Table of contents

The problem of 4f-5d interaction in doubly ionised lanthanides has been re-examined in LS coupling approximation by means of Racah's irreducible tensor method and F_k, G_k parameters available from atomic spectra. The results for most ions differ from those given by McClure and Kiss (1963) because of the different angular momenta used for the lowest term of the particular 4f^N5d configuration.

High-resolution gamma -ray diffraction measurements of the 200 rocking curve in TvVO₄ have been used to study the effective mosaic change above the Jahn-Teller phase transition and the misorientation between domains below the transition.

Preliminary experiments of inelastic neutron scattering confirm the hypothesis that the purely ferroelastic transition in Hg₂Cl₂ is induced by the condensation of a transversal acoustical model at one * point of the first Brillouin zone in the tetragonal phase. Second-order Raman spectra show that the frequency of the soft mode in the prototype phase is given by the relation nu ²=0.35 (T-184)(cm^-1)².

The velocity of transverse zero sound is adjusted to render mutually consistent a set of equations relating the velocities of longitudinal and transverse zero sound to each other and to the Landau parameters, Fl_n, at 0-33 bar. The result is incompatible with F₂^s<0 unless F₃^s approximately=0, but this conclusion is extremely sensitive to possible small inexactitudes in the Landau theory equations.

Torsion pendulum experiments in which the A-phase orbital relaxation times is comparable with the oscillation period show strong amplitude dependent damping in ³He-A that is absent in ³He-B. At large amplitudes the apparent superfluid density decreases towards zero, suggesting collapse of superflow as discussed by Bhattacharyya, Ho and Mermin (1977). A simple model is proposed to describe such collapse, at a rate limited by the Cross-Anderson orbital viscosity. This model accounts satisfactorily for the main features of the present experimental results.

Schottky barrier heights have been measured for a range of metals on atomically clean and on etched indium phosphide surfaces. The metal work function has little influence on the contact behaviour and the 'index of behaviour' S, often used to describe metal-semiconductor contacts, is not a meaningful parameter. It is suggested that this result applies in general to interfaces between metals and compound semiconductors. Chemical effects are shown to dominate the electrical properties of the contacts.

The small-r_s expansion is derived for the positron correlation energy and annihilation rate in a homogeneous electron gas. The correlation energy is found to be mu =-1.563*r_s^-12/+0.051 (ln r_s)²-0.098 ln r_s+0.976+higher terms.

The changes in the elastic properties of the thin monocrystalline platelets of CdSe, caused by periodical illumination with light from the intrinsic absorption range are experimentally investigated. The dependences of the amplitude of the light-excited flexural vibrations in the platelets on the wavelength and the intensity of the light absorbed are given. From these dependences the spectral distribution of the absorption coefficient is theoretically calculated.

The effective interionic potentials for liquid sodium rubidium and lead are calculated using previously proposed methods (see ibid., vol.9, p.L515, 1976) that extracts the effective potential from the static structure factor. The calculated potentials are examined in detail to test reliability of this method.

As a part of a continuing programme to study the general nature of the author's recently formulated deformable-ion model it is applied to study the lattice dynamics of ZnS, a II-VI compound of zincblende structure. The calculated phonon dispersion curves are in very good agreement with the experimental data. The differences between the present model and those used in some of the earlier calculations are pointed out. On the basis of the present and past studies with this model it is concluded that the deformable-ion model is a very general model that represents quite well the changing nature of the bond as one goes from ionic to covalent crystals.

The crystal-liquid interface, as computed by molecular dynamics at the triple point, is examined in the light of current models. Thermodynamic profiles of the density, potential energy and stress shows the interface is broad, extending over several atomic diameters. The density, configurational entropy, and diffusivity in the parallel planes, all change gradually through the interfacial region. In contrast to theoretical models, the density decrease arises primarily from an enhanced interlayer spacing, mainly on the crystal side, rather than from a depletion of the occupancy through vacancies. Trajectory plots obtained by computer graphics show the intermediate layers to be heterogeneous reflecting mountainous regions of registration extending into the liquid.

The authors examined the behaviour of a wide variety of systems undergoing structural phase transitions to a phase incommensurate with the high-temperature structure. With the aid of renormalisation group methods they argued that, in the cases examined, either the critical behaviour is effectively that of the n=2 component Heisenberg model or the phase transition is first order.

For pt.I see ibid., vol.11, no.17, p.3578 (1978). The authors studied the behaviour of systems undergoing structural transformations between commensurate and incommensurate ordered phases. They reviewed the theory developed by McMillan, and by Bak and Emery (1976) in which the order parameter for such phase transitions is visualised as a soliton density, and give a critique of this theory. They extended the analysis to those cases where the commensurate phase ordering is characterised by a wavevector of the Lifschitz type, such as the Brillouin zone corner. In both cases they found the phase transition to be first order, in agreement with experiment. They also briefly analysed an n=2 component model system which is shown to exhibit a continuous commensurate-incommensurate phase transition entirely analogous to one promoted by an acoustic phonon instability.

