Showing papers in "Reports on Progress in Physics in 1982"

Commensurate phases, incommensurate phases and the devil's staircase

Abstract: Surveys recent theories on the transition between commensurate (C) and incommensurate (I) phases, and on properties of the I phase. The devil's staircase concept for the I phase is described. Differences between theories in two and three dimensions are discussed, together with those on chaotic structures.

Hydrides formed from intermetallic compounds of two transition metals: a special class of ternary alloys

Abstract: The authors review available experimental information on the existence, thermodynamic stability and physical properties of hydrides formed by the absorption of hydrogen gas in intermetallic compounds of two transition metals. The emphasis is on stability. It is shown that empirical models for the stability of ternary hydrides can be reconciled with ideas based on the results of band structure calculations of (binary) metallic hydrides. It is concluded that metallic hydrides can be looked upon as alloys of metallic hydrogen. In addition to the thermodynamic properties of ternary metallic hydrides the authors discuss experimental information on electronic properties (magnetic, super-conductivity, band structure features) and on crystallographic and metallurgical properties (neutron scattering, nuclear and electron spin resonance, Mossbauer spectroscopy, diffusion). Applications are briefly reviewed.

Defects in silicon

Abstract: The method of obtaining pure polycrystalline silicon is described, followed by short accounts of how this material is converted into single-crystal form either by the Czochralski (CZ) pulling method or the float-zone (FZ) method. It is shown that the silicon contains various impurities including oxygen, carbon, boron and possibly hydrogen. The defects and impurities often show a nonhomogeneous distribution in the form of helical swirls. Heat treatment of silicon-containing oxygen leads to the clustering of this impurity. At 450 degrees C there is formation of small complexes that act as shallow donors. Investigations using IR and ESR spectroscopy have so far failed to determine the atomic configuration of the defects. Heating at higher temperatures causes wide-scale precipitation of oxygen. There are interactions with carbon and there can be formation of silicon carbide precipitates. Contamination from Cu, Au, Fe, etc., can occur during these treatments and methods for gettering these metals are discussed, involving dislocations and silica precipitates. Low-temperature irradiations produce vacancies and self-interstitials which combine with impurities to form complexes on heating from 4K to 300K. Evidence is presented to illustrate the possible charge states of self-interstitials. Damage produced by fast neutrons is discussed next, followed by a brief account of neutron transmutation doping whereby neutrally occurring 30Si is converted to 31P by the capture of thermal neutrons. Some aspects of high-temperature diffusion are discussed and attempts are made to correlate the data with that derived from the irradiation studies.

Optical bistability and related devices

Abstract: Gives a broad description of optical bistability: from practical applications, as an optical transistor or optical memory element, to its phase transition interpretation. The theory is divided into three parts. The first is a simple discussion that covers most of the basic experimental effects and concepts of practical importance. The second part applies to atomic system where a semiclassical as well as a quantum-mechanical approach is possible. The third one discusses the mechanisms compatible with large nonlinearities observed in InSb and GaAs as semiconductors constitute the most promising materials for applications. Current experimental progress in all-optical and hybrid devices is also discussed and scaling example is presented to indicate the possibilities in high-speed all-optical signal processing.

X-ray-excited Auger and photoelectron spectroscopy

Abstract: This article reviews developments in the understanding of X-ray-excited Auger and photoelectron spectra in the light of theoretical developments in atom, molecular and solid-state physics. After reviewing progress in XPS and AES separately emphasis is placed on the inter-relationship between the two fields: Auger rates, for example, are the dominant contribution to core-level XPS linewidths and by combining XPS and AES it is possible to deduce information about Coster-Kronig processes which are difficult to study directly. An account is given of how the combination of measurements of environmentally dependent shifts in XPS and AES energies allows one to isolate initial- and final-state contributions which can then be related to the results of other experimental techniques. There is a brief discussion of many-electron effects which, in some instances, dominate both XPS and AES spectra. The author discusses how the combination of XPS and AES spectra involving valence levels enables one to study the effects of hole-state localisation.

