Table of contents

Correlations between parameters which characterize the molecular structure of amorphous selenium and a number of physical properties of this material have been investigated. The radial distribution function has been determined from electron diffraction data and measurements of viscosity, solubility in CH₂I₂ and the activation energy for crystallization have been made. The degree of disorder in the structure of amorphous selenium increases with the temperature of the melt which is quenched to form the glass. It is suggested that the observed changes in the molecular structure of amorphous selenium result from a decrease in the average length of polymer molecular chains, Se_n, and a decrease in the number of ring molecules, Se₈.

Compares two methods to calculate the density of states n(E) of disordered alloys, both of which use the continued fraction expansion of the Green functions and have the same number of exact moments. In one of the procedures the authors approximate n(E) by the average of local densities of states, each calculated by the continued fraction expansion. In the other, the moments of the alloy are calculated first and from these n(E) is obtained from a single continued fraction. It is shown that, for a tight-binding one-dimensional alloy, the first method is definitely better due to the pronounced structure of the alloy density of states. For more realistic Hamiltonians and for amorphous and liquid systems, both methods may be more comparable.

The authors show that, as a consequence of the linear coupling between the magnetic and elastic degrees of freedom, rotational spin-reorientation type phase transitions in three dimensions will be described by classical critical exponents.

Polarized Raman spectra of oriented single crystals of RbAg₄I₅ have been measured for all scattering geometries from 448K down to near the second-order phase transition at 208K. Unpolarized spectra were continued down to 136K. The authors have observed both the vibrations of the Ag⁺ ions relative to their cages of I^- ions and the breathing modes of the cages themselves.

Photoemission studies using incident photon energies of 21.2 eV and 40.8 eV have been performed on the first row transition metal intercalates, Mn_0.26NbS₂, Fe_0.33NbS₂, Co_0.32NbS₂ and Ni_0.34NbS₂. The valence band densities of states show a systematic broadening of the d conduction band due to charge transfer and suggest that a rigid band model approach to intercalation is a valid first approximation. Structural data, derived from electron and X-ray diffraction studies are also included.

The Fourier transform phi (r), of the ESAFS is expressed as the convolution of a structure factor and a scattering factor, xi (r). The authors show that the xi are long range and sharply oscillatory, so that the interference of neighbouring shells can substantially hinder the determination of structural information from phi . A procedure to localize the xi without significant loss of spatial resolution is recommended. This reduces the interference and enables the extraction of the scattering signal of a single shell of atoms. The local environment of the excited core then follows directly from a simulation of the complete phi . This analysis is illustrated with crystalline Ge.

Unlike the alkali halides with NaCl structure, the CsCl-structure alkali halides present several problems regarding the cohesive energy, the static lattice structure, the dielectric properties and the phonon dispersion relation. From a critical discussion of the problems it is seen that these differences from the rest of the alkali halides become all the more prominent, when any attempt is made to reproduce the above properties simultaneously in any of the existing models. An attempt is made to calculate simultaneously the lattice statics and dynamics of the CsCl and CsI crystals from a single model with a single set of model parameters. The results of the calculation show that a consistent description for most of the major properties is obtained in the model. The source of the remaining discrepancy is discussed.

Measurement of the heat capacity from 1.8 to 30K and the thermal expansion from 1.5 to 35K, from 65 to 90K and at room temperature have been made on a sample of Sn/50.4 at.% Te. Values of the Debye temperature and the lattice Gruneisen parameter derived from these measurements exhibit strong temperature dependences below 20K which are attributed to thermal excitation of the low-lying transverse optic mode.

Many suggested approximations for the ensemble averaged Green function of a disordered system are not analytically acceptable. Here Kraichnan's (1961) idea of using 'realizable' approximations is explored and generalized. The method is used to prove that a self-consistent cluster multiple scattering approximation in the theory of disordered alloys is analytic, a result which is of both practical and conceptual importance.

The present controversy regarding the validity of the AC and DC rate-equation formulations of hopping conductivity is examined within the exact resolvent theoretic framework of quantum transport theory. It has been argued that terms representing field-induced charges in the local value of the chemical potential which appear in rate-equation theory should not be present: this would destroy the percolation theoretic interpretations of the DC conductivity. The present theory, based on the Kubo formalism, demonstrates the validity of the rate equations in the weak-coupling, low-frequency and DC limits. The controversy is resolved by the observation that the anti-rate-equation theories have neglected an infinite sequence of terms appearing in perturbation theory which restore the disputed chemical potential terms to the rate-equation formulation.

A new decoupling of the Green functions in a Heisenberg ferromagnet is proposed, based on an identity derived from the Dyson transformation. The decoupling scheme introduces a new decoupling parameter (in addition to that of Callen) lambda which has been chosen empirically by means of a modified Tahir-Kheli expression for the Curie temperature. The results for the Curie temperature are calculated for SC, BCC and FCC lattices. It is seen that for a wide range of spin values the results agree very closely with the Pade approximant results. The agreement is much better than that of previous decoupling schemes.

