Table of contents

The authors have made X-ray diffraction and visual observations of mixtures of methane and nitrogen near the melting point. Together with the phase diagram of the condensed phases of these mixtures, these provide evidence that mixtures composed of two molar concentrations of nitrogen to one of methane form a layered liquid structure near the melting point.

The Raman active phonon in CeH_{approximately 3} is anomalously soft as compared with that in CeH_{approximately 2}. It is shown that this can be attributed to enhanced electron-phonon interactions arising from changes in the electronic structure with composition: when going from CeH₂ to CeH₃ a new s-like valence band is formed, and the valence electrons screen the Raman active mode via virtual transitions into Ce d states. Frequency renormalisation due to the electron-phonon interaction has been calculated from the electronic energy bands.

The well converged p/d gaps calculated with the self-consistent symmetrised orthogonalised plane wave method and the Hedin-Lundqvist potential are reported for TiS₂ and TiSe₂. The various factors having an effect on the size of the gap are discussed. It is concluded that the ideal TiS₂ and TiSe₂ are small-gap semiconductors.

A phenomenological Langevin-like equation for spin glass dynamics is proposed and solved. The equation reproduces the static spherical model results for spin glass in the long time limit. The magnetisation, order parameter and on-site correlations are found to decay exponentially above the transition temperature and algebraically at and below it, as t to infinity . A closed-form expression for the dynamic susceptibility valid at all temperatures is also obtained.

The author conjectures that the equation (ln(1+(q-1)exp(-qJ/k_BT_c))/lnq)_p+(ln(1+(q-1)exp(-qJ/k_BT_c))/lnq)_p,=1 provides a very accurate approximation of the as yet unknown critical frontier of a fully anisotropic homogeneous quenched bond-mixed q-state Potts ferromagnet in a square lattice, where the random coupling constant J is distributed according to the laws P(J) and P'(J) for 'horizontal' and 'vertical' bonds respectively. This equation contains as particular cases a great number of exact (or believed so by us) results as well as a few recent conjectures (which are definitely only approximate).

3d core-level X-ray photoemission spectrum of CeNi₂ exhibits two distinct sets of structures corresponding to trivalent and tetravalent cerium ions. This study provides direct experimental evidence for the presence of mixed valency in CeNi₂.

The statistical distribution of the percolation threshold in two-dimensional samples of finite size is investigated by a Monte Carlo method. Conducting elements are added at random positions in an N*N array, the percolation threshold being identified as the fractional area occupied by the conductor when the first continuous conducting path can be traced across the array. Studies were made on arrays having eightfold coordination with N=5, 10, 20, 40 and on arrays having fourfold coordination with N=20. The average value of the percolation threshold is about 0.4 for eightfold coordination and 0.6 for fourfold coordination. For eightfold coordination the width of the distribution decreases with array size as N^-1v(p)/ with v_p=1.78+or-0.03. This is a slower rate of decrease than for arrays with fourfold coordination for which various authors report v_p=1.33-1.36. The difference is surprising since it is not consistent with the assumption of universality. Extrapolation of the data suggests that the percolation threshold for arrays with eightfold coordination can be determined experimentally to a precision of 1% by taking N approximately 2000.

It is found that the temperature T_c of transition to the superionic state of some fluorites is depressed by doping with trivalent cations, falls of up to approximately 200K being observed for doping concentrations of up to approximately 9 mole per cent. Calculation suggests that complexes formed by the impurity cations and charge-compensating interstitial anions act as traps for thermally-generated anion interstitials, thus reducing the energy of formation of anion Frenkel pairs and also of T_c.

Measurements on the F- and M-centre production during electron irradiation of NaCl at temperatures between 15 and 95 degrees C indicate the existence of an apparent F-centre diffusion process which is thermal, short-range and proportional to dose rate. Its importance in F-centre aggregation is discussed, together with the implication it has on the basic F-centre formation mechanism.

A self-consistent calculation of the band structure of silicon has been performed using the method of linear combination of atomic orbitals with a wavevector dependent optimised orbital basis set. Nineteen points in ¹/₄₈th of the Brillouin zone were used to determine the charge density in each of the nine iterations needed to reach self-consistency. The resultant self-consistent band states are then used to predict such ground state properties as Compton profiles and X-ray structure factors.

