Table of contents

Studies analytically 1D complex scalar fields with the phase anisotropy in the strong anisotropy region above the bifurcation point. The precise WKB formula for the three-well problem is applied to the pseudo-angular part of a set of 1D Schrodinger equations derived from the transfer-matrix eigenvalue equation. The authors find that non-topological solitons play the dominant role in the low-temperature statistical mechanics in the strong anisotropy region near the bifurcation point, in marked contrast to the recent results by Trullinger and DeLeonardis (1980). Generally, the correlation length is found to be proportional to 1/(t_NT²+ t_T²)^1/2 with t_NT and t_T being tunnelling terms, arising respectively from non-topological and topological solitons.

The idea of multiphonon transitions as tunnelling through a potential barrier is used to find the barrier width as a function of energy, from the experimental dependence of the multiphonon transition rate on temperature. The method is illustrated by the example of the B centre in GaAs where the available experimental data are explained by a multiphonon process with non-linear electron-phonon interaction, taking into account anharmonic effects.

A resistivity minimum, with a log T dependence, has been observed in a single crystal of Pb_0.99Ge_0.01Te. This behaviour is interpreted in terms of scattering from double potential wells which represent the alternative positions of the Ge²⁺ ions below the structural phase transition. Specific heat measurements on the same sample show a low-temperature anomaly consistent with this interpretation.

Reports resistivity against temperature measurements along the high-conductivity a axis of TMTSF₂CIO₄, TMTSF₂SbF₆ and TMTSF₂TaF₆, under pressure and as a function of applied magnetic field, that show that all three compounds exhibit superconducting phase transitions near 1K, when sufficient pressure is applied. TMTSF₂SbF₆ and TMTSF₂TaF₆ become superconducting above critical pressures of approximately 10 and 11 kbar respectively whereas the authors observe a superconducting phase transition in TMTSF₂ClO₄ at much lower pressures (<3 kbar) they find that the critical pressure above which the SC phase is stabilised can be correlated with the separation between the sheets of TMTSF molecules and anions.

XPS and L_III X-ray absorption edge studies regarding the valence state of cerium have been carried out on the intermetallic compounds CeCo₂, which becomes superconducting at low temperatures. It is observed from XPS that the surface shows both Ce³⁺ and Ce⁴⁺ valence states, while the X-ray absorption edge studies reveal only Ce⁴⁺ in the bulk. Thus valence fluctuation and superconductivity do not coexist in the bulk of this compound.

A review is given of elastic spin-polarised low-energy electron diffraction (SPLEED) from crystalline surfaces. For the two relevant spin-dependent interaction mechanisms, spin-orbit coupling, and exchange, dynamical theories are presented with emphasis on symmetry principles, potential models and multiple scattering formalisms. Brief accounts of computational aspects and experimental apparatus (polarised-electron source, spin detectors) are followed by a survey and discussion of experimental and theoretical results for non-magnetic and magnetic surfaces. Finally, the impact of SPLEED on electron emission methods is pointed out, and the prospects of SPLEED as a powerful tool for surface analysis are assessed.

The strong coupling corrections to the shear viscosity of superfluid ³He-B at 20 bar are discussed. The calculated viscosity with strong-coupling corrections is smaller than that calculated with the weak-coupling theory over the whole temperature range and the plateau region of the viscosity is pushed down to lower temperatures. There is good agreement between these theoretical results and the slip-corrected experimental data of Archie et al. (1981).

Reports on pseudopotential calculations of the electronic structure at various line defects in crystalline silicon. The authors have determined the density of states and the spatial distribution of charge densities with a view to establishing a link with the existing analogous results for surfaces and point defects (vacancies). In particular, they have focused attention on the localised dangling bond states and have attempted to establish the magnitude of the parameters determining the stability of the line defects considered. They have obtained a consistent picture which is in accord with the intuitive view concerning the short-range nature of the covalent bond.

