Table of contents

The MCD data for the GR1 zero-phonon line of diamond have been analyzed on the basis of a vibronic model. The values of the matrix element ( eta mod L_z mod xi ) for the angular momentum and of its reduction factor K(T₁) in the excited T₂ state are extracted and compared with results of the Lowther-Stonham calculation (1978).

The character of the incommensurate-commensurate phase transitions is discussed with the framework of the Landau-type theory. It is shown that the variation of the order parameter amplitude could induce an attractive interaction between phase solitons, and consequently lead to a discontinuous phase transition. In models relevant to corresponding transitions in ferroelectrics and liquid crystals, the tricritical point can thus be reached by varying the temperature and the anisotropy parameter.

The authors have made a detailed numerical analysis of neutron diffraction lineshapes across the constant-coverage melting transition of incommensurate monolayers of methane on graphite. They find that in the liquid phase the liquid lineshape is well fitted with a lorentzian structure factor. While the exact nature of the transition very close to T_m cannot be commented upon, the 'freezing' of liquid appears to be initially continuous, with an increase in correlation length to hundreds of Angstroms before the appearance of a solid phase.

Contrary to the statement in a recent publication by A.I.M. Rae (1982), the present authors demonstrate that both the quasi-harmonic lattice approximation and orientation strain order parameter approach to the phase transition in sym-triazine give identical results. Both treatments rely on a Landau mean field expansion of the free energy in the vicinity of the phase transition. The usefulness of one theoretical treatment over the other is a function of the details of the experimental findings and just which properties are chosen to be modelled.

For comment see J. Raich and E. Bernstein, ibid., vol.15, no.10, p.L283 (1982). The application of quasiharmonic theory to the ferroelastic phase transition in s-triazine is discussed. Contrary to a recent comment by J.C. Raich and E.R. Bernstein, the theory described by those workers is not identical to that previously developed by the present author.

Calculations are presented of the surface structure and surface tension of the electron-hole liquid (EHL) in (111)-stressed Ge. The dramatic changes which the EHL undergoes as the Ge band structure changes with stress make this system ideal for studying the relationship between the surface and bulk properties of liquids. The Cahn-Hilliard scaling rule (1958) kappa sigma approximately L is found to be obeyed under strikingly diverse conditions. A case also occurs where no general relation between the surface tension and the bulk properties can hold.

Extending a dynamical theory for a classical particle moving in an array of randomly distributed fixed scatterers to d>or=2 dimensions, the critical volume fraction for the suppression of diffusion is found to be p_c=1-exp(-d). The diffusivity below the percolation edge is discussed and the localisation length is determined near the edge and for the high-density limit.

The critical behaviour of an m-component (m>or=2) spin glass in a magnetic field is studied near six dimensions. The critical behavior associated with the freezing of the transverse degrees of freedom (Gabay-Toulouse transition) is shown to be identical, to all orders in epsilon =6-d, to that of an isotropic (m-1)-component spin glass in zero field.

Pulse echo overlap measurements of ultrasonic wave velocities have been used to obtain the hydrostatic pressure derivatives delta C_IJ/ delta P of the elastic constants of the scheelite structure fluorides LiYF₄ and LiY_0.5Tb_0.5F₄. It is shown that the acoustic symmetry axes do not shift appreciably with pressure in either material. Hence the pressure derivatives delta C_IJ/ delta P, like the elastic constants C_IJ themselves, can be transformed (with usual simplification) to reference bases comprised of acoustic symmetry axes (( kappa and kappa + pi /2) or ( gamma and gamma + pi /2)) and the Z axis. A method of calculation of the acoustic model Gruneisen gammas in the long-wavelength limit, based on the transformation of the elastic constants and their pressure derivatives to the acoustic symmetry axis reference frame, is developed for TII Laue group crystals and results are given for LiYF₄ and LiY_0.5Tb_0.5F₄. Shear acoustic mode softening under pressure along the kappa and kappa + pi /2 acoustic symmetry axes has been observed in both these scheelites.

A method is discussed whereby self-consistent calculations of quantities characteristics of specific optic phonons can be carried out ab initio. An application of the method is made to KCl which utilises the X alpha exchange scheme and the APW method. Results are presented for the first ever self-consistent charged density changes which accompany optic phonon excitation. These call into question methods for computing lattice vibration spectra which rely on assuming that only the outermost electron shells deform and move relative to the atomic nuclei during lattice vibrations.

The authors have determined the frequencies of the optically active phonons of several perovskite-type layer structures, namely of K₂ZnF₄, K₂NiF₄ and K₂CuF₄ by means of far-infrared reflection spectroscopy and for K₂ZnF₄ also by means of Raman scattering. The experimental results are compared with rigid-ion model calculations, where the short-range force constants are not only treated as adjustable parameters but also successfully calculated with interionic potentials derived mainly for alkali halides.

