Table of contents

Calculation of longitudinal optical (LO) mode potential functions and dispersion curves are made for a AlAs/GaAs/AlAs quantum well using a macroscopic model. The interface boundary conditions employed are continuity of potential and normal components of both electric flux density and relative ionic displacement. In the non-dispersive limit the model yields unique potential functions of two types: confined modes and interface modes. The confined mode potential functions are almost identical to those calculated for a microscopic model by Huang and Zhu (1988). The interface modes are identical to these predicted by both the microscopic model and the dielectric continuum model. The introduction of bulk dispersion in GaAs produces modes which are hybrids of the confined and interface phonons.

A model based on a binary mixture of solid- and liquid-like phases can explain the empirical values for the frozen-in enthalpy and the frozen-in entropy in glasses. It is suggested that a binary mixture of equal amounts of the two phases can provide both spatially extended regions of high molecular mobility and structural rigidity on a macroscopic scale.

The problem of two electrons on a one-dimensional (1D) lattice is solved for the case when the lattice unit cell contains two unequivalent sites a and b with two corresponding electron-electron correlation potentials: attractive U_a and repulsive U_b. It is shown, that the kinematics of the electron-electron scattering in such a system are remarkably different from those of the ordinary 1D Hubbard model because of the presence of the Umklapp processes. Analytical properties of the S-matrix are studied. Ground state properties and excitation spectrum are analyzed as functions of U_a and U_b.

Finite-size calculations are used to study the magnetic field dependence of the ground state of interacting 2D electrons. It is found that the pair correlation function acquires the hexagonal symmetry of a quantum Wigner crystal when the filling factor is between ¹/₅ and ¹/₅. The energy of the hexagonal state is compared with variational results for the Wigner crystal and the nature of the transition to hexagonal symmetry is briefly discussed.

The structures of molten mixtures of zinc chloride and potassium chloride have been investigated by pulsed neutron diffraction. Total structure factors have been determined for natural samples of the two pure salts and for mixtures across the composition range using the Liquids and Amorphous Materials Diffractometer at the Rutherford-Appleton Laboratory's ISIS source. The results show that the tetrahedral coordination of anions around zinc ions is present right across the composition range and species of a lower coordination number are not formed. Furthermore, the intermediate-range ordering found in pure zinc chloride is persistent in the mixtures right up to high potassium chloride concentrations. The results are consistent with a recent clustering model, proposed as an explanation for the origin of medium-range ordering in molten salts.

The P-T phase diagram of Te has been investigated by means of thermobaroanalysis (TBA) and electrical resistance measurements. For crystalline Te the boundary of the transition Te_III-Te_IV has been found and the location of triple point Te_III-L'-Te_IV has been established (P=7.6+or-0.3 GPa; T=835+or-15 K). The melting curve of Te was investigated up to 10 GPa. In the Te melt the transition L-L' has been found from the TBA signal and by jumps in the electrical resistance. The triple point L-Te_III-L' has been found to be located at P=4.7+or-0.3 GPa and T=800+or-15 K. At pressures lower than P=1.6+or-0.4 GPa and temperatures higher than T=1120+or-30 K the TBA anomalies and the jumps in resistance become smooth and unmeasurable. The model of a phase transition in a liquid is presented based on the existence of crystal-like clusters in the melt.

The transitions in the Bi melt were found under high pressures. These transitions were accompanied by anomalies of electrical resistivity and volume. The (P, T) phase diagram of liquid Bi was investigated up to 7.7 GPa and 1020 K.

The anion-excess fluorite (Sr,Y)Cl_2.03 has been studied by coherent diffuse neutron scattering techniques. At ambient temperature the disorder is found to be static within instrumental resolution. The parameters of defect cluster models have been fitted directly to the data. Excess chlorine ions are found to aggregate into cuboctahedral clusters whose ionic coordinates agree with those calculated from a simple hard sphere model. At elevated temperatures the scattering exhibits quasi-elastic energy broadening indicating the dynamic nature of the disorder. Data obtained at 1050 K show that the cuboctahedral clusters have broken up, the scattering being reminiscent of that obtained from pure fluorites in the fast-ion phase. It is possible to account for the high temperature scattering in terms of 'snapshot' models of the diffusing anions and their associated relaxation fields.

