Showing papers in "Reports on Progress in Physics in 1994"
TL;DR: In this paper, the observational methods for detecting and measuring extragalactic magnetic fields are discussed, along with some new indirect methods which could be used for inferring field strengths at large redshifts which are otherwise beyond the reach of direct measurement.
Abstract: The observational methods for detecting and measuring extragalactic magnetic fields are discussed, along with some new indirect methods which could be used for inferring field strengths at large redshifts which are otherwise beyond the reach of direct measurement. Various cosmological seed field generation mechanisms are reviewed, which could generate seed fields for the subsequently formed galaxies. The question of whether the original seed fields were produced in galaxies, or the pre-recombination early Universe must await a clearer picture of how the first stars and galaxies formed.
949 citations
TL;DR: In this paper, the experimental structure of small particles is reviewed, the emphasis being an attempt to correlate experimental information with theoretical models, and a general model merging thermodynamic and kinetic factors is presented to rationalize the available data, followed by a brief discussion.
Abstract: Data on the experimental structure of small particles is reviewed, the emphasis being an attempt to correlate experimental information with theoretical models. First, a general discussion of some of the controlling factors is presented, primarily equilibrium shapes of small particles, the effect of surface stresses, kinetics, and the role of chemisorption and the substrate. Experimental techniques for obtaining information about small particles are then described, primarily electron microscopy approaches. Experimental data on the static structure of small particles is then reviewed, both single crystals and the many, complicated twinned structures in face-centred cubic materials. An overview is then given of some of the more recent results on dynamic phenomena in small particles. Finally, a general model merging thermodynamic and kinetic factors is presented to attempt to rationalize the available data, followed by a brief discussion.
815 citations
TL;DR: Low energy electron microscopy (LEEM) is a surface imaging technique in which the surface is illuminated by an approximately parallel electron beam at near normal incidence as mentioned in this paper, and the image is formed with those electrons which are elas- tically backscattered into a small angular region around the surface normal.
Abstract: Low energy electron microscopy (LEEM) is a surface imaging technique in which the surface is illuminated by an approximately parallel electron beam at near normal incidence. The image is formed with those electrons which are elas- tically backscattered into a small angular region around the surface normal. The limitation to a small angular region is necessary because of the large aberrations of the objective lens which produces the primary image. This lens is a so-called cathode lens which not only has imaging properties but at the same time decelerates the fast electrons of the illuminating beam to the desired low energy at the specimen and re-accelerates the backscattered electrons to high energies again. In order to achieve this, the specimen is at a high negative potential which differs from the potential of the emitter of the electron gun of the illumination system by V 0 = E 0/e, where E 0 is the energy of the electrons at the specimen. Typical energy values are E = eV = 15−20 keV for the fast electrons and 0 < E 0 < 50 eV at the specimen. There are three fundamental quantities which are important in LEEM: resolution, intensity and contrast. These will be discussed in Sect. 12.1. Section 12.1 also describes how LEEM can be combined with other surface characterization techniques, such as low energy electron diffraction (LEED), photoemission electron microscopy (PEEM) and other emission microscopies. Section 12.2 illustrates the applications of LEEM and of the associated techniques to the study of clean surfaces, while Sect. 12.3 presents examples of the power of LEEM in the study of surface layers. Section 12.4 gives an outlook for possible future developments. A brief summary (Sect. 12.5) concludes this chapter.
586 citations
TL;DR: In this paper, the authors provide an overview concerning the physical mechanisms and paradoxes taking place in non-radiative detection, and explain the connection between Heisenberg uncertainty relations and the Rayleigh criterion.
Abstract: The early eighties have experienced a revolution in the perception of physical phenomena. This revolution is the birth of a new generation of imaging systems based on the detection of non-radiating fields. The near field optical microscope is the latest of this family. Like its prestigious brothers, the STM and the AFM, it allows one to see the physical world with new eyes. The objective of this article is to provide an overview concerning the physical mechanisms and paradoxes taking place in non-radiative detection. We will first explain the connection between Heisenberg uncertainty relations and the Rayleigh criterion. The fundamental role of evanescent fields will be pointed out through the plane wave expansion. Finally a catalogue of the different configurations currently in use will be given and illustrated with some experimental results.