For pt.II see ibid., vol.11, no.17, p.3591 (1978). The authors discussed the nature of the excitation spectrum of a structurally incommensurate phase, both when the displacement field is describable by a single Fourier component and when it may be described in terms of an array of phase solitons. In each case they established the character of the Goldstone phase mode, and in the latter case they found an additional phase excitation whose properties merge into those of a phase mode in the commensurate structure as the 'lock-in' phase boundary is approached. They examined phase mode-acoustic mode interaction and give a brief discussion of phase mode pinning.

The authors report the first experimental determination of the electron surface barriers for dense hydrogen and deuterium. The barrier was determined by injection of electrons from a planar gold surface into the liquid and dense vapour phases. The density dependence of the barriers agrees well with a Wigner-Seitz calculation for molecular densities greater than 1*10²² cm^-3.

The valence band parameters for zinc telluride are obtained by fitting shallow impurity excited states with effective mass theory. To obtain a satisfactory fit it is necessary to assume that two-hole bound exciton transitions to excited acceptor states with predominantly d-character are very weak. They conclude that the valence band parameters mu , gamma and delta are respectively 0.58, 3.8 and approximately 0.12. The static dielectric constant is close to 9.2 and the effective mass binding energies of donors, acceptors and free excitons are respectively 18.4 meV, 62.5 meV and 13.3 meV neglecting special polaron coupling effects expected for the acceptor ground state.

Within the local energy independent pseudopotential theory, a convergent supercell calculation of the position of the discrete impurity level produced by the neutral undistorted vacancy in silicon has been performed. Translational symmetry was reintroduced by placing the impurities on a Bravais lattice. The author made calculations with unit cells containing up to 2662 atoms plus vacancy. A deep level in the lower part of the fundamental band gap is found, lying 0.18 eV above the top of the valence bands. The results also indicate that the localised impurity wavefunction decreases rather slowly and extends to 30 AA.

The previously developed theory, by incorporating the alternant-molecular-orbital method to the Matsubara-Toyozawa scheme for impurity conductivity in heavily doped semiconductors, has been applied to Si:P, Ge:Sb and CdS:Cl. The results agree well with experiments.

The authors studied the effects of second-neighbour interactions on the critical temperature of the bond-diluted Ising model within the framework of the decorated Ising model. A suitable second-neighbour bond problem can be formulated and solved in the annealed limit. This limit is known to be quite reliable for Ising models without competing interactions. Plots of the critical-temperature and zero-temperature magnetisation versus concentration are given for various ratios of the second to first neighbour exchange coupling. When this ratio is very small, but finite, there is a tail in the phase boundary.

Specific heat of two-dimensional Ising-like antiferromagnets Rb₂Co_cMg_1-cF₄ was measured. The dependence of Neel temperature, T_N, on the concentration of nonmagnetic Mg atoms was obtained by measuring the peak positions of the magnetic specific heat and also of the magnetic diffuse-scattering intensities of neutrons. The results are in fairly good agreement with the prediction from the concentration expansion method over a full range of c. The asymptotic behaviour of the specific heat in the vicinity of T_N is also discussed.

The two-sublattice Hubbard model has been solved in approximation schemes which are exact in both the zero bandwidth and small interaction energy limits. It is found that the finite temperature antiferromagnetism, which appears in the Hartree-Fock approximation, does not exist in this scheme. An improved solution which includes effects due to finite lifetime also rules out local moments at finite temperatures. The similarity of this solution with the coherent potential approximation for an AB model is discussed. Inclusion of the resonance broadening effect does not appear to alter the physical nature of the solutions.

The magnetic excitations of Fe_1-pMg_pCl₂ have been studied using a numerical method involving the time integration of the equations of motion. Calculations were performed on a 10000 site, randomly disordered two-dimensional model alloy. The spectral response function was calculated for the whole range of dilution p at various points in the Brillouin zone. The results obtained are compared with experimental data and with other theoretical calculations, in particular the moment expansion method of Fujiwara (1977).

Taking into account the magnetostatic interactions is equivalent to introducing a new term in the anisotropy field as long as only spin waves perpendicular to the wall are studied. Within this restriction, the authors studied the localised magnons in a Bloch wall, both in the sharp boundary limit when K/J>⁴/₃, and in the regime where the wall is a few atomic distances wide. They showed that the mode arising from the translation mode of the continuum limit varies continuously as a function of anisotropy over the whole range. The other mode below the bulk spin waves plays the role of a soft mode at the transition K/J=⁴/₃, between a Bloch wall and a sharp boundary between domains. The polarisations of both modes are discussed.

¹²⁹I Mossbauer spectroscopy has been performed in the ferromagnetic phase of CrI₃ in the presence of an external magnetic field. The reorientation of the microcrystals along the applied field has been observed for a loose powder sample and is in agreement with the high magnetic anisotropy of CrI₃. The magnetic hyperfine interaction transferred at the iodine nucleus through covalency and overlap effects has been interpreted in terms of an anisotropic component arising from the presence of a finite spin density in the 5p iodine orbitals and a negative isotropic component originating from the exchange polarised 5s orbital of iodine or/and the core polarisation by the unpaired 5p electrons. The relative importance of the several contributions to the transferred field is discussed.