Table of contents

The authors use Monte Carlo methods to investigate a purely dynamical model for structural glasses. They observe stretched exponential decays of the equilibrium autocorrelation function and measure the late-time relaxation times tau . These diverge with temperature following a Vogel-Fulcher law. They also study systems which are quenched deeply and then re-heated. This gives a peak in the effective specific heat with properties matching those of the glass transition. The model thus reproduces the main phenomenology of glasses and the glass transition.

A k.p band-structure formalism is presented to describe photonic dispersion relations in periodic dielectric structures, within the scalar wave approximation. A reciprocal effective dielectric tensor is defined and an expression analogous to the f-sum rule for semiconductors is derived. Application of the present formalism is discussed using a two-band model. The generalization to vector waves is outlined.

Phase transitions in TiD_{approximately 0.75} subjected to high-pressure treatment were investigated by simultaneous real-time measurements of neutron diffraction and small-angle neutron scattering. The neutron spectra were taken on heating the samples in the temperature ranges 100 to 300 K and 300 to 760 K followed by cooling to 430 K. A sequence of structural transitions was observed, which involves seven different phases and intermediate states with the hexagonal close-packed, face-centred cubic, face-centered orthorhombic or body-centred cubic metal sublattices and hydrogen atoms distributed on octahedral or ordered/disordered tetrahedral interstices.

The heat flux between two solids is calculated, taking into account the impurity monolayer separating the crystals. It is found that the weak impurity-crystal interaction results in a resonance-type frequency dependence of the phonon energy transmission coefficient. The calculation enabled a description of the low-temperature heat transfer anomalies recently measured in point contacts to be given.

For the scalar phi ⁴-lattice model of structural phase transitions the freezing transition at a temperature T_c* above T_c is found in the framework of mode-coupling theory. To study the critical behaviour of this transition the frequency and temperature dependencies of the linear and quadratic dynamical susceptibility are investigated by using a self-consistent numerical solution of the mode-coupling equations. Two cases are considered: (i) a pure crystalline system with a B transition at T_c*, and (ii) a system with randomly distributed defects with an A transition at T_c*. The experimental aspects of the presented results are discussed.

Self-consistent LAPW band structure calculations were performed for the martensitic phases B19'-NiTi and B19-PdTi. The resulting densities of states (DOS), partial charges, partial densities of states, electron densities and Fermi surfaces were compared to the results of a former calculation for B2-NiTi and B2-PdTi. The DOS at the Fermi level is lower for the martensitic than for the austenitic phases. The most significant changes at the phase transition were found for the Fermi surfaces and derived properties. Nesting between electron and hole sheets of the Fermi surface is greatly reduced in the martensites. The generalized susceptibility and the function zeta (q) approximating the electron-phonon contribution to the dynamical matrix were also calculated for three symmetry directions. Maxima of zeta (q) show up only in the (111) direction. Furthermore, calculated XPS spectra and optical conductivities were compared to experiment, if available.

The electronic and magnetic behaviour of transition metal impurities is simulated by performing ab initio band structure calculations assuming ordered supercells with the composition Pd₃₁TM (TM=Cr, Mn, Fe, Co, Ni). The results show the formation of narrow spin-split impurity bands originating from the TM with magnetic moments M_TM deviating from the Slater-Pauling curve. Due to the high susceptibility of the Pd host the TM atom polarizes the surrounding Pd atoms (with M_Pd) causing the 'giant moments' when all the magnetic moment is attributed to the TM impurity. This polarization is described by a covalent polarization (CP) model, which the authors use for explaining both the change in sign of the Pd polarization in the TM series between Cr and Mn, and the linear dependence of the ratio M_Pd/M_TM on the number of valence electrons of the TM. In addition, they estimate the Curie temperature by treating the localized TM moments in terms of a Weiss mean-field model and the itinerant electrons of the Pd host having spin fluctuations.

The electrical conductivity of (Zn_1-xMn_x)₃As₂ solid solutions with 0<or=x<or=0.13 has been measured in the temperature interval of 4.3<T<200 K. It is shown that the conductivity is of activational nature. The data are analysed with a model based on the disorder of Mn atoms over the vacancy-type sites of the metallic sublattice. At low temperatures the conductivity is determined by hopping of holes over the states in the lower Hubbard band whereas at high temperatures the holes are activated to states higher than the mobility edge of the upper Hubbard band. The observed complex dependence of the conductivity and the high-temperature activation energy on x are explained in terms of an increasing contribution of multiple spin-flip scattering of holes during their transitions between acceptor centres when x is increased.

