Table of contents

The dynamic localization of electrons induced simultaneously by DC and AC electric fields in the classical limit is investigated. Scattering processes are taken into account. It is shown that the localization induced by one field can be suppressed by another one when n Omega = omega ( omega is the Stark frequency and Omega is the AC field frequency). The localization is also effectively destroyed by scattering processes.

The authors present a calculation of the Lyapunov exponent for transverse magnon modes in a Heisenberg spin-glass chain with positive and negative exchange interactions, both analytically as a microcanonical average and also numerically in the usual canonical statistical ensemble. In the positive- omega regime they find that the localization length obeys xi ( omega ) varies as omega ^-1 at omega to 0 if p>q, where p, q=1-p are the +1 and -1 spin concentrations, respectively. It is shown that the previously reported xi ( omega ) varies as omega ^-2/3 singularity, related to the anomalous dispersion law omega varies as k^3/2, holds only in the limit of equal concentrations p=q=0.5. The corresponding densities of states diverge as rho ( omega ) varies as omega ^-1/2 and varies as omega ^-1/3 for p>q and p=q, respectively.

The authors investigate the accuracy of the Hartree-Fock (HF) and Hartree (H) approximations for calculating energies in an N-electron quantum dot. The HF and H energies calculated from an analytically solvable model Hamiltonian are compared to exact energy solutions. The HF and H approximations become less accurate with increasing number of electrons, decreasing magnetic field, increasing dot size and increasing electron-electron interaction strength. The dependence on electron number is contrary to the well known infinite electron gas result.

The entire crystal structure of each member of the series of R_{1+ epsilon} Fe₄B₄ compounds is built up from the interpenetration of two incommensurable substructures: the R substructure and the Fe-B substructure. In their (uv0) electron diffraction patterns, satellite reflections due to the modulation occur in addition to the basic reflections corresponding to either the Fe-B substructure or the Nd substructure. To study the systematic extinction conditions for the observed X-ray and electron diffraction patterns, symmetry analyses have been performed by means of two approaches-by considering the three-dimensional space group of the commensurate supercells, and by considering the four-dimensional superspace group developed by de Wolff et al. (1981). The individual symmetry of the substructure has served as the basis for each approach in this paper. Examination of the symmetry under the commensurate approximation is the crucial step for the structure analysis of the Vernier structures. However, the occurrence and distribution of satellite reflections in the electron diffraction patterns cannot be interpreted within this framework. The superspace group characterization of these compounds provides a method of giving some insight into the general features of the conditions of the systematic extinction observed in the diffraction patterns of R_{1+ epsilon} Fe₄B₄ compounds.

The lattice and rotational dynamics of K_1-x(ND₄)_xI mixed crystals have been investigated by means of coherent inelastic neutron scattering as a function of the ammonium concentration and temperature. As the temperature is lowered, anomalies in the behaviour of transverse acoustic phonons propagating along the fourfold axis have been found. These are, firstly, a relaxational behaviour in the phonon energy and acoustic damping for x=0.42 and x=0.62 and, secondly, the existence of a dynamical central peak in the mixed and pure systems. For the three compounds investigated, the neutron data are interpreted in terms of coupling between dipolar orientational fluctuations and transverse acoustic phonons propagating along the fourfold axis. This rotation-translation coupling is efficient for wavevectors q*>or=0.5, but its strength is largest at the Brillouin zone boundary (the X point). Finally the overall dynamical response measured by neutron scattering suggests a collective slowing down of the ND₄⁺ dynamics on lowering the temperature.

Twelve convergent-beam electron diffraction patterns from an icosahedral phase specimen containing a dislocation along 12 different zone axes, which are equivalent according to icosahedral symmetry, have been photographed and simulated using dynamical theory. The splitting behaviours of higher-order Laue zone lines in these patterns have been analysed in detail. Some deviations between the experimental and theoretical patterns can be explained because the incident probe becomes too close to the dislocation core.