410 citations
TL;DR: The microstructural features of the Si-SiO2 system and the chemical physics of its defects are reviewed and examined in this paper, where the role of atomic and molecular sized entities is emphasized, and the latter are limited to those containing only Si, O, H or combinations thereof.
Abstract: The microstructural features of the Si-SiO2 system and the chemical physics of its defects are reviewed and examined. Topics are grouped by scientific commonality, rather than by the usual technological manifestations. The role of atomic and molecular sized entities is emphasized, and the latter are limited to those containing only Si, O, H, or combinations thereof. Most of the reported researches involve x-ray or electron diffraction, Auger or photoelectron spectroscopy, Rutherford backscattering, electron spin resonance, or capacitance-voltage or deep-level transient spectroscopy. Several forms of crystalline and amorphous vitreous silica are considered as a basis for discussion of thin film thermal silica on silicon wafers. Local lattice symmetry, stoichiometry, bond lengths and angles, vacancies and voids, dangling orbital centres, and fixed and migratory hydrogen species are treated extensively. Elements of relevant theory are summarized. Overall, it is hoped to provide a solid data base for future development of general models for essential electronic phenomena in the Si-SiO2 system.
365 citations
TL;DR: A review of the background physics of the process and the development of the technique is described in this article, and a brief discussion of the high energy forward scattering version of the X-ray photoelectron diffraction-XPD is included, but the most of the review is concerned with the lower energy backscattering method more relevant to the determination of detailed adsorption sites on surfaces.
Abstract: Photoelectron diffraction is the name given to the phenomenon resulting from the coherent interference of the directly emitted component of an electron wavefield, emerging from an atom as a result of core level photoemission, with other components elastically scattered by surrounding atoms. Experimental characterization of this effect provides information which can be used to provide quantitative determinations of the structure of surfaces, and particularly of adsorbed species on surfaces, in an element-specific fashion. Since the initial. Demonstration of the phenomenon in the late 1970s, an extensive methodology for surface structure determination has been developed. In this review the background physics of the process, and the development of the technique is described. A brief discussion of the high energy forward scattering version of the technique (X-ray photoelectron diffraction-XPD), which utilizes zero-order diffraction effects, is included, but the most of the review is concerned with the lower energy backscattering method more relevant to the determination of detailed adsorption sites on surfaces. In addition to the general theoretical, experimental and methodology background, a number of the more recent developments are described including use of 'direct inversion' methods for (approximate) structure determination, including a survey of photoelectron holography, and the realization of chemical shift photoelectron diffraction to allow structure determinations of surfaces including atoms of one element in more than one inequivalent site. All of the developments are illustrated with specific examples, mainly of molecular and atomic adsorbates on metal surfaces.
279 citations
TL;DR: In this paper, the available data about the structure of bimetallic surfaces where the constituent elements are intermixed at atomic level is reviewed, including the surface of bulk alloys and systems resulting from diffusion at metal-metal interfaces.
Abstract: This paper reviews the available data about the structure of bimetallic surfaces where the constituent elements are intermixed at atomic level. These systems include the surface of bulk alloys ('alloy surfaces') and systems resulting from diffusion at metal-metal interfaces ('surface alloys'). All cases where a complete surface crystallographic determination exists are listed and discussed, as well as selected cases where the structural data are not complete but can lead to a model of the atomic structure.
228 citations
TL;DR: In this paper, the central 100 parsecs of our Galaxy were studied and the importance of magnetic and gravitational forces, evidence for stellar formation and a central massive black hole, and the origin and nature of ionization, outflows and interstellar gas dynamics.
Abstract: The subject of this review is the central 100 parsecs of our Galaxy, with a strong focus on the central few parsecs. Observations of the electromagnetic spectrum over 13 orders of magnitude in wavelength show a broad range of phenomena involving a number of physical processes. We discuss the stellar and interstellar components, the importance of magnetic and gravitational forces, the evidence for stellar formation and a central massive black hole, and the origin and nature of ionization, outflows and interstellar gas dynamics.