The Edwards and Anderson model for spin glasses is investigated by a decimation rescaling transformation applied to a spin-¹/₂ Ising model. The technique reproduces the exactly known results for a linear chain and, in particular, predicts ordering to a spin-glass type ground state at T=0. For a two-dimensional square lattice it is shown that approximations which give qualitatively correct results in other situations predict no spin-glass transition at any temperature. A comparison is made with results for spin glasses on a Bethe lattice and with an infinite-range spin-glass model, which has been found to exhibit a transition. It is argued that the difference between infinite-range and short-range models is that the former has entirely extended eigenvectors of the random J_ij matrix whereas the latter has at least some localized eigenvectors.

The authors interpret the spin-wave lineshape in the two-dimensional ferromagnet K₂CuF₄ by a theory developed for small wavelengths, as is relevant for inelastic neutron scattering, applicable for temperatures both above and below the Curie temperature. The agreement between theory and experimental results is rather good. However, at high temperatures, when the spin-wave lineshape is very broad, the present theory, which is available for energies near the spin-wave energy omega _q, gives only qualitative results.

The temperature and frequency dependence of the methyl proton relaxation in solid acetone is presented and compared in detail with the theoretical model of methyl group relaxation developed by Punkkinen (1974-75). While showing substantial agreement in some areas, this comparison yields inexplicable inconsistencies in others. In terms of the properties of the acetone molecule, the temperature-dependence data are found consistent with a methyl group hindering barrier of 4.6 kJ mole^-1 and the frequency-dependence data are best interpreted by assuming a tunnelling splitting between 50 MHz and 100 MHz. There is no evidence to suggest a marked temperature dependence of this tunnelling splitting.

A simple rate equation is used to calculate the Mossbauer line-broadening in anisotropic materials where the diffusing atoms may occupy two different types of site. Numerical calculations are presented for single crystals and polycrystalline material, both of which can exhibit large departures from a Lorentzian lineshape. It is emphasized that measurements on single crystals could yield much detailed information on diffusion mechanisms because of the large predicted angular dependence of the diffusional broadening.

Results of laser Raman studies of the E-modes in KH₂AsO₄, RbH₂AsO₄ and CsH₂AsO₄, their deuterated isomorphs, and KH₂PO₄ are presented. In the ferroelectric phase, the results reveal splittings for many of these modes, which arise from non-reststrahlen effects and which are consistent with the predictions of the recent pseudospin-two-phonon theory of Young and Elliott (see ibid., vol.7, p.27121 (1974)). These results are used to discuss the temperature dependence of the central component amplitude in these materials.

Structure has been observed in the infrared absorption spectrum of low-resistivity n-type ZnSiP₂ in the region of 0.76 eV. This is attributed to transitions between the Gamma ₃ and Gamma ₂ conduction bands which corresponds to the well known X₁-X₃ transitions which occur in n-type GaP. A peak has also been observed in the same wavelength region in the photomodulated absorption spectrum of samples with a wide range of resistivities. In this technique, an intense beam of light is used to modulate the number of carriers in the lowest conduction band, and the corresponding change in transmission measured. This is thought to be the first time this technique has been used in this way.

Using the spin-wave spectrum of a two-dimensional ferromagnet with spins lying in the basal plane, the variation in the intensity of exciton-magnon combination bands in the visible absorption spectrum of the ionic ferromagnet K₂CrCl₄ (T_c approximately 65K) is quantitatively explained. The near-neighbour exchange integral of 3.5 cm^-1 taken from the series-expansion fit to the high-temperature susceptibility of Gregson et al. (1975) gives a good fit to the intensity of the 15900 cm^-1 band from 4 to about 15K.

Hall measurements have been made on Bridgman and liquid-phase epitaxial silicon-doped gallium arsenide. The samples were then rendered transparent by e^--irradiation and the absorption due to the localized vibrations of Si_Ga donors and Si_As acceptors was measured. These data were used to obtain values of 1.99e and 1.75e respectively for the apparent charges of the two defect modes, with the assumption that there were no other donors or acceptors present. This calibration is very similar to an earlier one due to Spitzer and co-workers (1968-69), but is significantly different with respect to the interpretation of electrical data for LPE layers.

The excitation spectrum of the IR emission ⁵A₁(⁵E) to ⁵B₂(⁵T₂) of substitutional Cr²⁺ in ZnSe displays, as in the case of ZnS, band-band transitions, photoionization of activator centres, and a structured region of inner transitions towards higher levels of the Cr²⁺ ions. In the crystal-field approximation, a fitting of the eigenvalues of the Tanabe-Sugano matrices to the excitation bands supplies the crystal-field parameter Dq and the Racah parameters B and C. This fitting and the fact that both for ZnSe and for ZnS, one specific excitation band depends on pre-treatment, suggest a slightly modified interpretation of the ZnS excitation spectra.