Showing papers in "Reports on Progress in Physics in 1990"
TL;DR: The theory of diffusion and the principal methods of determining surface diffusion coefficients are presented and their strengths and weaknesses discussed and areas of agreement and disagreement between various measurements are discussed.
Abstract: The theory of diffusion and the principal methods of determining surface diffusion coefficients are presented and their strengths and weaknesses discussed. A summary of major experimental results for diffusion of metallic and non-metallic adsorbates on metal surfaces is given and areas of agreement and disagreement between various measurements are discussed. A brief overview of principal conclusions, problem areas and future directions concludes the review.
867 citations
TL;DR: In this paper, the basic general constraints controlling surfactant aggregation and the formation of interfaces are summarized and related to interfacial film properties and the intermolecular interactions inside them.
Abstract: From simple micelles in water, nearly spherical aggregates of amphiphilic molecules, to bicontinuous microemulsions, oil and water microheterogeneous mixtures stabilised by a surfactant film with both local and large-scale disordered structures, the world of surfactant-containing systems is fascinating. Depending on a subtle balance of attractive and repulsive interactions between molecules at interfaces, an extraordinarily rich polymorphism of aggregated structures can be observed. After summarising the basic general constraints controlling surfactant aggregation and the formation of interfaces, different structures of micelles and microemulsions are reviewed and related to interfacial film properties and the intermolecular interactions inside them.
387 citations
TL;DR: In this paper, the main part of the review is devoted to the statistical decay of equilibrised very hot nuclei, and a new mathematical method, microcanonical Metropolis sampling, which allows the explicit calculation of micro-canonical observables directly out of the micro canonical partition sum with high statistics, is presented.
Abstract: Heavy ion collisions are the only tools that enable us to reach compressions of nuclear matter up to a factor of about three. Theories of the collision dynamics that are necessary in order to extract the equation of state for nuclear matter are discussed. The main part of the review is devoted to the statistical decay of equilibrised very hot nuclei. A new mathematical method, microcanonical Metropolis sampling, which allows the explicit calculation of microcanonical observables directly out of the microcanonical partition sum with high statistics, is presented. Thermodynamics of finite nuclei can thus be established from the first principles. This is important as nuclei subjected to long-range Coulomb forces do not allow extrapolation to the thermodynamic limit. This peculiarity has interesting consequences for the phase transitions and the critical behaviour of nuclei. It is also related to the property of nuclei to break into large clusters at high excitation.
374 citations
TL;DR: In this paper, the authors review the history of the discovery of white dwarfs and the realisation that their structure is determined by the physics of the degenerate electron gas, and show a complicated pattern dominated by diffusion processes and their interaction with accretion, convection and mass loss.
Abstract: White dwarf stars, compact objects with extremely high interior densities, are the most common end product in the evolution of stars. The authors review the history of their discovery, and of the realisation that their structure is determined by the physics of the degenerate electron gas. Spectral types and surface chemical composition show a complicated pattern dominated by diffusion processes and their interaction with accretion, convection and mass loss. While this interaction is not completely understood in all its detail at present, the study may ultimately lead to important constraints on the theory of stellar evolution in general. Variability, caused by nonradial oscillations of the star, is a common phenomenon and is shown to be a powerful probe of the structure of deeper layers that are not directly accessible to observation. Very strong magnetic fields detected in a small fraction of white dwarfs offer a unique opportunity to study the behaviour of atoms under conditions that cannot be simulated in terrestrial laboratories.
360 citations
TL;DR: In this paper, the authors present a coherent summary of the current understanding of chemisorption phenomena, stressing theoretical concepts and developments rather than experimental techniques, in a search for physical concepts that can systematise the observational data and perhaps make it possible to predict the behaviour of systems that have not yet been studied.
Abstract: Presents a coherent summary of the current understanding of chemisorption phenomena, stressing theoretical concepts and developments rather than experimental techniques. The rapid experimental development of surface science requires a more systematic approach to the explosion of chemisorption data. To this end, the common characteristics of chemisorption systems are first reviewed before the differences are discussed and trends identified, in a search for physical concepts that can systematise the observational data and perhaps make it possible to predict the behaviour of systems that have not yet been studied. The major topics discussed are the adiabatic potential energy surface, the electronic structure problem, the Newns-Anderson model, atomic and molecular chemisorption, and reactions and heterogeneous catalysis. A comprehensive review of experimental results is not attempted within the concept-oriented approach of this study. It is shown that simple models are able to describe semi-quantitatively the chemisorption bond for simple gas atoms, and that there is some understanding of the surface and adsorbate parameters that determine important experimental observables such as the chemisorption energy, bond lengths, and vibrational frequencies. For molecular chemisorption and the dissociation of molecules on metal surfaces the understanding is less well developed, but there is some qualitative understanding of a number of trends.