Electron density deformation maps of Ti₂O₃ and V₂O₃ at room temperature have been obtained by Fourier analysis of carefully measured X-ray diffraction intensities from spherical single crystals. The corresponding theoretical maps have been calculated on the basis of band structure results, including self-consistently the effect of intra-atomic coulomb and exchange interactions parameters. The agreement between theory and experiment is good concerning the density deformation in the vicinity of the metal atom sites. It serves as the first direct evidence for the difference in band structure and metal-metal bonds in these substances, which are related to the different natures of their metal-insulator transition.

A cluster-Bethe model for the study of deep impurity levels in a semiconductor is used. The coupling between the donor wavefunction and the lattice distortion is introduced via a dehybridisation mechanism. In addition to the position of the donor deep level and its wavefunction, minimum values of the ionic displacement were evaluated. The resulting stabilization energy is also in good agreement with experiment.

The non-compact analogue O(2,1) of the rotation group in three dimensions is generated by three scalar operations that create two phonons, annihilate two phonons, and count them. These generators commute with those of the symmetry groups for the octahedral systems E(X) epsilon , Gamma ₈(X) tau ₂, and, under conditions of equal frequencies and equal couplings, Gamma ₈(X)( epsilon (+) tau ₂). This permits an independent method for finding the matrix elements of the interaction term of the Jahn-Teller Hamiltonian to be devised. The parallel construction of the basis states for these three Jahn-Teller systems makes it transparent why the energy matrices should be the same (apart from trivial differences). The symmetries found by O'Brien (1977), for some isoscalar factors O(5) are related to the substitution P to -P-1, where P, the quasispin, defines representations of O(2,1). Analogues of O'Brien's symmetries exist in O(3) and in the groups used in atomic shell theory.

The authors have studied the site diluted triangular antiferromagnet by constructing ground state spin configurations for a variety of defect concentrations. They find there is no spin glass ordering at any temperature in the dilute limit or near the percolation threshold and argue that there is none at intermediate concentrations. This conclusion is also supported by a real-space renormalisation treatment.

Pure and diluted anisotropic quantum Heisenberg models are treated by renormalisation group scaling methods. Detailed discussions are given of the dependence of transition temperature on anisotropy and concentration, and of the dependence of correlation length on anisotropy, concentration and temperature. Particular emphasis is given to the weakly anisotropic system near the percolation limit and to the weakly anisotropic and Ising-like situations near the pure limit. One-, two- and three-dimensional cases are treated in turn. Some comparisons are made with neutron scattering results for the correlation length of quasi one- and two-dimensional systems and with the measured transition temperature curve for a diluted anisotropic layer system.

The ground state in the spin-flop phase is investigated by means of numerical calculations of the static correlation functions for short chains of up to 12 atoms. In the light of these results the authors suggest a picture in which the deviations form extended quasi-particles with an effective long range repulsive interaction. The model gives reasonable results for the magnetisation as a function of field and for the spin wave spectrum in the spin-flop phase.

The aggregation of Mn²⁺-Na⁺ vacancy dipoles and precipitation of Mn²⁺ has been studied in Mn²⁺-doped single crystals of orthorhombic NaNO₂ using EPR. The aggregation process seems to take place in two stages, being extremely slow in the second stage compared with the first stage. In crystals grown in aqueous solution a new intermediate phase seems to be precipitated during growth itself.

Nuclear spin relaxation rates due to magnetic dipole coupling between spins of a single species undergoing relative diffusion depend on two-spin correlation functions J^(p)( omega ). The high- and low-frequency limits of J^(p)( omega ) are discussed for liquids, cubic crystals and two- and one-dimensional systems. It is shown that the low-frequency form of J^(p) ( omega ) is J^(p)(0)-A₃ omega ^1/2 in liquids and crystals, A₂ln omega ^-1 in two-dimensional systems and A₁ omega ^-1/2 in one-dimensional systems and expressions for the coefficients A₁, A₂ and A₃ are derived which are valid for any diffusive model whose long-range, long-time behaviour may be described by the diffusion equation. The high-frequency form of J^(p)( omega ) is derived for a simple-hopping model of spins on one-, two- and three-dimensional lattices and in each case J^(p)( omega ) is of the form B omega ^-2. Expressions are derived for the coefficient B, for each dimension, which correctly include the effect of correlations between the hops of the spins.