Using the localisation ideas in the kq representation for perfect lattices, an equation is postulated defining perturbed localised modes for the impurity problem in lattice dynamics. The equation is solved by perturbation theory in the unit-cell scheme, up to second order. Multibranch and one-branch effective equations of motion are derived. The formalism is illustrated through the example of a molecular diatomic chain with a strongly localised impurity and in the tight-binding limit. A first-order correction is found for the displacement function of the optical localised mode.

The authors report detailed experimental studies of chloranil (C₆Cl₄O₂) soft-mode behaviour both at constant pressure and variable temperature, and at constant temperature and variable pressure. The phase diagram is inferred. When pressure is increased, the transition is linearly shifted to higher temperature with a slope of about 7.5K kbar^-1. This behaviour may be related to the very short distance between the carbon and oxygen atoms of adjacent molecules. Over the whole experimental pressure range the soft-mode temperature dependence is found to be in agreement with Landau theory down to a frequency of 2 cm^-1. Below this frequency the soft mode has been qualitatively observed with a Fabry-Perot interferometer up to the transition temperature.

Total and ionisation energies have been calculated for a number of simple atoms using spin density functional theory in the local-spin-density approximation (LSDA) as well as by methods that attempt to correct it for electron self-interaction effects. The LSDA total energies using the new improved results of Vosko et al. (1980) for the correlation energy of a spin-polarised homogeneous electron gas are further from experiment than those based on other frequently used, but less accurate, parameterisations. On the other hand ionisation energies are improved and compare as well with experimental as the self-interaction corrected results. By using the transition-state concept the authors examine the non-local correction to the LSDA single-particle potential of a valence electron and show that it can be relatively large.

The exact nature of the conduction band states in the ferromagnetic semiconductors EuO and EuS at T=0 is investigated. No corrections to standard band theory occur for majority spin states, but shifts and broadening occur in minority spin states due to electron-magnon interaction. Previous work incorporating this effect assumes a non-degenerate s band instead of the more realistic d band used here, but qualitative comparison is possible.

A simple model for identifying a deep level in semiconductors is suggested. It is shown that the relative positions of the critical points of the host can be related to the maximum of the photoionisation spectrum.

Capacitance bridge measurements of loss on irradiated phosphorus-doped silicon specimens at low temperatures are reported. A peak in AC conductance at 39K is seen after irradiation. Some annealing characteristics are described. It is argued that the behaviour is not associated with divacancies or single-vacancy defects.

The effect of impurities on the critical behaviour of the electrical resistivity due to spin-electron scattering is investigated within the context of the random mean spherical model, an exactly solvable model with one-dimensional disorder, exhibiting a smeared phase transition due to Griffiths singularities. It is shown that the singular part of the temperature derivative of the resistivity will have the same behaviour as the specific heat at the transition point, a relationship that is not self-evident for random systems.

Measurements of the pressure dependence of the Hall coefficient and conductivity of TiS₂ indicate that TiS₂ is an extrinsic semiconductor. The strong temperature dependence of the conductivity is matched by a strong pressure dependence that is consistent with electron-phonon scattering. In contrast, TiSe₂ is a semimetal, with a strongly pressure dependent p-d band overlap. The transition temperature of the structural phase transition in TiSe₂ falls at -1.2K kbar^-1.

The ground-state properties and low-lying excitations of spin systems with competing interactions are studied for various finite two-dimensional S=¹/₂ Heisenberg systems of 8 spins and corresponding Ising systems of up to 60 spins. The response of the spin systems on the conflict in the interactions is studied using numerical exact results. In the Ising model the competition of the interactions influences the ground state and the excitations essentially. Contrary to conventional Ising systems in disordered strongly frustrated Ising systems the low-energy density of states is smooth, the gap between the ground state and the first excitations vanishes. The low-lying excitations are characterised by a large portion of multiple spin excitations. Thermodynamic consequences are indicated. In the Heisenberg case the influence of the conflict in the interactions is less obvious, the character of the ground state and of the first excitation has similarities with that of conventional antiferromagnets, the spins have an enlarged number of possibilities to evade the conflict in the interactions.

The results obtained from thermally detected EPR experiments on semi-insulating chromium-doped GaAs are reported. The main lines in the spectra show tetragonal symmetry and arise from type I substitutional Cr_s²⁺ ions in tetragonal sites. Weaker isotropic and anisotropic lines have also been found. Type II Cr does not give rise to significant spectra. The line intensities are accounted for mainly in terms of magnetic-field-induced transitions for Cr_s²⁺. Electric-field-induced transitions are very weak in GaAs.

A detailed theoretical model for substitutional (type I) Cr²⁺ in GaAs based on the multimode dynamic Jahn-Teller effect is described. It is shown that random strain accounts for the observed tetragonal EPR spectra and the low-frequency phonon scattering (<or approximately=5 GHz). The model also explains why the APR spectra observed from n-type crystals in the dark is weak. Mention is made of the problem of type II chromium and a brief discussion is given of the application of the model to Cr²⁺ in II-VI compounds.