195 citations
TL;DR: In this paper, the authors present different suggestions for applications of photorefractive crystals and SLMs for holographic interferometry and 2D image processing, based on various phase-conjugate geometries.
Abstract: This chapter reviews different suggestions for applications of PRCs. Due to the high sensitivity, high diffraction efficiency, reversibility, and the ease of handling, these dynamic photosensitive media attracted the attention of researchers working in different areas of coherent optics. Early proposals on the use of photorefractive crystals and SLMs for holographic interferometry and 2D image processing are still interesting. Recently, a number of new ideas on PRC applications based on various phase-conjugate geometries have appeared, too.
142 citations
TL;DR: Low temperature scanning electron microscopy (LTSEM) allows one both to investigate interesting physical phenomena in superconducting thin film samples with a spatial resolution of about 1 mu m and to perform a functional test of superconding devices and circuits at their operation temperature as mentioned in this paper.
Abstract: By extending scanning electron microscopy to the temperature regime of liquid helium and nitrogen a powerful technique for the imaging of the local properties of superconducting thin films and Josephson junctions is obtained. Low temperature scanning electron microscopy (LTSEM) allows one both to investigate interesting physical phenomena in superconducting thin film samples with a spatial resolution of about 1 mu m and to perform a functional test of superconducting devices and circuits at their operation temperature. We discuss the technical and physical background of the LTSEM imaging technique including the electron optical and cryogenic requirements, the interaction of the electron beam with the superconducting sample, the dynamics of the electron beam induced non-equilibrium state, and the electron beam induced signal. The origin of spatial structures in superconducting thin films and Josephson junctions and their spatially resolved analysis by LTSEM is reviewed. The use of LTSEM in the functional test of both low- and high-temperature superconducting thin films, devices, and circuits is summarised.
114 citations
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TL;DR: In recent years, sand has become a paradigm of complexity in physics; despite its everyday familiarity, it has come to typify what is increasingly regarded as a 'new' state of matter, namely matter in the granular state as mentioned in this paper.
Abstract: In recent years, sand has become a paradigm of complexity in physics; despite its everyday familiarity, it has come to typify what is increasingly regarded as a 'new' state of matter, namely matter in the granular state. Granular matter shows behaviour that is intermediate between that of solids and liquids and manifests fascinating properties like dilatancy and hysteresis. The last few years have seen an explosion of theoretical and experimental activity in the study of its dynamics, in particular to do with the relaxational behaviour of vibrated powders, segregation phenomena and sandpile avalanches. This article sets out to review the progress that has been made, and to present our current understanding of the dynamics of sand.
TL;DR: The spin-polarized electronic structure of magnetic transition metallic materials is shown to be a fundamental part of spin density functional (SDF) theory which is able to give a quantitative account of many ground-state magnetic properties.
Abstract: The spin-polarized electronic structure of magnetic transition metallic materials is shown to be a fundamental part of spin density functional (SDF) theory which is able to give a quantitative account of many ground-state magnetic properties. Recent developments in the implementation of this theory are mentioned and comments made concerning the comparison of the electronic structure with spectroscopic measurements. The consequences of spin-orbit coupling effects on the electronic structure for magnetic anisotropy are uncovered via a relativistic generalization. The electronic structure of crystalline, magnetic and transition metal alloys is discussed in some detail and the implications for low-temperature, magnetic and related properties given. These include magneto-volume effects and the connection between magnetism and compositional order. Recent work on amorphous, metallic magnets, magnetic overlayers, thin films and multilayers is briefly described. The theory for low-temperature magnetic excitations is outlined in terms of the dynamic, spin susceptibility, which is also based on the electronic structure. This gives an account of spin waves in ferromagnets and spin fluctuations in paramagnets. The picture of the paramagnetic state of transition metal ferromagnets at high temperatures is described in which spin fluctuations are. Modelled as 'local moments', SDF theory is consequently extended to finite temperatures. The underlying electronic structure is shown to be modified in some cases by these collective electronic effects. Throughout the article, the successes and limitations of the theoretical results, when compared to experimental measurements, are set out.