276 citations
TL;DR: In this article, the authors describe the continuum of metal-induced gap states (MIGS) which are derived from the virtual gap states of the complex semiconductor band structure, and the physical mechanism primarily determining barrier height is provided by the decay of the metal's electron wavefunctions into the semiconductor in the energy range between the top of the valence band and the Fermi level.
Abstract: Almost all metal-semiconductor or Schottky contacts exhibit rectifying behaviour which is caused by a depletion layer on the semiconductor side of the interface. The electronic properties of a Schottky diode are characterised by its barrier height which is the energy difference between the top of the valence band of the semiconductor and the Fermi level at the interface. The physical mechanism primarily determining barrier height is provided by the decay of the metal's electron wavefunctions into the semiconductor in the energy range between the top of the valence band and the Fermi level where the metal conduction band overlaps the semiconductor band gap. These wavefunction tails are described as the continuum of metal-induced gap states (MIGS) which are derived from the virtual gap states of the complex semiconductor band structure.
245 citations
TL;DR: In this article, the detailed structures of the various discotic phases and some of their important physical properties are described. But they are classified into two distinct categories, the columnar and the nematic.
Abstract: Recent studies have established that many compounds composed of disc-shaped molecules exhibit stable thermotropic liquid crystalline phases. They are now referred to as discotic liquid crystals. Structurally, most of them fall into two distinct categories, the columnar and the nematic. The columnar phase, in its simplest form, has long-range translational periodicity in two dimensions and liquid-like disorder in the third, whereas the nematic phase is an orientationally ordered arrangement of discs without any long-range translational order. This review article describes the detailed structures of the various discotic phases and some of their important physical properties.
221 citations
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TL;DR: Several projects involving Type I supernovae and their impact on nucleosynthesis are described in this article, where a major effort is underway to compute the spectrum at epochs near maximum light.
Abstract: Several projects involving Type I supernovae and their impact on nucleosynthesis are described. 1) A major effort is underway to compute the spectrum at epochs near maximum light. 2) Dynamical calculations have been done exploring the range of possible outcomes of the evolution of binary white dwarfs. 3) Consideration is given to the need to eject iron from the Galaxy to avoid overproduction by Type I events. 4) The nature of the class of peculiar Type I supernovae is explored.
169 citations
TL;DR: In this paper, a review of point defect parameters in ionic solids by atomistic simulation is presented, where both static and dynamic methods are discussed, and the effect of the long-range electrical terms on the defect energies and entropies is discussed.
Abstract: This review considers the calculation of point defect parameters in ionic solids by atomistic simulation. Both static and dynamic methods are discussed. After a brief overview of the field, the author considers how to obtain a reliable model of the crystal potential. This matter is fundamental to any adequate simulation. The author then turns to consider the calculation of defect energies and entropies. The following section discusses how the results of such simulations are to be interpreted. This requires a discussion of the reaction rate theory for defect motion, in particular recent work on what the theory means and how its shortcomings can be corrected. The author then turns to the interaction of point defects with interfaces and to their interaction with dislocations. The effects of the long-range electrical terms are shown to be particularly important in these circumstances. Finally, the author summarizes the review and discuss future directions in the field.
145 citations
TL;DR: In this paper, the authors summarized current thinking about thermally activated magnetic processes, and put them in the context of recent discoveries about magnetic domain structure in crystals containing only one or a few domains.
Abstract: Rock magnetism is concerned with the remanent magnetisation of ferrimagnetic fine particles in weak fields ( approximately=0.1 mT), at temperatures ranging from ambient to the Curie point, and over times of geological length. Since most magnetisations in rocks or their synthetic analogues are thermally activated, the blocking temperature below which the approach to equilibrium is effectively frozen is of particular importance. The author summarises current thinking about thermally activated magnetic processes, and puts them in the context of recent discoveries about magnetic domain structure in crystals containing only one or a few domains. Pseudo-single-domain effects in such crystals, which bridge the gap between classic single-domain and multidomain behaviour, are one of the current frontiers in rock magnetism.
112 citations
TL;DR: In this paper, the main types of electrostatic analysers are reviewed and the data for the energy resolution are obtained from a general approach, for first-and second-order focusing analysers of any size or geometry and whose optical coefficients are known.