Brillouin scattering in condensed matter

Abstract: Surveys the developments in Brillouin scattering during the past fifteen years. Mainly classical effects are described, therefore excluding stimulated and resonance Brillouin scattering. The study is limited so that the frequency interval of the acoustic excitations lies in the hypersonic regime.

Surface electronic structure

Abstract: The theory of the electronic structure of clean metal and semiconductor surfaces is reviewed, starting from an effective one-electron Schrodinger equation. Methods for solving the Schrodinger equation at surfaces are briefly described, and the effects of the surface on the electronic wavefunctions are discussed using simple models. The results of detailed calculations of the surface electronic structure of s-p bonded metals, transition metals and semiconductors are reviewed, with an emphasis on the effect of the local environment on the density of states. Properties like the work function and surface energy depend on the surface electronic structure, and their variation with material and surface is discussed; the surface energy contains an important contribution from the interaction between electrons, and this will be considered in some detail. The change in electronic structure compared with the bulk leads to changes in atomic structure, with surface reconstruction on semiconductor and some metal surfaces, and this is also discussed. The interplay between theory and experiment is very important in surface studies, and the theoretical surface energy bands reviewed compare well with experimental photoemission results; this comparison has proved particularly useful for understanding surface reconstructions.

The fourth test of general relativity

Abstract: In the 'fourth test of general relativity' the gravitational acceleration of celestial bodies-the Earth and the Moon-were experimentally compared in the gravitational field of the Sun. Because such bodies obtain an appreciable fraction of their total mass-energy from their internal gravitational self-energy (5*10-10 for the Earth), this comparison of free-fall rates measures, among other things, how gravity pulls on gravitational energy and how gravitational energy contributes to the inertial mass of celestial bodies. Using high-precision laser ranging between Earth and reflectors on the Moon's surface, it was found that the Earth and Moon's acceleration in the Sun's gravitational field are the same to one part in 1011. Hypothesising that the gravitational to inertial mass ratio of a celestial body may differ from one by the order of the gravitational self-energy content of the body divided by the total mass-energy: MG/MI=1+ eta (UG/Mc2) eta being a dimensionless constant determined by gravitational theory, the lunar laser ranging experiment limits mod eta mod to less than 1.4*10-2. This experiment is consistent with general relativity which predicts eta =0, but scalar-metric tensor theories, such as the Brans-Dicke theory, vector-metric tensor theories and two-tensor theories of gravity are inconsistent with this experiment unless sufficient adjustable parameters are used.

Self-trapped states of positrons and positronium in dense gases in liquids

Abstract: In dense media positrons and positronium atoms are bound in self-trapped states. These states are density fluctuations stabilised by the light quantum particles. The positronium atoms are trapped in a bubble, while positrons are trapped in a cluster. Transitions to these states at changes of the density or temperature of the medium produce an essential effect on the annihilation rate. It can be considered as a local phase transition in the vicinity of the positron (or positronium). The present article is a review of the present state of the physics of positron and positronium self-trapped states in dense gases and liquids. Basic properties of the positronium bubbles and positron clusters are considered, as well as the conditions under which they exist and their effect on the annihilation rate. Experiments in which the self-trapping is considered to be closely related to other similar electronic phenomena, known for dense gases, liquids and other dense media. The main results from the study of positron and positronium interactions with isolated atoms are also presented. The density effects in moderately dense gases anticipating self-trapping are considered.