TL;DR: In this article, the authors reviewed the current knowledge on collisions between complex nuclei in the range 10-100 AMeV bombarding energy and compared with the essential features characterizing reactions below 10 AM eV and-partly above 100 ameV.
Abstract: The current knowledge on collisions between complex nuclei in the range 10-100 AMeV bombarding energy is reviewed in comparison with the essential features characterizing reactions below 10 AMeV and-partly-above 100 AMeV. The data are inspected for deviations from fusion and binary reactions-quasi-elastic and deep-inelastic-prevailing below 10 AMeV and, in particular, the latter are found to persist far into the energy range considered. The sequential statistical decay of the primary fragments, due to increasing excitation energies, is important in the entire energy range and must carefully be taken into account. The break-up of the lighter collision partner into two or more fragments during the interaction phase (direct break-up) is found to set in at about 10 AMeV and to develop fully until 40 AMeV. It appears to be the typical process for the lower half of the energy range considered, while in the upper half systematic information is still lacking; however the rapidly growing knowledge allows, at best, some trends to be recognized. As an alternative to and probably competing with direct break-up we also discuss frequently used scenarios where the observed ejectiles originate from a statistical decay of an intermediate hot subsystem. We find that the experimental evidence for such a concept, though broad, is not compelling and some inconsistencies persist. Rather, the intermediate-energy regime can be understood in terms of the reaction mechanisms already established at low energies, when combined with the proper extension of the explicitly treated degrees of freedom to those of suitable constituents of the collision partners. The concept of dissipation and friction stays useful in the proper dynamical context. Microscopic modelling is still far from being perfect. We face the competition of mean-field effects and nucleon-nucleon collisions, specific many-particle correlations as intermediate-size phenomena playing an essential role.
TL;DR: In this article, a review of the present status of heavy-ion collisions at intermediate energies is presented, and a detailed discussion of the flow effects with a broad comparison with the available data is presented.
Abstract: This is a review of the present status of heavy-ion collisions at intermediate energies. The main goal of heavy-ion physics in this energy regime is to shed some light on the nuclear equation of state (EOS), hence we present the basic concept of the EOS in nuclear matter as well as of nuclear shock waves which provide the key mechanism for the compression of nuclear matter. The main part is devoted to models currently used for describing heavy-ion reactions theoretically and to observables useful for extracting information about the EOS from experiments. A detailed discussion of the flow effects with a broad comparison with the available data is presented. The many-body aspects of such reactions are investigated via the multifragmentation break up of excited nuclear systems and a comparison of model calculations with the most recent multifragmentation experiments is presented.
TL;DR: In this paper, the total cross section and diffractive processes of high-energy hadron colliders were measured and compared with earlier results at lower energy. And the general properties of the scattering amplitude, as derived from fundamental principles, were discussed, together with the current models and with the recent theoretical developments based on QCD.
Abstract: Measurements of the total cross section and of diffractive processes, which have been performed in the last decade at the high-energy hadron colliders, are presented and compared with earlier results at lower energy. The general properties of the scattering amplitude, as derived from fundamental principles, are discussed, together with the current models and with the recent theoretical developments based on QCD.
TL;DR: In this article, the authors demonstrate that foodstuffs can be usefully and excitingly studied within the framework of physics and demonstrate that the same issues that exercise the mind of researchers in traditional areas of soft condensed matter can be found within these homely materials, issues such as percolation, the nature of the glass transition and mechanisms of phase separation.
Abstract: The aim of this article is to demonstrate that foodstuffs can be usefully and excitingly studied within the framework of physics. Many of the same issues that exercise the mind of researchers in traditional areas of soft condensed matter can be found within these homely materials, issues such as percolation, the nature of the glass transition and mechanisms of phase separation. By applying the conventional tools of the physicist new insights can be obtained into the structures and responses of foodstuffs. In turn these insights may lead to improvements in the overall quality of the food to be consumed-by the knowledge contributing to improvements in processing, texture or storage for instance. The article begins with a brief introduction to the different classes of molecules usually found in food: carbohydrates, lipids and proteins. This is followed by a description of some of the methods frequently used to characterize foods which may lie beyond the physicist's remit. The next sections discuss in detail some of the generic characteristics of gels, foams, emulsions and powders using case histories to expand on the general principles of these states of matter. Finally there is a brief description of some common processing methodologies.
