Table of contents

The authors discuss guided phonon propagation in extremely narrow wires with position modulated radius and free boundaries. The modulated radius represents a randomly rough surface associated with the fabrication of small wires. A coordinate transform technique is used to introduce the surface roughness into the vector differential equation for an elastically isotropic medium. By solving the secular equation numerically the authors have been able to determine the dependence of the phonon mean free path on wave vector, roughness amplitude and correlation length.

Macroscopic and microscopic effects of anharmonic lattice vibrations in Pd have been studied. The interatomic forces are modelled according to a phenomenological potential which includes three-body angular forces in the harmonic part. The cubic and quartic force constants are fitted to the experimental linear thermal expansion coefficient and constant-pressure specific heat, respectively. Thermoelastic properties are evaluated in quasi-harmonic approximation while caloric properties require computation of constant-volume anharmonic contributions. The phonon lineshifts and linewidths have been calculated along the principal symmetry directions, in the framework of the second-order perturbation theory. The room-temperature experimental anomalies in the ( xi xi 0)T₁ branch are reproduced by the theory.

Several samples of a-SiO₂/c-Si(100) with different oxide layer thicknesses and different silicon surface qualities were examined by X-ray diffraction. The layer thickness as well as the surface roughness and the interface roughness were studied by measuring the reflectivity of the samples near total external reflection. A separate and detailed determination of the interface roughness is obtained by analysing the intensity near the Si(400) reciprocal lattice point. There is a pronounced diffuse scattering parallel to the normal of the interface, the frequently called crystal truncation rods. Both the reflectivity and the isointensity contours around the (400) reflection demonstrate the presence of an intermediate layer between a-SiO₂ and c-Si in a crystalline state.

The collective excitations of a double heterostructure in the presence of a magnetic field are examined in detail. The polarization function of the quasi-two-dimensional electron gas in the presence of a perpendicular quantizing magnetic field is calculated within various long-wave approximations and in the random-phase approximation. Using these polarization functions the dispersion curves of the coupled intra- and inter-subband magnetoplasmon-phonons are calculated and plotted in graphical form. The calculation of the dispersion relation is done for GaAs-Ga_1-xAl_xAs double heterostructures and include both quantum size effects and the correct spectrum of the long-wave optical phonons. In this paper the higher resonances of the quasi-two-dimensional magnetoplasma, the Bernstein modes, are also investigated. It is shown that the intra- and inter-subband magnetoplasmon-phonons are free of Landau damping for finite perpendicular quantizing magnetic fields.

The authors construct a diagram technique for the (t-J) model Hamiltonian expressed in terms of Hubbard operators. This technique combines features of the diagram technique for normal Fermi systems and those for the Heisenberg model with spin operators. The general graphic structure is identified for one-particle Green functions composed of Hubbard operators. The (t-J) model has logical consistency, is reasonably simple and can serve as a working tool in exploring various properties of this model. To an approximation that resides in summing ladder-type diagrams with antiparallel electron lines, the authors calculate the system's magnetic susceptibility. A formula is derived which reflects the dual nature of the magnetic states in the Hubbard model; this duality manifests itself in the presence of a localised and an itinerant contribution in the bare susceptibility. The authors trace how the relative value of the localised contribution varies as the electron concentration is increased from 0 to 1. With n>2/3, the paramagnetic phase of the system becomes unstable with respect to the occurrence of ferromagnetic or antiferromagnetic ordering. A magnetic phase diagram for zero temperature is constructed on the (t/U, n) plane.

The authors investigate the properties of the two-dimensional quantum S=1/2 Heisenberg antiferromagnetic model on a square lattice by means of a block approach. The lattice is divided into four spin square blocks and the Hamiltonian is expressed in terms of low-energy block states. In the framework of the mean-field theory based on the 'auxiliary-boson' formalism; they show that the spin excitation spectrum contains one singlet mode with the temperature-dependent gap and three degenerate triplet modes. The asymptotic low-temperature spin-spin correlations are proportional to R^-3/2 exp(-R(1/ xi ₁+1/ xi ₂)), where xi ₁=exp(A/T), xi ₂=constant and A approximately=0.5J. The uniform susceptibility tends to zero in agreement with numerical results; the energy per site is close to -0.57J. Beyond the meanfield theory the authors find gapless collective excitations analogous to the gapless mode in the neutral Fermi gas with attraction.

