Table of contents

Previous work on the angular distribution of phonons emitted by a heated two-dimensional electron gas is extended to include the effects of an in-plane (Hall) electric field such as would be present in a dice in which the heating is due to the passage of an electric current. The anisotropy is seen to be such that there is an enhancement of the phonon emission in the direction of current flow.

The authors compute the width of the Rayleigh surface-wave peak in the Brillouin spectrum of a superconducting simple metal. The electron-phonon interaction is considered to be responsible for the attenuation. The decay constant presents a discontinuity at T_c and may have a second discontinuity below T_c depending on the phonon frequency.

An X-ray crystallographic structure determination has been carried out for the square root 5* square root 5 Mo(001)/O surface. This is the penultimate structure in the sequence of ordered states obtained by high-temperature desorption from the oxidized surface. The oxygen induces a reconstruction with one Mo atom missing from the surface unit cell and O atoms occupying the exposed threefold sites. The O sits asymmetrically in these sites and induces a rotational distortion of the remaining top layer Mo atoms, which cluster together. The asymmetry can be attributed to partial direct bond formation between surface Mo-O clusters, since apparently this is not mediated by any significant second-layer displacements.

The dissociative chemisorption of water vapour on the Si(111) 7*7 surface at 150 K and the effect of annealing have been studied using X-ray photoelectron spectroscopy and energy loss spectroscopy. From the data the authors may deduce that at 0.25 L the water molecule begins to adsorb on the adatom site and 'rest'-atom site of the unit cell and immediately dissociates to form a silicon hydroxyl Si-OH group (and probably an SiN group), but no evidence for molecularly adsorbed water is observed. After subsequent annealing from 150 K up to 900 K, they observe decomposition of the Si-OH group, oxidation of the silicon surface and then re-establishment of the clean surface.

In this paper, some properties of a surface exciton in polyatomic polar crystals are studied. Effective Hamiltonians in the ground state of the surface exciton for both strong-coupling and weak-coupling-polyatomic polar crystals are obtained by the method of a linear combination operator and simple unitary transformation. The effective mass of a strong-coupling surface exciton is derived using a Lagrange multiplier method. The self-trapping energy and effective potential of the surface exciton could be written as a series in alpha _s^-1, the first term being proportional to alpha _s, the coupling constant. The self-trapping energy and effective potential contain an extra contribution due to crossed terms between the different phonon branches. For a surface Wannier exciton the increasing part of the effective mass is proportional to alpha _s.

Surface spin waves in a semi-infinite magnetic superlattice with a single-ion uniaxial anisotropy are investigated by use of the transfer-matrix method. The dispersion equations for surface spin waves are obtained. The authors find that not all of the structures of the superlattice can excite the surface spin waves and that the anisotropy term need not favour the excitation of surface spin waves, but that it will certainly influence the value of the energy of the excited surface spin waves.

The authors present high-resolution measurements of several of the LMM and LMN Auger spectra of Nb, Mo, Pd and Ag excited by bremsstrahlung. Because of their high energy (2.0-3.5 keV), these spectra have not been reported in the literature. Assignments of the peaks are made based upon energy and intensity considerations.

The authors study a spin-one-half Ising model of an alternating magnetic superlattice. They use the mean-field approximation and express their results for the Curie temperature in terms of two types of determinants. The dependence of the Curie temperature on the film thickness is obtained. The effects of surface modification on finite superlattices are studied numerically for two types of modification to the surface exchange constants.

The kinetics of the correlated annealing of pairs of neutral (F-H) and charged ( alpha -I) Frenkel defects in alkali halides is treated theoretically, taking into account defect diffusion, annihilation at short distances, and elastic or Coulomb interactions. It is shown that elastic interactions considerably affect the annealing kinetics and survival probability even for defect pairs that are third or fourth neighbours in the crystalline lattice. A widely used description of the correlated annealing in terms of the pseudo-first-order reaction fails for close defects, yielding effective energies E* for which in fact differ essentially from the activation energy E_a of diffusion, even when it is corrected by an interaction energy. The effect of the defect initial distribution over relative distances is studied. It is demonstrated that pulsed creation of spatially well correlated defects can lead to the non-exponential decay of concentrations usually expected for an isothermal annealing. The theory developed is compared with actual experimental data for KBr. It is concluded that the several-stage annealing of defect concentrations often observed in thermostimulated experiments does not necessarily mean recombination of dose Frenkel defects which differ in their initial distances.

The positron-electron momentum density in germanium was studied by a full-scale use of the two-dimensional angular correlation of positron annihilation radiations (2D-ACAR). The term, a full-scale use of 2D-ACAR, means that the three-dimensional momentum-space density rho (p) of the annihilation positron-electron item is directly determined from a set of experimental 2D-ACAR spectra, followed by the reconstruction technique. The obtained electron momentum density, rho (p), showed, as a whole, good agreement with the electron momentum distribution of the fully filled Jones zone with the exception that a deep dip in the origin appears, which has been observed in the 1D-ACAR or semi-2D-ACAR integrated spectra in germanium. In the present work, the electron momentum density distribution shows a ridge along the (110) line and a vary deep valley exists along the (100) line, which is also the deviation from the Jones zone. In germanium, the valence 4p electrons are mainly participating in bonding sigma bonds, and the distribution in space is prominently anisotropic. This anisotropy is reflected on the electron momentum density distribution in the present work. The analysis based on group theory was performed by Saito et al. in order to investigate the origin of the anisotropy in the present work.