TL;DR: The last decade has seen substantial progress in the use of solid state detectors, which are now exploited on a large scale in high energy physics experiments for precise positional measurements as mentioned in this paper, and the major classes of detector and the methods employed to construct them and instrument the large systems presently in use for particle tracking.
Abstract: The last decade has seen substantial progress in the use of solid state detectors, which are now exploited on a large scale in high energy physics experiments for precise positional measurements. This article surveys the major classes of detector and the methods employed to construct them and instrument the large systems presently in use for particle tracking. Some of the most topical areas of current research and an indication of future important applications are described.
TL;DR: In this article, the Rydberg constant Rinfinity, the scaling factor for all transition frequencies in hydrogen, has been determined with a precision approaching 10-11, which means that all other transitions in hydrogen can be considered as metrological standards to this same level of precision.
Abstract: Hydrogen is the simplest atom, comprising only a proton and an electron. This simplicity means its properties can be calculated theoretically with impressive accuracy, currently of order 10-11. At the same time, rapid advances in the techniques of laser spectroscopy have paved the way for experimental measurements at a comparable level of precision. This article begins with an outline of the problems encountered in the theoretical calculations and explains where the current uncertainties lie. Next, methods of Doppler-free laser spectroscopy are described and recent key experiments based on them reviewed. In the latest measurements, the Rydberg constant Rinfinity , the scaling factor for all transition frequencies in hydrogen, has been determined with a precision approaching 10-11. This means that all other transitions in hydrogen, ranging from the microwave to the ultraviolet, can be considered as metrological standards to this same level of precision.
TL;DR: In this article, the current status of thermometry below 1 K from a metrological point of view is reviewed, and a considerable fraction of the paper is devoted to temperature reference standards and the actual state of their underlying scales.
Abstract: This paper reviews the current status of thermometry below 1 K from a metrological point of view. It emphasizes the relation to thermodynamic temperature without completely ignoring practical questions. Thus principles and limitations are discussed in detail for primary thermometers as well as for those secondary thermometers that can be employed to extend the temperature range by extrapolation. Practical thermometry is covered in a few instructive examples which illustrate the effects of experimental problems on temperature measurement. A considerable fraction of the paper is devoted to temperature reference standards, and the actual state of their underlying scales, that are used to reproduce accurately temperature values for calibration of secondary thermometers without running primary thermometers.
TL;DR: In this paper, the authors present the formalism which describes the spin-polarization observables in terms of spin-state transition amplitudes in a manner that reveals the required correspondence between the theoretical and experimental definitions of the observables.
Abstract: This review has two aims: (i) to present the formalism which describes the spin-polarization observables in terms of spin-state transition amplitudes in a manner that reveals the required correspondence between the theoretical and experimental definitions of the observables; then (ii) to emphasize that spin physics, the experimental and theoretical investigations of spin-polarization effects in scattering and reactions, has become a clear unifying element among the otherwise seemingly disparate fields of nuclear, particle and electron-scattering physics. Illustrative examples of research results in these fields are used to demonstrate this commonality. The important role of intrinsic spin in providing experimental investigations of parity conservation, charge symmetry and time-reversal invariance is discussed.
TL;DR: The physical motivations to search for new particles and new effects are reviewed in this paper, where the main emphasis is put on the study of electroweak symmetry breaking, in particular the search for Higgs bosons, and on supersymmetry.
Abstract: The physical motivations to search for new particles and new effects are reviewed The status of searches, direct and indirect, performed at accelerators, is described, as well as the foreseeable future at upgraded and planned machines The main emphasis of the review is put on the study of electroweak symmetry breaking, in particular the search for Higgs bosons, and on supersymmetry
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