⁵⁷Fe emission Mossbauer spectra from a source consisting of SrLaCoO₄ doped with ⁵⁷Co are compared with spectra obtained with an absorber consisting of the same oxide doped with ⁵⁷Fe. There is evidence that the electronic structure of the ⁵⁷Fe³⁺ Mossbauer ion is determined by its local environment alone, in both types of experiment.

An IR study of the absorption near 0.36 eV of the negative divacancy (V₂^-) in silicon has been made under additional optical excitation for neutron-irradiated silicon as a function of the initial type and doping, and also of the neutron fluence. For some n-type samples, comparisons were made with electron irradiation and with the IR absorption of the P impurity near 0.04 eV. When the ratio of the neutron fluence to the initial carrier concentration r_n is <or approximately=0.4 cm, the 0.36 eV bands show no or weak photodependence in n-type Si; in p-type Si, the bands are not observed even under optical excitation. For r_n>2 cm, the bands are not seen under equilibrium, but they can be made to appear in both n- and p-type Si under pumping of the sample with photons in the 0.8 to 1 eV range; the bleaching out of the bands is obtained by pumping near 0.7 eV. The pump energy dependence for the appearance of the bands indicates that at least two mechanisms can turn V₂⁰ into V₂^-: (i) the direct ionisation of V₂⁰ in the valence band; (ii) an indirect process in which electrons are released by some defects and trapped by V₂⁰. The threshold for the vanishing of the band is connected with the photoionisation of V₂^- in the conduction band. Under equilibrium conditions, the authors have measured an optical cross section of V₂^- at 0.343 eV of 1.45*10^-15 cm² in n-type Si at 8K. This is a limiting value corresponding to a reduced neutron fluence of less than 0.4 cm, related to Gossick's simple model of neutral damages in semiconductors where a core, made intrinsic by high defect concentration, is separated from the undisturbed crystal by a space charge region. An empirical determination of the fraction of the crystal outside the space charge region was obtained by comparing the equilibrium P concentration, measured by IR absorption, before and after neutron irradiation.

A recently published assessment of the reproducibility of experimental LEED intensity data from the tungsten (100)(1*1) surface by Stevens and Russell (1981) is evaluated. I-V curves are presented which suggest that the independently obtained LEED spectra which they compared, including their own, were not at precisely normal incidence, this being the major cause of the differences among the curves. The importance of a valid criterion for establishing normal incidence is emphasised, especially when studying subtle changes in surface-structure symmetry. This is demonstrated by data which suggest the W(100) surface above 370K shows a very subtle loss of mirror-plane symmetry in the direction parallel to the steps existing on the crystal surface studied. Just as below 370K the observed 2 mm point group symmetry of the ( square root 2* square root 2)R45 degrees structure can be attributed to a preferred orientation of p2mg domains, this apparent 1 m point group symmetry of the (1*1) structure is suggested to be a direct step-induced preference for one p1m1 orientation rather than a consequence of deviations from normal incidence.

The energy dependence of the reflection of low-energy electrons in the 0-11 eV range from basal plane surfaces of 2H-NbSe₂ single crystals is presented. The method used was total current spectroscopy. The results have been explained in terms of inelastic electron-electron scattering accompanied by interband transitions, using published densities of states for NbSe₂.

A theory is given for the interaction of a helium atom with a metal surface. The basic theory is a generalisation of that used by Zaremba and Kohn (1977) for jellium surfaces, and relates the interaction to shifts of the band energies due to the presence of the helium. The authors show that these shifts can be determined via perturbation theory in the non-local helium pseudopotential. The first-order expression is particularly simple and demonstrates the physical origin of the interaction in terms of a balance between the kinetic energy cost when the Bloch tails orthogonalise to the helium 1s orbital and the potential energy gained in the field of the helium atom. For jellium, higher-order corrections are shown to contribute only approximately 15% to the interaction. The first-order result should therefore give a reasonably accurate picture of the corrugation profiles probed by low-energy helium particles.

A new cumulant expansion is constructed for the excitation probability in a solid caused by an external time-dependent potential. The new systematic expansion is discussed in the context of an external particle interacting with a surface. It is shown that earlier key results are obtained from the first term in the expansion.

(110), (100) and (111) single crystals of silver were cleaned by argon ion bombardment and annealing in ultra-high vacuum, the chemical purity and atomic arrangement of the surfaces being monitored by Auger electron spectroscopy and low-energy electron diffraction. The work functions of the crystals, determined photoelectrically, were 4.14+or-0.04 eV, 4.22+or-0.04 eV and 4.46+or-0.02 eV respectively. The importance of eliminating contamination by sulphur, which diffuses from the bulk crystal, is emphasised; such contamination increases the work function by several tenths of an eV.

Results of inverse photoemission in the 20-40 eV range are presented for a polycrystalline platinum sample. This choice of incident electron energies avoids interference effects which can occur between the radiative emission and the fluorescence emission following the 5p_1/2,3/2 core-hole excitation. The theoretical density of states based on LMTO band-structure calculations is found to describe the energy distribution curve of the unfilled states above the Fermi level fairly well.