Abstract: Describes the realization and use of a modern electron spectrometer. The discussion includes practical guidelines accessible to non-specialists who wish to design their own instrument. The basic characteristics and the design principles of such devices are discussed, with emphasis on the key component: the energy analyser. The main types of electrostatic analysers are reviewed. Formulae, tables and graphs are presented for the determination of their main features, in particular their energy resolution. The data for the energy resolution are obtained from a general approach, for first-and second-order focusing analysers of any size or geometry and whose optical coefficients are known. A figure of merit is proposed for their comparison and various optimization criteria are discussed. Different electron-optical aspects dealing with the calibration of analysers, the modes of operation for various types of measurements, and fringing field shielding are also surveyed. An effort is made to recall the fundamental laws and concepts of electron optics. A presentation is given of the basic data needed for the choice and the design of a lens system suitable for the efficient transport of the electrons. The case of electron monochromators is the object of a specific discussion. The last part is a survey of the latest developments in the techniques of electron spectroscopy.
TL;DR: In this paper, the geometrical properties of random close packings of grains are studied using the tools of stereology and the distribution of contacts which controls the transport characteristics in the grain phase, and the rigidity of granular structures which can be mapped onto a lattice model and can be approached by a central force percolation model.
Abstract: Many studies originally made in the field of statistical physics have led to results which are directly related to different features of random packings of granular materials. The authors outline some of these developments: geometrical properties of packings, electrical transport in the pore or the grain space, mechanics of granular arrays. They mainly focus on the following points: (i) The geometrical properties of random close packings of grains which can be studied using the tools of stereology. (ii) The distribution of contacts which controls the transport characteristics in the grain phase. (iii) The rigidity of granular structures which can be mapped onto a lattice model and which can be approached by a central-force percolation model. (iv) The non-linear properties due to the asymmetry of the contact behaviour between two grains, which can be studied on randomly depleted lattice models. Such properties, which come from the existence of non-linear local laws, are extended to the strong irreversible behaviour of fractured structures. (v) Finally, they consider some minimal structure approaches which model the distribution of stresses along a granular system.
TL;DR: An introductory review of the cholesteric blue phases is presented in this paper, where the emphasis is on the basic concepts of the theoretical framework and the recent achievements of theory and experiment.
Abstract: An introductory review of the cholesteric blue phases is presented. The emphasis is on the basic concepts of the theoretical framework and the recent achievements of theory and experiment. The author discusses a number of controversial issues indicating where future research is expected to shed light on the remaining unresolved problems.
TL;DR: In this article, the physics of silicon dioxide layers, with thicknesses below 100 nm, are presented in a review which focuses on the developments in the 1980s and deals with the whole range of SiO2 properties and behaviour.
Abstract: The physics of silicon dioxide layers, with thicknesses below 100 nm, are presented in a review which focuses on the developments in the 1980s. The study deals with the whole range of SiO2 properties and behaviour, beginning with oxidation kinetics and the physical structure of SiO2, and ending with oxide breakdown and reliability test methods.
TL;DR: In this article, the authors review quantum interference effects that have been observed in ultrasmall structures and their implications for future electronic devices, and discuss some of the unresolved questions that have to be answered in order to develop accurate models for quantum device simulation.
Abstract: Device analysis has traditionally been based on the semiclassical Boltzmann transport equation. Despite its impressive successes, this approach suffers from an important limitation; it cannot describe transport phenomena in which the wave nature of electrons plays a crucial role. A variety of such quantum effects have been discovered over the years, such as tunnelling, resonant tunnelling, weak and strong localisation, and the quantum Hall effect. Since 1985, experiments on ultrasmall structures (dimensions 100 nm) have revealed a number of new effects such as the Aharanov-Bohm effect, conductance fluctuations, non-local effects and the quantised resistance of point contacts. For ultrasmall structures at low temperature, these phenomena have clearly shown that electron transport is influenced by wave interference effects not unlike those well known in microwave networks. New device concepts are being proposed and demonstrated that are based on these wave properties. The authors review quantum interference effects that have been observed in ultrasmall structures, and their implications for future electronic devices. They also review the theoretical understanding of such phenomena and discuss some of the unresolved questions that have to be answered in order to develop accurate models for quantum device simulation.
TL;DR: In this paper, a review of recent developments of Skyrme's original version of the model and their applications to the structure of the nonstrange and strange baryons as well as to the interacting two-baryon system is presented.
