pt.I, see ibid., vol.7, no.16, 2901 of 1974. The method developed in part I is used to investigate the effects of spin-phonon interactions on the phonon spectrum in a Heisenberg antiferromagnet. The calculations employ a field theoretical perturbation technique and an expansion in powers of the high-density parameter 1/z. Results are derived for the renormalized phonon energy and for the phonon damping (which is proportional to the attenuation of sound) over a wide range of temperatures below T_N. Calculations are also presented for the spin-lattice relaxation rate, which can be closely related to the phonon damping. Some numerical estimates are made for MnF₂ and compared briefly with experiment.

The EPR spectrum of Cu²⁺ ions in K₂SO₄ and Rb₂SO₄ has been measured in detail at a large number of orientations of the applied magnetic field. Results are in substantial agreement with previously reported work when interpreted using an orthorhombic model. Particular attention has been focused on a detailed fit, covering all orientations, using a more appropriate spin Hamiltonian for arbitrarily low symmetry. An evaluation of the procedure is given and some of the difficulties outlined. The final estimate of off-diagonal elements in the Zeeman and hyperfine tensors is not precise, but they are similar to those required to fit the spectrum of Cu²⁺ in K₂SeO₄. A non-coincidence in Zeeman and hyperfine tensor axes of 8 degrees is determined for the xy plane.

The theory of thermal mixing between nuclear Zeeman interactions and electronic spin-spin interactions of impurities in dielectric solids, used to interpret the mixing between the Zeeman interactions of two nuclear spin species as the result of their separate coupling with the electronic spin-spin reservoir. The predictions of the theory are compared with the experimental observations made in lithium fluoride doped with F centres in the domain of high electronic polarization. They account for the field and temperature dependence of the mixing rate, as well as for the order of magnitude of that rate.