The H₂^- centre in NaF is studied by electron spin resonance (ESR). The ESR spectrum shows a triplet due to the hyperfine interaction of an unpaired electron with the two equivalent protons. Each line of the triplet is further split owing to the superhyperfine interactions with the neighbouring nuclei of the first two shells (as in the case of the F centre) and the centre may also be described as being a neutral H₂ molecule embedded in an F centre.

Several Pb⁺-type centres are produced in SrCl₂:PbCl₂ crystals by X-ray irradiation at 80 K and subsequent thermal and optical treatments. From the analysis of the electron-spin-resonance (ESR) and production properties, the dominant spectrum, with trigonal symmetry around (111), is attributed to a Pb⁺ ion located on a cation site with an anion vacancy in a nearest-neighbour position: the so-called Pb⁺ (1) centre with the laser-active-type structure. Specific thermal treatments lead to two more Pb⁺ centres with the same symmetry. The analysis of their hyperfine interaction parameters shows that in both centres the Pb⁺ ion is subject to a strong odd crystal field too, but their defect structures could not be further elucidated. An additional spectrum with a small orthorhombic distortion is observed in a more heavily doped crystal and is attributed to a lead dimer centre consisting of a Pb⁺ (1) defect perturbed by a nearest-neighbour substitutional Pb²⁺ impurity.

For the F-centre CN^- defect complex ('F_H(CN^-) centre') in CsBr, the polarization dependence of anti-Stokes Raman scattering is investigated. Comparison with calculated intensities shows that the results are inconsistent with the suspected C_4V point symmetry of this centre. Possible explanations are considered, including structural models with lower symmetry and reorientational effects of the CN^- molecular ion.

In their high-temperature cubic phase the alkali hydroxides NaOH and KOH and the corresponding deuterated compounds are described by an elastic dipole model for the reorienting OH^- groups. The theory describes thermo-diffuse scattering, Huang scattering and phonon softening. The inelastic data and the dispersions of the acoustic modes of KOD are shown and interpreted. Similarities to cooperative Jahn-Teller compounds and alkali cyanides are discussed.

1D lattice dynamics are considered for a pseudo-binary atomic chain in order to investigate the effects of thermodynamical clustering due to non-ideality of the solid solution A_xB_1-xC on the phonon density of states (DOS). Using an exact dimensional distribution function for impurity clusters, phonon spectra are calculated numerically for 1D models of (Cd,Hg)Te and (Al,Ga)As of different compositions and temperatures of alloy preparation. The phonon DOS is also obtained analytically near the limits of the frequency range, and in the vicinity of the local mode. It is shown that clustering strongly suppresses the local mode. Moreover, for the decaying alloy where impurity clusters can be quite long, the DOS looks like a superposition of the phonon DOSs of pure materials, and positions of the maxima of the alloy DOS with respect to those in pure materials are sensitive to the degree of clustering.

The study by kinetic mean-field techniques of a d-dimensional Ising system characterized by a sort of dynamical disorder reveals a rich phase diagram which exhibits a non-equilibrium tricritical point (only) for d>2, and re-entrance phenomena. The system time evolution is stochastic due to the simultaneous action of several independent spin-flip mechanisms, each corresponding to a different applied magnetic field. Such competition brings about randomness and a type of frustration that may occur also in real systems. In fact, this models the actual case of a magnetic system under a very rapidly fluctuating field, for example. Furthermore, the system may be interpreted as a non-equilibrium random-field system which, unlike the familiar quenched and annealed cases, contains a fast random diffusion of disorder.

The authors present a model relating the migration enthalpy H_v^m for nearest-neighbour vacancy jumps in cubic metals to the phonon dispersion. The migration enthalpy is split into two parts, one depending only on the lattice structure, the other on the vibrational properties of the particular metal. This latter term can be written in terms of the static lattice Green function, i.e. of the omega ^-2 moment of the spectrum. It can thus be calculated directly from measured phonon dispersion curves. For FCC metals, excellent agreement between calculated and measured values of H_v^m is found. For BCC metals, where H_v^m is known from experiments only in a few cases, predictions are made wherever the phonon dispersions are available. The model takes into account the unusually low-lying phonon branches in some of the BCC metals and yields, where phonon frequencies shift with temperature, temperature-dependent values of H_v^m.