The phase diagram of a three-dimensional Ising model in a magnetic field with commensurate and incommensurate phases is studied at low temperature within a mean-field approximation. This analysis was motivated by problems in condensed matter physics concerning, for example, bipolaronic charge-density waves, neutral-to-ionic transitions in organic salts, staging in intercalation compounds, magneto-elastic materials, stacking in discotic liquid crystals, etc. The interactions between the Ising spins are long-range exponentially decaying, and are antiferromagnetic in one direction and ferromagnetic in the perpendicular planes. The ground state of this model is known to be a commensurate or an incommensurate structure with a wavevector that varies as a complete devil's staircase as a function of the magnetic field. Provided some conditions hold on the model parameters, the authors prove rigorously that, for small enough temperature, the mean-field variational form of the model still yields a minimum that is a commensurate or an incommensurate structure, the wavevector of which also varies as a complete devil's staircase as a function of the magnetic field or temperature.

The authors study the electronic structure of the GaAs/AlAs Fibonacci superlattice using a semi-empirical sp³s* tight-binding method. They find that a self-similar energy spectrum can be seen in the band structure, although the energy spectrum depends strongly on the wavevector parallel to the layers. Furthermore, they find that a localization character is enhanced due to the band hybridization, producing a spiky density of states and a localization-like effect of the wavefunctions in the hybridized energy region. The reasons for the localization-like behaviour are discussed briefly.

A real-space renormalization-group approach for calculating the exact electronic Green function of an infinite Thue-Morse chain is presented. This approach is based on a renormalization transformation introduced according to the self-similar structure of the chain and the recursion relations for the matrix elements of the Green function. Numerical results for the local density of states for the off-diagonal model show the existence of the extended states.

A semiempirical approach to the evaluation of corrections to LDA electronic states and effective masses in semiconductors is described. Applications to GaAs, AlAs, Ge, Ga_1-xAl_xAs VCA alloys are presented, and a scaling law for the scissor operator is discussed.

The boundary problems of the widely used cluster method for crystal computations have been investigated, and problems in the cluster calculations of partial ionic crystals were attributed to the charge-transfer problem. The proposed solution, the charge-transfer method, applied to ZnSe and ZnS systems, obtained results comparable to the band-structure calculations for the distributions of the density of states, the energy gaps and the bonding strengths even for the smallest cluster and showed small size effects, which overcame one of the largest deficiencies of the cluster methods. The influences of the charge transfer on the distributions of the density of states were also discussed. The results indicated that the charge-transfer problem is an important factor that should be considered in the cluster imitation of partial ionic crystals.

The effective spin bonds in a material with both charge-transfer and Hubbard-Mott gaps are investigated. With the assumption that there are localized spins which can be described by a spin-only Hamiltonian in real space, the spin-coupling strengths of the bonds are analytically calculated by an exact mapping from an extended Hubbard model to a Heisenberg model for both the undoped and the doped cases. The result also provides some information on the form of the spin-only Hamiltonian for the description of the spin degrees of freedom of the system.

The seven pentagonal and the two icosahedral point groups that represent the symmetries of quasicrystals in two and three dimensions are expressed in terms of composition series. Utilizing these series, eleven magnetic variants induced by the nine generating point groups are derived, employing the concept of colour generators. Using a computed character to represent a magnetic property, the idea of factor groups contained in a composition series is explored to enumerate simultaneously the maximum number of non-vanishing and independent constants required to describe a chosen magnetic property for all the point groups involved in the composition series and such of the magnetic variants that may exist. The case of magnetoelectric polarizability is worked out in detail for two composition series. The piezomagnetic, pyromagnetic and magnetoelectric polarizability tensors for all twenty magnetic quasicrystal classes have been obtained and tabulated. The results of this study are briefly discussed.

Two groups of EPR hyperfine lines of a V⁴⁺ ion substituting for a Ti⁴⁺ ion corresponding to two kinds of sites which are Ti(1) and Ti(2) sites in the KTiOPO₄ single crystal, are observed at room temperature in the X band. The principal values and direction cosines of the principal axes of the g and A tensors are computed using a least-square fitting procedure for two kinds of sites, and the principal axis of the g tensor of all eight Ti ion sites substituted in a unit cell of KTiOPO₄, of which orientations are physically distinct, are revealed.

Proton line shapes were acquired for the easy-axis antiferromagnet CsFeCl₃.2H₂O. The line shapes were acquired at 250 K for a complete 360 degrees rotation about the b axis of the crystal, and at various temperatures between 250 K and 23 K, for two orientations. A model for the line shapes has been created based on the temperature dependence of the interaction between protons and the local magnetic field differences at each proton site. The field differences lead to symmetric line shapes whose peaks have a separation that increases as the temperature decreases. This model accounts well for the major component of the observed line shapes, provided that the proton-proton separation is increased over the separation obtained from published data. The residual portion of the observed line shapes appears to be a single line which results from water molecules that behave as though they are not part of the crystal structure.