Catastrophe theory in physics

Abstract: Presents a broadly based discussion of 'catastrophe theory,' a mathematical discipline commonly associated with the names of Thom and Zeeman, placing emphasis on the development feedback between the mathematics and its applications, especially to the physical sciences. The author aims to present a typical selection of current work. Among the more prominent of the concepts that have emerged from this work are co-dimension, determinacy, unfoldings and organising centres. He shows, using specific applications as motivation, how these concepts may be used, and generalised to areas not obviously within the formal purview of 'catastrophe theory' as it is often presented. Structural stability, a concept from topological dynamics that provides a philosophical background for catastrophe theory, is also discussed. The mathematics of the subject has advanced considerably over the past decade, and in doing so has lost many of its original limitations. Some of these new directions are exhibited. On occasion, the catastrophe-theoretic methods are compared to more traditional ones.

The Drell-Yan process

Abstract: The Drell-Yan process is an electromagnetic effect in which a quark and antiquark from a pair of interacting hadrons annihilate to give a lepton pair. A brief description of hadron structure and the parton model is given to provide a necessary background. Then the general features of lepton pair production are described and the Drell-Yan formalism is set up. Experimental techniques are described next; these are rather simple since it is only necessary to detect the leptons emitted from hadron interactions. Predictions and tests of the basic model show that, in general, it works well. The anomalous features are the overall cross-section level (high by a factor of 2) and the unexpectedly large mean transverse momenta. Quantum chromodynamics (QCD) offers the explanation of these anomalies, and measurements of dilepton production provide important quantitative tests of QCD. Such measurements have also been used to determine nucleon structure functions and, for the first time, meson structure functions.

Low-energy electron diffraction for surface structure analysis

Abstract: Low-energy electron diffraction (LEED) is the most important technique for studying the atomic structures of crystalline surfaces This review aims to give an introduction into the experimental, theoretical and analytical procedures needed for a successful structure determination After a brief introduction that gives some historical background and sets the problem in perspective, the experiment is described The fundamentals of two-dimensional crystallography are discussed and their application to LEED are described The calculation of diffracted intensities requires the development of N-beams dynamical theory, which is discussed together with the need for appropriate computer programs Since the analysis is based on trial-and-error methods some attention is paid to the development of structure models, the problems caused by the co-existence of equivalent domains, and the procedures for evaluating the postulated models A brief discussion of the accomplishments is given which, rather than comprehensive, is an assessment of the present state of the art in surface crystallography

Electric-dipole moments of elementary particles

Abstract: Elementary particles can have electric-dipole moments only if there is a failure of symmetry under both parity (P) and time-reversal (T) transformations. It is now known experimentally that there are in nature failures of both of these symmetries, but so far the only experimental manifestations of failures of T symmetry have been limited to the neutral kaon system. Most theories that are consistent with the observed T-violating decay of KL0 also predict non-zero values for the electric-dipole moments of elementary particles, with many of the predicted values being within the range of experimental verification. However, so far no electric-dipole moment has been observed for any elementary particle. Experiments setting limits on the electric-dipole moment, eD, of elementary particles are reviewed. The most sensitive tests of theory are with the neutron, for which D

The study of lattice defects by channelling

Abstract: The channelling of ions in crystals is described and its application to the study of a variety of lattice defects is outlined. Ions which are channelled along different crystallographic axes and planes interact with displaced atoms in distinctive ways, enabling the atomistic nature of lattice defects to be determined. Three main areas of study are considered. (a) The trapping of vacancies and self-interstitials by solute atoms and the identification of the resulting trapping configurations. (b) The displacement of host atoms from lattice sites (e.g. ion-induced amorphisation of semiconductors). (c) The relaxation and reconstruction of surfaces.

Statistical surface physics: a perspective via computer simulation of microclusters, interfaces and simple films

Abstract: Surveys selected computer simulations or 'experiments' relating to the statistical physics of surface phenomena. An introduction to the Monte Carlo and molecular dynamics simulation techniques is presented, followed by chosen computer simulation applications which have been done mainly at the IBM Research Laboratory over the last several years. The examples are taken from studies of the structure and thermodynamics of microclusters, liquid-vapour and liquid-solid interfaces, and two-dimensional simple films. An attempt is also made to provide an introductory theoretical framework for describing or understanding certain relevant features of a given simulation study. An up-to-date bibliography of the various topics is given at the conclusion.