Body-centred tetragonal (BCT) antiferromagnets are a central topic of both theoretical and experimental research. The Heisenberg BCT antiferromagnet shows full frustration induced by the lattice structure for all physically meaningful values of the exchange parameters. A previous analysis of the minimum-energy configuration indicated full frustration only if no coupling between next-nearest-neighbour planes is present. However, the analysis was restricted to a classical helix configuration whereas infinite non-helical configurations characterized by an arbitrary angle between the spin at the centre and the spins at the corner minimize the classical energy of the model in a substantial part of the parameter space. In any case the relevance of quantum fluctuations in solving the infinite degeneracy is confirmed. The first quantum correction to the magnon energy spectrum is shown to lift the degeneracy of the soft lines which are present in the classical approximation.

The classical calculation of the Bloch wall mobility, based on the Landau-Lifschitz-Gilbert equation, leads to a result which is 1000 times larger than the experimental value found for very pure yttrium iron garnet. The authors criticize this approach and propose a novel mechanism. It is argued that the non-zero transverse magnetization of the wall induces a magnetic back-flow around the wall, and hence a drag force proportional to the velocity. A correct order of magnitude of the mobility is obtained if one assumes that the intrinsic relaxation times of the medium are very different for high frequencies (where resonance experiments are performed) and low frequencies, relevant to the wall motion.

The transmission coefficient for tunnelling through double barriers and superlattices of GaAs-Ga_1-xAl_xAs, under transverse-magnetic-field action, has been calculated using a transfer matrix model. In this work, the one-dimensional effective-mass equation has been first solved analytically by means of the confluent hypergeometric functions as envelope functions

A model potential for some quasi-one-dimensional organic materials is introduced. In this model, electrons are nearly free in the longitudinal direction, but in the transverse direction there are high rectangular periodic barriers which prevent the movement of electrons between adjacent slender fibres. By application of the random phase approximation (RPA), the authors show that non-damping acoustic plasmons (NDAP), similar to 'slender' acoustic plasmons (SAP) proposed by Y.C. Lee and co-workers (1983), appear in the system. As an example, they have carried out the calculation on (TMTSF)₂PF₆, and are assured of the appearance of NDAP for its electrons under certain conditions. Finally, the authors show that NDAP play a principal role in the superconductivity of quasi-1D organic materials, and the authors discuss the conditions that favour the appearance of NDAP.

A sharp step-like structure, which is seen in the current-voltage characteristic of a laterally confined AlGaAs/GaAs resonant tunnelling structure, is a novel manifestation of a quantum effect arising when one-dimensional wires feed electrons into a zero-dimensional quantum box. The authors can analyse the data to deduce a tunnelling time, an inelastic scattering time and the transmission probability for electrons in this system. The height of the current steps, Delta I, gives the electron tunnelling time, tau_e, via Delta I=e/2 tau_e, while the differential conductance gives the peak transmission probability, T₀, via G approximately (e²/h)T₀. The authors must invoke inelastic scattering to explain the low value of T₀ approximately 9% obtained from the data, and so they find that tunnelling in their structure is sequential.

The frequencies and the absorption cross sections of infrared active phonons show anomalies related to the transition between the normal and superconducting phase in YBa₂Cu₃O₇. The excess intensity of the absorption line and the frequency shift of the mode near 585 cm^-1 are proportional to each other. Correlating these parameters with the thermodynamic order parameter using hard model spectroscopy leads to an order parameter of the superconducting phase which follows formally mean field theory of the BCS or Landau-Ginzburg type.

The sudden reduction of the photoluminescence linewidth observed at low temperature in coupled quantum wells is analysed in view of possible 2D phase transitions occurring at different density limits. The linewidth calculated within a BCS-like transition is found to be proportional to the gap below T_c. This can be regarded as a 'fingerprint' for identifying the nature of the condensed phase.