The difference between relative changes in macroscopic length Delta L/L and X-ray lattice parameter Delta a/a has been measured over a wide temperature range from room temperature to 25 K below the respective melting point in pure silver and in some dilute silver-tin alloys between 0.25 and 8.60 at.% Sn. At higher temperatures, Delta L/L- Delta a/a increases, showing an Arrhenius behaviour due to vacancy formation. The absolute vacancy concentration at the melting point increases continuously from 5.2*10^-4 for pure silver to 2.5*10^-3 for a Ag-8.6 at.% Sn alloy. The vacancy formation enthalpy decreases from 1.05 to 0.74 eV simultaneously. The binding enthalpy and entropy for the first vacancy-impurity complex were found to be h_BI=0.15+or-0.02 eV and s_BI=-(0.5+or-0.2)k_B. The impurity-impurity interaction is repulsive and of the order of ¹/₁₀ eV, similar to other noble metal alloys with positive excess charge. The binding Gibbs free energy of the first complex was determined as g_BI=0.20+or-0.02 eV in good agreement with earlier measurements of resistivity and silver self-diffusion enhancement in silver-tin alloys but without some of their constraints since the vacancy concentration has been determined directly. All results can be interpreted within the framework of the model of Berces and Kovacs (1983).

The atomic configuration, electronic states and a local vibrational mode of the Li-saturated vacancy in Si are determined using ab initio calculations. The author finds in agreement with an earlier theoretical suggestion, that this defect consists of a tetrahedral combination of a negatively charged vacancy and four positively charged Li interstitials. A C_3V configuration is found to be higher in energy. This confirms recent experimental suggests that a symmetry lowering, which is found in the luminescence spectrum, is due to excitonic effects. The geometric simplicity of this defect complex suggests that it may be universal for irradiated Si doped with column I and (from chemical symmetry) column VII elements. As an example of the latter he considers the structure of the Br-saturated vacancy.

The authors investigate the thermal conductivity in one-dimensional quasi-periodic Toda lattice by means of the molecular dynamics technique. The lattice consists of two kinds of atom with different masses which are arranged according to the Fibonacci sequence. The temperature profile exhibits exponential behaviour as does the diatomic Toda lattice presented in their previous paper. The thermal conductivity is evaluated by separating the ballistically propagating part from the total heat flow. Heat conduction in the Toda lattice with quasi-periodic mass distribution in found to obey Fourier's law. The resultant thermal conductivity is inversely proportional to local temperature and the magnitude is reduced remarkably in comparison with that of the diatomic Toda lattice.

The authors extend the surface-embedded Green function technique for calculating the electronic structure of surfaces and interfaces by presenting a method for determining substrate embedding potentials which makes no approximations to the substrate potential. They first present an alternative derivation of the surface-embedded Green function method, to clarify the use of a planar surface in simulating embedding on a more complicated surface and illustrate this with rigorous tests. Considering the case of a region embedded on two surfaces, they determine the conditions under which the resulting Green function may itself be used as a substrate-embedding potential, and thereby derive a procedure for obtaining an embedding potential which makes no approximation to the substrate potential. In the case of a substrate with semi-infinite periodicity this reduces to a self-consistency relation, for which they describe a first-order iterative solution. Finally, a particularly efficiency scheme for obtaining local properties within a surface or interface region is outlined. This constitutes a full-potential solution to the one-electron Schrodinger equation for systems of two-dimensional periodicity, whose calculation time scales linearly with the number of atomic planes.

A general and analytical method of evaluating the zero-field splitting parameter D originating from the spin-spin interaction is presented. Application of this method to the self-trapped excitons (STE) in alkaline earth fluorides identifies the geometry of the STE in an unambiguous way and gives values of the zero-field splitting parameter D in good agreement with experiment. A recently proposed model of the STE in alkali halide crystals involving rotations is briefly discussed on the basis of the zero-field splitting.

By incorporating lateral tunnelling between adjacent quantum wires as well as taking into account the dependence of the areal electron density and modulation strength on the varying gate voltage, the authors have calculated the magnetoplasmon excitation energies of a lateral multiwire superlattice. They have also examined the role played by modulation strength (i.e. barrier height) and applied magnetic field on the collective modes. Their model successfully reproduces several interesting features recently observed with the use of far infrared spectroscopy. Also, a novel commensurability relation between cyclotron and skipping orbits is predicted.