The temperature and frequency dependences of the spin-lattice relaxation rates of quadrupolar nuclei in incommensurate systems have been evaluated both for direct and for Raman processes in the plane wave and the multisoliton limits. In contrast to translationally periodic systems, the spin-lattice relaxation rate varies over the incommensurate NMR line and is determined by phason and amplitudon fluctuations. The average T₁ should be anomalously short and nearly independent of temperature over the whole incommensurate phase, and should sharply increase at the lock-in transition reflecting the change in the phason spectrum. The variation of T₁^-1 over the incommensurate NMR line allows for a separate determination of the amplitudon and phason contributions as well as a discrimination between direct and Raman processes.

Optically detected magnetic resonance of phosphorus cluster (P₄) centres in ZnSe:P is reported. The P₄ centres are associated with an emission band peaking at 1020 nm at 2K. The band is due to the radiative transfer to the P₄ centre of an electron from a shallow donor. The transfer shunts other recombination processes, including one which causes an emission band at 637 nm. Thus ODMR signals which are observed as microwave-induced increases in infrared intensity are observed as decreases in the intensity of the visible emission. The P₄ level which participates in the shunt process is located 1.2 eV beneath the conduction band.

Mossbauer spectroscopy using polarised gamma rays was applied for the investigation of a KFeF₃ single crystal at 4.2K. Using this methodology, it was shown that one may reduce the ambiguity range of the hyperfine parameters inherently occurring in the case of complex magneto-crystalline structure.

The far-infrared absorption spectrum of stoichiometric silver beta -alumina shows that the silver ions occupy predominantly a single type of site at low temperature. By contrast, in non-stoichiometric silver beta -alumina three principal configurations are found at low temperatures. The spectra of both materials change markedly on warming to room temperature. Explanations of the infrared data are proposed which are consistent with results of neutron and X-ray structural studies.

In a previous paper the authors (see J. Physique, vol.41, p.707, 1980) showed by kinetics measurements a donor-acceptor transition peaked at 1.59 eV in CuGaSe₂. They confirm here the fundamental parameters of this emission by measurements in terms of temperature and excitation rate. They show the influence of weak phonon coupling on intra-pairs recombination. An original deconvolution method which takes account of this influence permits the authors to find the experimental spectra again.

A study of the L_III X-ray absorption spectra of the rare earths Nd(Z=60), Sm(Z=62), Gd(Z=64), Dy(Z=66), and Yb(Z=70) in their sesquioxides is reported. The preabsorption observed in the spectra of Sm and Gd is attributed to the vacancies created in the 4f levels due to exposure of the sample of continuous X-rays. The near-edge structure of the spectra is interpreted on the basis of molecular orbital levels in these systems. The metal-oxygen average bond lengths are calculated using the Levy method and the results are found to be in good agreement with those obtained using a diffraction technique. The variation of bond lengths across the rare-earth series is discussed.

The electronic structure of the (111) and (111) surfaces of GaP, GaAs, and GaSb is investigated by extended Huckel theory. In order to examine relaxation effects on surface electronic structures, three different geometries are studied for each surface. The geometric model for surface relaxation is described in detail. It is found that the sensitivity of surface states on both kinds of (111) surface to a change in group-V atom is small for the three Ga compounds studied and that some new surface states associated with dangling- and backbonds appear upon inward relaxation for the (111) surfaces and upon outward expansion for the (111) surfaces. Surface energy bands and local densities of states are described in some detail and compared with recent photoemission and electron energy-loss experiments for the (111) and (111) surfaces prepared under various conditions.

Hall mobility at 13 and 300K was studied on MOSFET devices. The SiO₂ layers were thermally grown in O₂ with and without addition of trichlorethylene (TCE). For TCE devices additional scattering at neutral defects was found. These scatterers are inhomogeneously distributed and are predominantly located within 2 nm from the interface.