The static and dynamic magnetic responses of HoPO₄ and ErPO₄ have been studied by means of neutron spectroscopy and magnetic susceptibility measurements. The inelastic neutron scattering spectra exhibit well-defined transitions characteristic of crystal-field excitations of the rare-earth ions. A comparison of the neutron data with optical absorption and both non-resonance and resonance Raman scattering measurements has been made. The derived crystal-field-level structure provides a basis for explaining the low-temperature magnetic properties of both compounds. The calculated and measured paramagnetic susceptibilities agree well for HoPO₄ and ErPO₄ in the temperature range of 5-300 K. The highly anisotropic, saturated magnetization of the ground doublet in HoPO₄ may act as a 'bootstrap' for a long-range magnetic ordering of the moments parallel to the c-axis at low temperatures. The nearly spherically symmetric moments of the low-lying Kramers doublets of ErPO₄, however, tend to couple less effectively via exchange interactions.

Using real-time numerical propagation techniques, the authors consider the tunneling of electron wavepackets through potential barriers that possess internal dynamics. The models considered allow the study of the influence of localized surface vibrational modes on the tunnel current observed in scanning tunnelling microscopy. Non-resonant and resonant processes are considered. In the former case, the authors focus on the behaviour of the total tunnel current when the energy sweeps through an inelastic threshold. In the resonant case, they make contact with analytic theory and consider in detail, using a two-channel Breit-Wigner model, the physics underlying the resonant process, the physical origin of sum-rules obeyed by the tunnelling characteristic, and the conditions under which simple analytic theoretical results can be applied in general.

Inelastic effects on resonant tunnelling are considered for a model consisting of a double- delta -function potential with the region in between oscillating with amplitude V₁ and frequency omega . In the limit of small V₁/ omega , the authors obtain analytic expressions for the transmission coefficient for the elastic and the two nearest inelastic channels. Numerically, they obtain a set of resonant sidebands around the static resonances with magnitudes proportional to (V₁/ omega )² for both symmetric and asymmetric delta functions. The authors discuss the conditions under which the sidebands of contiguous static resonances can superpose, producing an increase in the probabilities of both elastic and inelastic events. They also describe the feedback mechanism between the scattering of elastic and inelastic events, which modifies the probabilities of elastic events.

The magnetization distribution in the ferromagnetic phase of CeFe₂ at 10 K has been determined using a combination of polarized and unpolarized neutron diffraction measurements on a single crystal. The results confirm previous results on powders, and the prediction of band structure calculations, that both the Fe and Ce atoms carry a magnetic moment, and that Ce is magnetized oppositely to Fe. The value of the Fe moment deduced from the measurements is mu _Fe=1.174(10) mu _B, somewhat smaller than that given by the theory. The value obtained for the Ce moment is strongly dependent on the assumption made about the relative numbers of unpaired 5d and 4f electrons. If the ratio of 5d/4f magnetization is taken as the value obtained in the band structure calculations (i.e. 0.75) the total Ce moment is -0.14(3) mu _B. The magnetization distribution around the Fe atoms deviates significantly from spherical symmetry; the form of the asymmetry indicates that the electrons occupy orbitals which are a coherent mixture of E_g-like and T_2g-like functions.

For pt. I see ibid. vol.5, p.1 (1993). The thermal dependences of Mossbauer spectra of various biferrocenium derivative salts is obtained using the cooperative spin-1 model presented previously. Both second- and first-order delocalized-localized transitions are reproduced accurately. An apparent thermal equilibrium between localized and delocalized states found in several cases is discussed within the framework of the model.

The influence of hydrostatic pressure on the temperature dependence of the optical birefringence and ultrasonic velocities is studied in the vicinity of the phase transition temperatures of Cs₂HgBr₄, Cs₂CdBr₄ and Cs₂HgCl₄ crystals. The new polycritical triple points which separate the normal, incommensurate and proper ferroelastic phases have been found in the P-T phase diagrams of all the compounds at applied pressures of 140 MPa, 100 MPa and 140 MPa, respectively. The origin of the triple points and the acoustic and optical properties near the phase transitions are discussed within the framework of phenomenological theory.

The luminescence properties of trivalent europium embedded in an isostructural SrRE₂O₄ (RE identical to Eu, Gd, Y, In) series have been studied. The energy level schemes as well as the crystal-field analysis of the two non-equivalent point sites present in the structure show clearly the smooth modifications of the europium local probe environment for only one point site versus the ionic radius of the cation, the environment of the other site being quite unmodified. The paramagnetic susceptibility of the europium compound is well reproduced by using the wavefunctions derived from the crystal-field calculation.

Using a time-of-flight technique the velocity of positronium (Ps) emitted from Ag(100) has been studied. The maximum velocity of Ps, arising from implanted positrons which have thermalized and diffused back to the emitting surface, has been determined yielding a formation potential for Ps of epsilon _Ps=-1.5+or-0.2 eV.

An exact solution of the scattering problem in 2D square-corner discontinuities is presented in an explicit form. This suggests an efficient computational procedure for characteristics of electron ballistic transport in square-corner shaped waveguides. Using the proposed method, bound-state energies, mode-to-mode as well as total and averaged transmission probabilities, in a square-corner right-angle bend have been calculated. Conductance and electron-filtering properties of the system are found to be challenging problems for its practical realization.