Hadron reaction mechanisms

Abstract: The mechanisms of hadron scattering at high energies are reviewed in an introductory fashion, but from a modern standpoint in which the authors try to combine the ideas of the parton model and quantum chromodynamics (QCD) with Regge theory and phenomenology. After a brief introduction to QCD and the basic features of hadron scattering data, the authors discuss scaling and the dimensional counting rules, the parton structure of hadrons, and the parton model for large momentum transfer processes, including scaling violations. Hadronic jets and the use of parton ideas in soft scattering processes are examined, and the authors then turn their attention to Regge theory and its applications in exclusive and inclusive reactions, stressing the relationship to parton exchange. The mechanisms of hadron production which build up cross-sections, and hence the underlying Regge singularities, and the possible overlap of Regge and scaling regions are discussed. The key to understanding hadron reaction mechanisms seems to lie in the marriage of Regge theory with QCD.

Advances in solar radio astronomy

Abstract: The status of the observations and interpretations of the sun's radio emission covering the entire radio spectrum from millimeter wavelengths to hectometer and kilometer wavelengths is reviewed. Emphasis is given to the progress made in solar radio physics as a result of recent advances in plasma and radiation theory. It is noted that the capability now exists of observing the sun with a spatial resolution of approximately a second of arc and a temporal resolution of about a millisecond at centimeter wavelengths and of obtaining fast multifrequency two-dimensional pictures of the sun at meter and decameter wavelengths. A summary is given of the properties of nonflaring active regions at millimeter, centimeter, and meter-decameter wavelengths. The properties of centimeter wave bursts are discussed in connection with the high spatial resolution observations. The observations of the preflare build-up of an active region are reviewed. High spatial resolution observations (a few seconds of arc to approximately 1 arcsec) are discussed, with particular attention given to the one- and two-dimensional maps of centimeter-wavelength burst sources.

Weak interactions of the neutron

Abstract: The most familiar manifestation of the weak interaction is nuclear beta decay and the most elementary beta -active nucleus is the neutron itself. The author examines the advantages and limitations of the various experimental techniques employed to study neutron beta decay and the implications of the results for fundamental theory and, to a lesser extent, for cosmology. The interaction responsible for nuclear beta decay is accurately described by a (V-A) theory with an absolute ratio of axial-vector to polar-vector coupling constants of mod lambda mod =1.257+or-0.009, and that the interaction is invariant under time reversal at the level of 10-3. Although, somewhat surprisingly, there still exists a considerable measure of uncertainty as to the precise value of the neutron half-life, it is not unfair to suggest that much of the experimental data on neutron beta decay is essentially complete so far as it relates to the fundamental structure of the weak charged currents. On the existence of weak magnetism or on the presence or absence of second-class currents, neutron beta decay, as distinct from nuclear beta decay, has told us nothing to date, and significant advances in experimental technique are probably required before that information begins to flow.

Physics of semiconductor power devices

Abstract: Semiconductor power devices are designed to rapidly switch or amplify high currents, to support high voltages, and to control electric power. Because of these requirements, their topographies and structures are different from those of small-signal devices. Specific designs are the result of the understanding of the physics of p-n junction HV breakdown, gain variation at high currents, current instabilities, etc. After introducing elementary semiconductor structures, the author reviews the basic principles of the operation of bipolar and field-effect power transistors and thyristors. This is followed by a discussion of phenomena of special interest for power devices; junction avalanche breakdown, the effect of the current level on the current gain, dynamic and static behaviour at high currents and thermal properties and instabilities. The review includes recent advances in device physics and introduces the reader to new methods of improving device performance. Power is a relative concept-at microwave frequencies at few watts is a very large quantity; this review therefore includes a section on microwave power devices.