The authors compute the Hall conductivity in a variety of model disordered two-dimensional systems by numerically evaluating the appropriate Kubo-Greenwood formula. Their models range from substitutional binary alloys to topologically disordered 'glasses', and include systems where the disorder is caused by the random small displacements of atoms from their positions in a crystalline lattice. Their Hamiltonian is reminiscent of the Kronig-Penney model in that delta -function-like atomic potentials are specified by a single parameter. The authors focus particularly on the sign of the Hall coefficient, and establish that the sign can be positive even when there is no well-defined dispersion relation for the electrons. They observe a correlation between the Hall coefficient and the derivative of the density of electron states.

Formulae are obtained for calculating the effective Hall coefficient in a weak magnetic field and for the effective Seebeck coefficient neglecting the secondary thermocurrent. A model of an infinite cluster on both sides of the percolation threshold with 'hot' and 'active' points is constructed. It is shown that the main contribution to the kinetic coefficient in the critical interval is given by a few separate small regions called 'active' or 'hot' points. For a given change of concentration in the critical interval, the number of these points correspondingly changes; near the percolation threshold there remains one active point in the volume rho ³, where rho is the correlation radius.

Second-order phase transitions are studied for a biaxial anisotropic disordered system composed of randomly distributed metal and non-metal regions, the former having tensor-type conductivity along the principal axes. Integrals are derived for the anisotropic Hall and Seebeck coefficients using Green functions. Computer calculations show that near the percolation threshold, the decrease of the biaxial anisotropy of the effective conductivity and Hall and Seebeck coefficients is described by new critical exponents. A model of 'special points' of the biaxial anisotropic infinite cluster above the threshold and the two-component infinite cluster below the threshold is developed.

The Hall coefficient and electric conductivity of black phosphorus prepared by the bismuth-flux method were measured at low temperatures. It is found that the conductivity shows log T-like behaviour below about 5 K and the Hall coefficient changes its sign from plus to minus at around 7 K with decreasing temperature. These results can be analysed successfully on the basis of a three-carrier model, in which bulk holes, variable-range-hopping holes and two-dimensional electrons take part in the electric conduction at low temperatures, except in the case of strong magnetic fields below 7 K. The present analysis supports the fact that magneto-electric properties observed at low temperatures in black phosphorus crystals originate from the 2D Anderson localization in an inversion layer on the surface.

The feasibility of computer simulation of electronic relaxation at a semiconductor-vacuum interface under a strong electric field is demonstrated. A kinetic description of conduction band electrons in a space-charge region layer with a self-consistent electric field under electron-impurity and electron-phonon scattering is developed by the particle method and Monte Carlo procedure. Results for the transient electronic process in a semiconductor field emitter are given.

A model is considered in which localized superconductivity (LS) is induced by a two-dimensional array of steps (twinning dislocations) at a non-coherent twin boundary in the bulk of a metallic crystal. It is proposed that the cores of the twinning dislocations are the sources of the local enhancement of the electron-phonon interaction parameter in superconductor. In the framework of the model the dependence of the LS critical temperature on the two-dimensional concentration of the steps and on the orientation angle of the twin boundary with respect to the crystalline plane of twinning is predicted.

The crystal and magnetic structure of the C15 Laves phase of HoMn₂ was restudied using powder neutron diffraction. High-resolution spectra showed HoMn₂ to remain cubic Fd3m below the magnetic transition temperature. The Ho spins assume a spin-canted ferromagnetic structure with 7.9 mu _B per Ho atom. One out of four Mn sites carries a moment of 0.6 mu _B induced by the strongly polarizing magnetic environment of ferromagnetically coupled near-neighbour (111) planes of rare-earth spins. A small thermal expansion anomaly accompanied by a spin reorientation is found at the Curie point of 25 K; the Neel point of the system lies at 31 K.