Abstract: The low-energy observables of baryons are analysed within the framework of the topological soliton model. This article is a review of recent developments of Skyrme's original version of the model and their applications to the structure of the non-strange and strange baryons as well as to the interacting two-baryon system. The approach is, in spite of the simplicity of the dynamical model, able to provide a description of baryon structure at a quantitative accuracy of the order of 20-30%.
TL;DR: In this article, a review of nuclear models which consider collective proton and neutron motion separately in terms of bosons is presented, and a general projection method is presented whereby any IBA-2 calculation for states of maximal F spin can be transformed into a simpler one in IBA, where the proton-neutron distinction is ignored.
Abstract: The present review deals with nuclear models which consider collective proton and neutron motion separately in terms of bosons. Extensions of the geometric Bohr-Mottelson model are sketched, followed by a more extensive discussion of the algebraic interacting boson approximation (IBA-2). In the latter the concept of F spin plays a central role, and detailed attention is paid to its tensorial properties. A general projection method is presented whereby any IBA-2 calculation for states of maximal F spin can be transformed into a simpler one in IBA-1, where the proton-neutron distinction is ignored. Data are presented on F-spin multiplets, and global fits with the IBA are discussed. A major portion of the review is concerned with M1 transitions. Strong M1s in deformed and spherical nuclei, with emphasis on the latter, are discussed as resulting from a change in proton-neutron symmetry. Weak M1s are considered as reflecting such symmetry changes in small amplitudes of the wave function. Attention is given to one-fluid boson models (IBA-1, geometric) simulating these two-fluid effects.
TL;DR: A review of the evidence for large-scale motions of galaxies out to a distance of approximately 5000 km s-1 is given in this article, where the authors present a review of evidence for the large scale motion of galaxies.
Abstract: The expansion of the Universe can be retarded in localised regions within the Universe both by the presence of gravity and by non-gravitational motions generated in the post-recombination Universe. The motions of galaxies thus generated are called 'peculiar motions', and the amplitudes, size scales and coherence of these peculiar motions are among the most direct records of the structure of the Universe. As such, measurements of these properties of the present-day Universe provide some of the severest tests of cosmological theories. The author presents a review of the evidence for large-scale motions of galaxies out to a distance of approximately 5000 km s-1.
TL;DR: In this paper, the basic principles of some important models are discussed: continuum, bead-rod-spring, transient network, reptation and configuration tensor models, with a consistent treatment of the fundamentals of the various models and their interrelationship.
Abstract: The understanding of the flow behaviour of polymeric liquids is of great interest from a practical as well as a theoretical point of view. An important part of the research in this field consists of the development of suitable models, describing the rheological properties of the materials. Depending upon its purpose, such a model may be based upon empirical knowledge of the macroscopic flow behaviour or on information about the microstructure of the materials. Moreover, for a given system, different types of modelling may be possible. In order to provide an overview of the various approaches in this area the basic principles of some important models are discussed: continuum, bead-rod-spring, transient network, reptation and configuration tensor models. Emphasis has been put on a consistent treatment of the fundamentals of the various models and their interrelationship, rather than considering any of them in much detail.
TL;DR: In this paper, a unified review of group theoretical analysis of angular and radial correlations is presented, pointing out also what remains to be done, particularly regarding radial correlations which have seen essentially no progress.
Abstract: A revival has occurred in the past fifteen years, connected with the emergence of two challenging problems of atomic dynamics. One deals with high doubly excited states of atoms in which two slow electrons move in a Coulomb field. The second is the study of highly excited states of a single electron in a magnetic field. Both involve non-separable Hamiltonians and competing interactions that are not perturbative. Thereby, analysis through conventional single-particle bases requires the mixing of a diverging number of angular and radial configurations. This has motivated a search for quasi-good symmetries that may provide alternative bases and alternative quantum numbers. Group theoretical analysis, involving primarily variants of O4, has been largely successful in the program regarding angular mixings in both problems. The author gives a unified review of this work, pointing out also what remains to be done, particularly regarding radial correlations which have seen essentially no progress. Links are provided throughout between the group theoretical work and other approaches to these problems.
TL;DR: A review of the techniques associated with low-temperature thermal equilibrium nuclear orientation, detected using nuclear radiations, is given in this article, with emphasis on recent developments and new experiments in the field.
Abstract: A review is given of the techniques associated with low-temperature thermal equilibrium nuclear orientation, detected using nuclear radiations. Following an introductory section that includes a historical summary, a brief description of the formalism and of some experimental aspects of nuclear orientation is presented. Various applications of nuclear orientation are then treated in detail, with emphasis on recent developments and new experiments in the field. The applications are grouped, according to the particular experimental techniques used, into static orientation, nuclear resonance methods, and time-dependent methods.