A 'variational' process and a 'recursion' scheme applied to a multimode dynamic Jahn-Teller Hamiltonian have recently been shown by Fletcher, O'Brien and Evangelou (see J. Phys. A: Math. Gen., vol.13, p.2035, 1980) to reduce into the single-frequency cluster model, corresponding to weak and strong electron-phonon coupling limits where narrow bands ( approximately 10² cm^-1) with a sharp-zero-phonon line and broad ( approximately 10³ cm^-1) absorption bands are observed respectively. A functional-integral formalism is presented for the optical response function of a general pseudo-Jahn-Teller model system which includes the coupling of the nearly degenerate 2s and 2p excited states to lattice phonons, appropriate to describe in some cases the F centre in alkali halides and the bound polaron problem. Band shapes derived in the independent ordering approximation (10A) do not give the details of the broad bands as shown in the comparison with the numerical spectra within the cluster model, at T=0K. It is further shown that the IOA does take into account the phonon dispersion of the host lattice at weak and strong coupling, in a manner equivalent to the two cluster model descriptions.

Measurements of photoconductivity and secondary excitation spectra have been used to determine the position of the Cr acceptor level in InP. A pair of complementary photoconductive onsets have been observed at 0.40 eV at 300K (0.47 eV at 6K), and at 0.93 eV at both 300K and 6K, which result from the photoexcitation of electrons out of and into the Cr trap level. This level lies 0.40 eV (0.47 eV) below the conduction band edge 300K (6K), which accounts for the relatively low resistance of Cr-doped semi-insulating InP. The trap populations have been changed by using both neutron transmutation doping, and secondary bias illumination. A maximum in the photoconductivity spectrum at 0.81 eV is provisionally interpreted as an intracentre ⁵T₂-⁵E transition of Cr²⁺ (3d⁴), where the upper state is located above the conduction band edge.

A simple, but general, model of magnon-phonon scattering is developed here as an extension of linear spin-wave theory in multi-lattice magnets. A method is presented for calculating the scattering amplitudes for one-phonon processes arising from modulation of the magnetic dipole interactions by lattice waves. Numerical results obtained for GdCl₃ are found to be in excellent agreement with the measured thermal conductivity; both the temperature dependence and the magnetic field dependence can be accounted for in this way. The dominant effect of the magnons is to scatter phonons via two-magnon-one-phonon processes. The one-magnon-one-phonon dipolar processes are shown to be of negligible importance in GdCl₃. Thermal transport by the magnons is found to be insignificant.

Electronic structures are calculated for two typical reconstruction models of the Si(111)-7 × 7 surface, i.e. the buckled and the vacancy models. The first-principles discrete variational DV-X alpha calculations are carried out for the surface cluster without a vacancy (Si₁₃H₁₅) and with a vacancy (Si₁₂H₁₅). Essential characteristics in the electronic structure are elucidated which are closely related to surface atomic arrangements. The low-density vacancy model recently proposed by Ino (1980) is pointed out to be a promising reconstruction mode for the Si(111)-7 × 7 surface.

Visible and infrared DCEL emission has been observed in thin films of ZnS:Cu:Nd:Cl prepared by co-evaporation in vacuum. The lifetimes of the excited states of the Nd³⁺ ion and the emission intensities at various wavelengths have been studied as a function of temperature and applied voltage. The energy distribution of the conduction band hot electrons taking part in the electroluminescence excitation process has been shown to follow a Boltzmann function and the mean energy of the hot electrons has been found to be 0.16 eV, a value which is consistent with the measured energy conversion efficiencies.

A theoretical calculation of angle-resolved photoemission from a system of bromine adsorbed on copper (100) face in a c(2*2) structure is reported. Comparison with experimental results of Richardson and Sass (1981) shows that for normal emission the significant features arising from the adsorption of bromine can be explained quite well. For off-normal emission the various trends in the experimental spectra are also qualitatively reproduced.