The ground state configuration of the two-dimensional (2D) Heisenberg model is a topic to which much theoretical effort is being applied with particular emphasis on the frustration effects caused by exchange competition between spins of different neighbouring shells. In this respect a central topic is the effect of quantum fluctuations on the soft modes present in the simple spin wave dispersion curve. A rigorous theorem based on the Bogoliubov inequality implies that a zero energy excitation must exist at the helix wavevector Q when long-range order is present. Recently it has been suggested that only the soft mode at k=0 survives quantum fluctuations in the square frustrated Heisenberg antiferromagnet, so that the Goldstone mode at k=Q could be recovered only by looking for an excitation different from the single-particle-like excitation. The authors show that this result is a spurious consequence of neglecting second-order perturbation contributions which are of the same power in 1/S as the first-order perturbation contribution accounted for evaluating the magnon self-energy. Indeed they show that a delicate balance of these contributions restores the Goldstone mode at the helix wavevector Q and substantially reduces the value of the quantum gaps that replace the accidental soft modes of the simple spin wave spectrum.

The authors performed band calculations by the linear muffin-tin orbital (LMTO) atomic sphere approximation (ASA) method for Mn₄N and BCC Mn using an exchange-correlation potential that depends on each of local spin directions at atomic spheres. They discussed the magnetic structure of Mn₄N on the basis of results for two prototypes among several models proposed experimentally for Mn₄N. The authors discussed the coexistence of ferromagnetic and antiferromagnetic states in BCC Mn.

The electron paramagnetic resonance spectra of Gd³⁺ in single crystals of (Gd_0.5Eu_0.5)₂CuO₄ show an anomalous anisotropy for temperatures below the magnetic ordering of the Cu ions (T approximately=280 K). The authors have performed a detailed experimental study of this feature, and they discuss their results in terms of a model that assumes a weak Heisenberg interaction between Cu and Gd moments. The authors show that that anisotropy is a consequence of the dynamic coupling of the Gd³⁺ paramagnetic mode with a weak ferromagnetic mode originating in the Cu magnetic moments.

It is believed that the high-field line of the double-peak ferromagnetic resonance spectrum observed in exchange coupled ferromagnetic bilayer films is due to an out-of-phase mode. By applying rigorous microscopic FMR theory to such systems the author shows that this mode is in fact due to an in-phase combination of sublayer modes, similar to the next (second) mode peak. The author also shows that when interface exchange coupling J^AB is antiferromagnetic, this high-field side peak is due to the interface-localized mode (in contrast to the bulk character of the next mode peak), and its intensity decreases when negative J^AB increases in absolute value. This explains the experimentally observed fact that some double-peak resonance spectra exhibit an inverted pattern of the peak intensities (i.e. I₂>I_I). Finally, the author also shows that ferromagnetic interface coupling generates only single peak resonance, irrespective of the strength of the coupling. It is concluded that when surface and/or interface intrinsic anisotropies are absent, the interface coupling underlying multipeak resonance bilayer spectra is antiferromagnetic.

Nuclear magnetic resonance spectra of ²⁰⁹Bi(I=9/2) in the ferroelastic BiVO₄ single crystal has been investigated by employing a wide-line Varian spectrometer. Only three lines due to a large quadrupole interaction were measured at a fixed frequency of 6 MHz in the principal planes at room temperature. From the experimental data the quadrupole coupling constant E²qQ/h=79.2+or-0.1 MHz and asymmetry parameter eta =0.70+or-0.01 are determined; however, these values turn out to be quite different from the previous report obtained with a ceramic sample at liquid-nitrogen temperature. The principal axes of the electric field gradient tensor are determined for the first time, the x, y and z axes being the crystallographic c+25 degrees , a+25 degrees , and b axes, respectively. Thes directions are discussed in terms of the Bi-O bonds in the Bi-O polyhedra.

The spectroscopy of Cu⁺ ions embedded in an alkaline earth fluoride (CaF₂) is reported for the first time. The absorption, emission and excitation spectra, as well as the lifetimes have been studied as a function of the temperature. It is shown that Cu⁺ ions occupy non-centro-symmetric sites. Furthermore, the analysis of the polarization dependence of two-photon absorption shows that, in fact, Cu⁺ ions appear to be located at sites of nearly cubic symmetry, in off-centre positions in the cube. The ¹E and ¹T₂ singlet states are responsible for the main absorption in the near-UV range, and the emitting level is shown to be the ³T₂ triplet state. This state is in fact split by the spin-orbit coupling into four components, but two close sub-levels could be responsible for the strong temperature dependence of the lifetime in the low-temperature range.