TL;DR: The main characteristics of various precision-machining processes are outlined in this article, and the sensitivity of various processes to particular types of machine and environment-induced disturbances is outlined.
Abstract: In the introduction, applications of parts with submicron accuracy are shown. The demands with regard to the accuracy of these parts are discussed. The main characteristics of various precision-machining processes are outlined. Specific advantages and problems associated with these processes are indicated. Different aspects of part accuracy and surface quality are treated in more detail. Many parts with submicron accuracy have an optical function, and special attention is paid to the requirements with regard to form accuracy and roughness of optical elements. A distinction is made between machine- and process-related problems. The machining operation is introduced as an overall system containing the machine and process as interacting subsystems. The author also discusses techniques used to minimise machine-related part errors in the design of precision machine tools. The sensitivity of various processes to particular types of machine- and environment-induced disturbances is outlined. Developments in the investigations of material removal mechanisms are described. These include the mechanical modes of material removal on which diamond turning, grinding and polishing (to some extent) are based as well as the basic chemical phenomena in processes involving etching. After these descriptions of the machine and process aspects, the influence of the interaction between these subsystems on the surface quality is outlined. The emphasis is on the coupled dynamic behaviour in diamond turning and grinding. The author concludes with an 'executive summary' outlining the areas in which significant progress has been made in recent years and the main unresolved problems.
TL;DR: In this article, a self-consistent Monte Carlo particle model aimed at a profound physical understanding of small semiconductor components of arbitrary geometries is presented, which consists briefly of following the transport histories of individual charge carriers in detail.
Abstract: A self-consistent Monte Carlo particle model aimed at a profound physical understanding of small semiconductor components of arbitrary geometries is presented. The simulation technique consists briefly of following the transport histories of individual charge carriers in detail. After a discussion of the stochastic distribution of the time of free flight, the scattering mechanisms and the scattering angles of the particles, a brief review of the relevant semiconductor physics is given. As an example to illustrate the salient points of the technique, the results of simulating a GaAs field-effect transistor will be discussed. The presentation of the transistor characteristics is used to establish a link between theoretical physics and the view of the electrical engineer. A short discussion of statistical phenomena such as noise, negative differential resistivity and substrate currents has also been included.
TL;DR: In this paper, the authors summarize the optical and radio techniques employed in the detection of high-redshift (z>3) quasars and attempt to reconcile the contradictory claims in the literature concerning the evolution of their space density at the highest redshifts.
Abstract: The detection of samples of high-redshift (i.e. distant) quasars provides direct information on the physical conditions existing when the Universe was only 10% of its present age. Quasars are the bright nuclei of certain galaxies and are the only astronomical sources that can currently be found in substantial numbers at redshifts z>2. It is probable that quasars are active for only a small proportion of the life of the host galaxy, in which case a substantial fraction of all galaxies must go through a quasar phase at some time in their history. Consequently quasars may be tracers of the galaxy population at high redshift, less active members of which are too faint for detailed study with present-day instrumentation. The main purpose of surveys for high-redshift quasars, then, is to map the evolution of the population, in luminosity and number density, back through the history of the Universe, thereby furnishing clues both to the nature of the quasar phenomenon itself and to the poorly understood process of how galaxies formed. Of special interest is the possibility of detecting the epoch of the birth of the quasar population. The authors summarise the optical and radio techniques employed in the detection of high-redshift (z>3) quasars and attempt to reconcile the contradictory claims in the literature concerning the evolution of their space density at the highest redshifts.
TL;DR: Knowledge of the atmospheres of Jupiter, Saturn and Titan has made huge progress thanks to the remote sensing exploration by the Voyager spacecraft, considerable information has also been obtained by ground-based infrared spectroscopy as discussed by the authors.
Abstract: Knowledge of the atmospheres of Jupiter, Saturn and Titan has made huge progress thanks to the remote sensing exploration by the Voyager spacecraft, considerable information has also been obtained by ground-based infrared spectroscopy. Jupiter and Saturn have a hydrogen-rich atmosphere with trace constituents in a reduced form. From the determination of elemental and isotopic ratios, one can derive information about their formation and probable evolution. Titan, Saturn's largest satellite, is a unique object in the Solar System. Its nitrogen-rich atmosphere and its surface pressure show some conditions similar to the Earth's atmosphere; the discovery by Voyager 1 of complex molecules (hydrocarbons and nitriles), produced by the dissociation of methane and nitrogen, suggests the possible development of a complex carbon chemistry.