The probability wave of the atomic configuration is used to study the ordered structures of ternary III-V semiconductor alloys. The atomic configuration of the most stable superlattices formed in the ternary III-V semiconductor alloys is discussed. Some features of the experiments are explained.

As a typical model exhibiting non-linear excitations, the continuum model for conjugated polymers is studied in the presence of a single impurity. Using the unperturbed Green functions the scattering matrix of the full problem is calculated and information about the mutual influence of non-linear and impurity intragap states is extracted. As the main difference the author finds that the localization length of the impurity state is determined mainly by the potential strength whereas for the non-linear state it is an intrinsic property.

The authors present magnetoconductivity results for a series of CaAl(Au) and CaAl(Ag) amorphous alloys. They have carried out a systematic study of the effect of varying the strength of the spin-orbit scattering in these alloys by introducing small concentrations of Au,Ag and Cu. The data are analysed using existing theories of quantum interference and electron interaction effects in disordered metals, from which estimates of the inelastic and spin-orbit scattering times have been obtained. The inelastic scattering is largely unaffected by the heavy-element doping while the spin-orbit scattering rate systematically increases and saturates at about 3% Au with a value of 1*10¹² s^-1.

The balance-equation theory recently developed for miniband Bloch transport in semiconductor superlattices is extended to systems with a strong lateral confinement. Carrier statistics, electron heating, and realistic scattering mechanisms due to impurities and phonons are included in the force- and energy-balance equations. Impurity- and phonon-limited linear mobilities are investigated as functions of the carrier occupation and temperature. The non-linear behaviour of the drift velocity against electric field for these one-dimensional superlattices, derived from the balance-equation theory, exhibits features different from that of a three-dimensional superlattice, but similar to the predictions of the Esaki-Tsu and Boltzmann theories.

A systematic theoretical study of Bloch electron transport in a superlattice miniband driven by a uniform external electric field parallel to the growth axis is carried out, based upon a recent extension of the balance-equation approach to arbitrary energy bands, with a dynamic force-balance equation for the centre of mass (CM), a variable-mass particle embodying the collective motion of the electrons, and a dynamic energy-balance equation. Analyses of both steady-state transport and the transient response to a step and an impulsive electric field are made for various superlattice systems, assuming tight-binding miniband structure and using parameters appropriate for Gamma -valley electrons in GaAs-based quantum-well superlattices at various lattice temperatures.

X-ray absorption near-edge structure of Cu K spectra of Cu₂O and CuO are analysed within the real-space multiple-scattering formalism. The crudest approximation (both in the cluster size and in the order of scattering) which can be made so that a particular spectral feature is still satisfactorily reproduced is found for each of the peaks. It was demonstrated that the peaks which correspond to one another both in the energy position and in the magnitude need not arise through the same scattering mechanism. Hence, caution must be exercised when the spectra of different compounds are compared. Multiple scattering involving non-collinear atoms can be important even in materials with inversion symmetry at approximately 20 eV above the edge.

The authors report a study of fluctuation effects in the resistance R, and the thermoelectric power S, of melt-processed BiSrCaCuO (2212) and sintered BiPbSrCaCuO (2223) polycrystalline materials. Applying several theoretical models to the analysis of experimental results, they extract a number of physical parameters for these systems: the Fermi energy, interlayer coupling strength and atomic Debye temperatures for Bi and Pb. The results are in good agreement with data obtained in other studies of similar materials.

