Showing papers in "Advances in Physics in 1986"

Aharonov-Bohm effect in normal metal quantum coherence and transport

Abstract: We review some of the recent surprising theoretical and experimental results obtained on the transport properties of small disordered metal samples. Even in the presence of disorder, the quantum mechanical interference of electron wavefunctions can still be observed. The Aharonov-Bohm effect is a particularly clear demonstration of this. In doubly connected structures (such as loops of wire) threaded by a magnetic flux, the electrical conductance oscillates because of the Aharonov-Bohm effect. In fact, because the electron trajectories are diffusive (i.e. random walks), even a lone wire (a singly connected structure) will exhibit a random pattern of conductance fluctuations as a function of the magnetic field because of the same interference effects. All that is required for the observation of these interferences is that the electrons retain ‘phase memory’ duing the period of transit through the sample. The length over which memory is maintained (the phase coherence length) can be much larger tha...

Computational statistical mechanics methodology, applications and supercomputing

Abstract: Computer simulation is adding a new dimension to scientific investigation, establishing a role of equal importance with the traditional approaches of experiment and theory. In this paper, we provide a text for understanding the computer simulation methodology of classical statistical mechanics. After developing the theoretical basis of the simulation techniques, the Monte Carlo and Langevin methods and various molecular dynamics methods are described. A very limited discussion is provided on interatomic potential functions, numerical integration schemes, and general simulation procedures for modelling different physical situations and for circumventing excessive computational burdens. The simulation methods are then illustrated using a variety of physical problems studied over the last several years at our laboratory. They include spinodal decomposition of a two-dimensional (2D) fluid, the melting of 2D and quasi-2D films, the structure and energetics of an incommensurate physisorbed film, and th...

Electronic structure of cerium and light rare-earth intermetallics

Abstract: We give an overview of the use of the impurity Anderson Hamiltonian to describe the spectroscopic and low-energy thermodynamic properties of cerium intermetallics, with emphasis on interpreting 4f photoemission spectra. We show Ce valence-band resonant photoemission, Bremsstrahlung isochromat and 3d X-ray photoemission spectra for CeRu2, CeNi2, CeIr2 and CeAl, and give a complete theoretical analysis of the spectra. We summarize the relation between the large and small energy scale properties. For each system, all the spectra, as well as the static magnetic susceptibility and Kondo temperature, can be described by the model using essentially the same parameters. We also present details of resonant photoemission spectra for five other cerium compounds, CeSi2, CeOs2, CePd3, CeCo2 and CeNi5, and discuss generally the problem of obtaining the experimental 4f spectrum. Alternative theories of 4f photoemission are examined critically and we give applications of the Anderson Hamiltonian theory to CeAl2,...

The structure and energetics of defects in liquid crystals

Abstract: The study of defects in ordered media has become a subject of considerable interest to condensed matter physicists in recent years. This article presents a systematic account of the structures, energies and interactions of defects in the nematic, smectic, cholesteric and discotic phases of liquid crystals. Relevant experimental observations are also described.

Properties of II–VI semiconductors associated with moving dislocations

Abstract: Moving dislocations in II–VI semiconductors carry a large electric charge. This charge is not in thermal equilibrium, but is due to the sweeping up of electrons from point defects. Its movement produces a dislocation current during plastic deformation, and conversely, the application of an external field changes the flow stress. This paper reviews the structure and properties of these dislocations, the theory of their charge and the phenomena which are a consequence of the strong mutual interactions of the dislocation and electronic sub-systems in these crystals. The materials show a large photoplastic effect (a change in flow stress under illumination), and related effects due to the injection of electrons at an electrode. Deformation produces reversible changes in the conductivity, pulsed and continuous luminescence and the emission of electrons from the surface.

The thermal oxidation of silicon the special case of the growth of very thin films

Abstract: The thermal oxidation of silicon is generally modelled by Deal and Grove's theory based on the assumption that the oxygen molecules dissolve in silicon in interstitial positions and migrate to the Si-SiO2 interface where they react with the silicon substrate. Experimental results for oxidation in dry oxygen agree with this theory only for thick oxide films. The growth of very thin oxide films exhibits particular features which are discussed in this paper. For these films, the growth mechanism is different from that of thick films; this difference is possibly associated with the transport of oxygen atoms through the silica network. The effect of hydrogenated impurities is also discussed.

Muon-spin rotation as a tool for investigating the diffusion of positive muons in type-II superconductors

Abstract: The measurement of the spin precession frequencies of immobile positive muons in the mixed state of uniformly magnetized type-II superconductors gives the distribution function of the magnetic field. Increasing muon diffusivity leads first to the suppression of the van Hove singularity associated with the field-strength maximum in the flux-line centres, then to an enhancement of the damping of the main precession frequency, and finally to a shift and motional narrowing of the precession frequency distribution. These phenomena can be used to determine muon diffusivities in Nb exceeding about 10−12m2s−1. The required information on the magnetic-field distribution function in the mixed state is presented in detail.

Theory of electromagnetic generation of acoustic waves in metals

Abstract: The theory of the anomalous skin effect in metals is used in order to study the direct generation of ultrasound by electromagnetic radiation incident on a metal surface, in the absence and in the presence of a magnetic field. The non-monotonic behaviour of the acoustic flux observed experimentally, as a function of the magnetic field can be explained within the framework of the free-electron model assuming that the electrons are scattered diffusely from the metal surface. The reason for this behaviour is traced to the variation of the relative phases of the collision drag force on the bulk ions and that on the surface with increasing magnetic field. The relations between existing theories of direct generation of ultrasound have been investigated within both the free-electron approximation and the effective-mass theory. We conclude that, besides the collision drag force, the Lorentz force and the Bragg reaction force, an additional ‘deformation’ force acts upon the positive ions. We find that the ...