Experimental verification of the Salam-Weinberg theory

Abstract: A review is given of the steps leading to the development of the Salam-Weinberg theory. The theory itself is defined and its relation to the general class of SU(2)*U(1) theories established. The discovery of weak neutral currents is described, as is the verification of the Glashow, Illiopoulos and Maiani quark coupling scheme in the discovery of charmed particles. Recent measurements of neutral current interactions of neutrinos are shown to lead to a unique set of quark couplings. The results of experiments on neutrino-electron scattering, when combined with data from the scattering of polarised electrons on deuterium, lead to a determination of the leptonic neutral current couplings. These couplings are consistent with data on weak effects in electron-positron collisions. The neutral current couplings thus derived favour a version of the SU(2)*U(1) theory with the Higgs mesons in doublets and quarks and leptons in right-handed singlets. In such a theory the data yield a value of sin2 theta w=0.233+or-0.009. The implications of this value for the grand unified theories of weak, electromagnetic and strong interactions are discussed.

Description of transitional nuclei

Abstract: The description of transitional nuclei, which have a deformation between spherical and strongly deformed nuclei, is reviewed. The author concentrates on models with a phenomenological core and one or two nucleons moving in its potential. For the core a rigid axial or triaxial rotor, the Bohr-Mottelson model or the interacting boson approximation is used. The motion of the additional nucleons is described as quasiparticles moving in the rotating deformed potential of the core. The properties of the odd nucleons are determined by two forces: (i) the coupling to the quadrupole moments of the core (strong coupling) and (ii) the influence of the Coriolis and centrifugal forces which like to decouple the odd nucleon from the motion of the core (decoupling). Models are presented which can describe at the same time decoupling and strong coupling and the competition between both. The model is applied to odd-mass nuclei, to the two-quasiparticle negative-parity states in even-mass nuclei, to zero- and two-quasiparticle positive-parity states in even-mass nuclei and to doubly odd-mass nuclei. Although it is the Os, Pt and Hg area is emphasised, results for other mass regions are discussed.

Progress in photovoltaic energy conversion

Abstract: The actual status of research on photovoltaic energy conversion with regard to its possible role in the production of electric power from solar radiation is reviewed. Silicon cells in single-crystalline or semi-crystalline forms are considered to be the dominant devices for the next decade. A variety of possibilities yielding lower fabrication costs and still high conversion efficiencies exist; an order-of-magnitude reduction from the present-day lost level turns out to be a realistic goal. This can be achieved by a combined effort in the fields of silicon material preparation, cell processing and module fabrication. Other longer term viable options are amorphous silicon, thin-film GaAs, II-VI compounds such as CdS and some advanced approaches using concentrated sunlight. It is believed that ultimately module efficiency will strongly determine the economic viability of a photovoltaic material.

On a quantitative definition of information and its impact on the field of communications

Abstract: Shannon (1948) gave a quantitative definition of information. Using this definition one can give bounds on the number of messages of a certain kind that can be stored and/or transmitted. The authors consider both perfect and noisy storage and transmission facilities. Aside from the above-mentioned bounds the theory also gives us explicit algorithms for the efficient storage and transmission of messages. First they consider the case of a single source and a single receiver. The theory is subsequently generalised to the multi-user situation where they have a communication network with many sources and/or receivers. They conclude by considering the overhead information in such a communication network that is necessary to protect the messages against unauthorised use, and to make sure that messages are delivered at the proper address.

An experimental view of jets (elementary particles)

Abstract: An account is given of the production and fragmentation of jets. Particular attention is paid to the way the major experimental features of their fragmentation can be due to very general properties of this fragmentation, so that these features cannot help to distinguish the production mechanism. Methods used for analysis of jet structures are described. The experimental features of jet structures in various processes are described, ranging from processes where jet structures are well-established and there is good evidence that these are due to fragmentation of quarks and gluons, to processes where this parentage is less certain or where quarks and gluons are believed to be involved but the resulting jet structures have yet to be observed.