The authors present a discussion of the dynamical correlation functions for Heisenberg magnets with a weak easy-plane anisotropy for temperatures just above the Kosterlitz-Thouless phase transition. Compared to the XY-model they find here vortices which have in addition to the well known in-plane structure a localized out-of-plane profile around their centre. For the ferromagnetic case this structure produces an effective magnetic field which acts as a gyro force on the other vortices. Assuming a dilute gas of weakly interacting vortices, they find analytic expressions for the vortex contribution to the dynamical correlation functions. These results are compared (i) with numerical Monte Carlo-molecular dynamics simulations on a 100*100 square lattice allowing them to determine values for the vortex correlation length xi and average vortex velocity u and (ii) with experiments performed on different quasi-two-dimensional magnetic materials.

The authors report a series of measurements on both polycrystalline and single-crystal samples of NpCo₂ in an effort to understand the low-temperature magnetic properties. Magnetization and Mossbauer experiments strongly suggest a transition to an ordered state. A variety of neutron experiments, including small-angle scattering and searches along the principal crystallographic directions with a single crystal, have failed to reveal any direct evidence for long-range ordering of the Np moments. These measurements do not exclude short-range order or a spin-glass state, although the latter appears to be excluded by the magnetization experiments. An account is given of the polarized-neutron experiments in an applied field that give direct evidence of hybridization between Np 5f and Co 3d electrons.

A general description of the thermo-induced dynamics of the interphase boundaries in ferroelectrics is proposed. The influence of applied magnetic fields on the velocity and width of the interphase boundaries is considered.

It is argued that the optical conductivity of face-centred icosahedral (FCI) quasi-crystals is dominated by band-structure effects. What makes FCI quasi-crystals different from conventional metallic alloys is the high multiplicity (namely, 42) of their reciprocal lattice vectors, which allows a better match between the Fermi sphere and the corresponding set of Bragg planes (the 'quasi-Brillouin zone'). This effect is also responsible for a strong reduction in the DC conductivity through a substantial optical mass increase, rather than anomalously strong scattering localization. The low DC conductivity, in turn, means that the weak Drude part of the AC conductivity is overwhelmed by stronger interband absorption resulting in a suppression of the Drude peak.

Bounds are given for the possible values of the following quantities: the reflection amplitudes r_ss, r_pp, r_sp, and r_ps; the transmission amplitudes t_so, t_se, t_po, t_pe; the Brewster angle given by the zero of r_pp; and the angle between the extraordinary ray and the extraordinary wavevector. These bounds are for arbitrary orientation of the optic axis relative to the reflecting surface and for any angle of incidence. Index matching enhances the effects of anisotropy, particularly in the reflection properties. A formula is given for the Brewster angle in the case where the optic axis lies in the plane of incidence. The conditions under which the four transmission amplitudes can be zero are also discussed.

Infrared spectra of ammonium dysprosium sulphate tetrahydrate (NH₄Dy(SO₄).4H₂O) were investigated at 95<T<315 K with a view to understanding the dynamics of NH₄⁺ ions in the lattice. The observed vibrational behaviours, i.e. splitting of the vibrational modes, shifts in frequencies, and linewidths of the ammonium vibrations in the lattice, were tested against two vibrational exchange dephasing models. The authors results suggest that at 95<T<200 K the behaviour of the ammonium modes is consistent with the Harris-Shelby-Cornelius model, where the high-frequency modes dephase independently via specific low-frequency modes that are undergoing exchange with the thermal bath. The dephasing channels were identified with the librations of the ammonium ions. At 200<T<275 K, vibrational dephasing channels are more complex and can no longer simply be equated with ammonium librational modes, thus suggesting additional motion of the ammonium ions in the lattice.

A microscopic model for evaluating the Huang-Rhys factor S(a_1g) associated with the symmetric mode of a transition-metal impurity M in insulators is proposed. It is applied to the first excited states of Cr³⁺, V²⁺, Mn²⁺ and Cu²⁺. Theoretical calculated values of the exponent n indicate that it varies slightly along the ligand series F^- to Cl^- to Br^- giving rise to an increase in S(a_1g). This explains why the Stokes shift of systems involving MBr_N units can be similar or higher than the corresponding shifts of MF_N complexes.