Showing papers in "Reports on Progress in Physics in 1991"
TL;DR: In this paper, a review focusing on nematic liquid crystals is presented, where three main kinds of effects can be distinguished: the perturbation of the liquid crystalline structure close to the surface, the bulk liquid crystal structure is recovered with an orientation which is fixed by the surface and critical adsorption or wetting can occur at surfaces.
Abstract: As their name indicates, liquid crystals simultaneously exhibit some characteristics common to both ordinary isotropic liquids and solid crystals. This ambivalence is also found in the effects of surfaces on these systems which lead to a great diversity of phenomena. These phenomena are reviewed focusing on nematic liquid crystals which have the simplest structure among the many existing types and which have been the most extensively studied. Three main kinds of effects can be distinguished. The first concerns the perturbation of the liquid crystalline structure close to the surface. Beyond this transition region, the bulk liquid crystalline structure is recovered with an orientation which is fixed by the surface: this phenomenon of orientation of liquid crystals by surfaces is the so-called anchoring. Finally, close to bulk phase transitions, critical adsorption or wetting can occur at surfaces as is also seen in isotropic systems.
754 citations
TL;DR: A review of the formation, the crystal structure and the magnetic properties of several classes of rare earth based intermetallic compounds that lend themselves as starting materials of permanent magnets is given in this article.
Abstract: A review is given of the formation, the crystal structure and the magnetic properties of several classes of rare earth based intermetallic compounds that lend themselves as starting materials of permanent magnets. These compounds include R2Fe14B, R2Fe14C and R2Co14B and the large class of ternary rare earth compounds having the tetragonal ThMn12 structure. Special emphasis is given to the changes in magnetic properties of R2Fe17 compounds observed after interstitial solution of C or N atoms. The magnetic properties of all these compounds are discussed in terms of current models based on intersublattice and intrasublattice exchange and the interplay between the rare earth sublattice anisotropy and 3D sublattice anisotropy. A substantial portion of the review is devoted to manufacturing routes of permanent magnets and a description of the coercivity mechanisms operative in the magnets. A comparison is made of the performance and economic costs of various types of magnets and novel applications are briefly discussed.
500 citations
TL;DR: The electron momentum spectroscopy (EMS) as mentioned in this paper is a technique that uses electron-impact ionization reactions near the Bethe ridge at total energies of the order of 1000 eV or higher.
Abstract: Unique information about the motion and correlation of valence electrons in atoms, molecules and their ions is obtained from electron-impact ionization reactions near the Bethe ridge at total energies of the order of 1000 eV or higher. This is electron momentum spectroscopy. The history, theory and practice of the field are discussed and its value is shown by numerous examples.
416 citations
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TL;DR: In this article, a review of developments in the theoretical investigation of nuclear double-beta decay is presented, in particular the neutrinoless mode, since it is sensitive to lepton number violation as predicted by gauge theories beyond the standard model and it is expected to give important information on the nature of the neutrinos and the weak interaction.
Abstract: Developments in the theoretical investigation of nuclear double-beta decay are reviewed. In particular, the neutrinoless mode is discussed in detail, since it is sensitive to lepton number violation as predicted by gauge theories beyond the standard model and it is expected to give important information on the nature of the neutrinos and the weak interaction. Various approximations made in the theoretical treatment of neutrinoless and two-neutrino double beta decay are examined, and the limits on the effective Majorana mass of the electron neutrino as well as the coupling constants of the right-handed leptonic current are presented.
332 citations
TL;DR: An overview of the current understanding of multiphoton ionization of atoms can be found in this article, where a large part of the review is devoted to some nonperturbative theories.
Abstract: Presents an overview of the current understanding of multiphoton ionization of atoms. It begins with an introductory section to explain the background of the subject. Then the article develops the three topics which have been central themes of discussion in multiphoton ionization of atoms these past few years: multiply charged ion production, very high order harmonic generation, and above-threshold ionization, a name given to the absorption of a very large number of photons by an already ionized electron. A large part of the review is devoted to some theoretical aspects of multiphoton ionization of atoms and especially non-perturbative theories. Finally the article considers the very near future prospects of laser-electron interactions and more generally laser-matter interactions at 1018-1019 W cm-2, an intensity range now within reach due to new short pulse laser technology.
215 citations
TL;DR: In this article, the authors consider the evolution of an open quantum mechanical system which together with its environment forms a closed system and perform numerical calculations for the nuclear system at low as well as at high level density.
Abstract: The author considers the evolution of an open quantum mechanical system which together with its environment forms a closed system. The numerical calculations are performed for the nuclear system at low as well as at high level density. In each case, the relevant modes are discussed and compared to the results of the standard methods developed for their description. The influence of the respective remaining modes is compared with existing experimental data. The transition from the resonance reaction mechanism at low level density to the direct reaction mechanism at high level density takes place at a stochasticity threshold. The many-body properties are conserved, at high level density, in long-lived traps but the spectroscopic information is lost. The evolution to the thermal equilibrium takes place via the formation of quantum chaos in accordance with the second law of thermodynamics. The evolution is accompanied, in the open system, by the formation of a new order with less degrees of freedom. These modes are far from thermal equilibrium. They screen the long-lived modes which are near to thermal equilibrium.
211 citations
TL;DR: The properties of hot electrons in systems where electrons and phonons experience quantum confinement are reviewed in this paper, particularly with reference to the principal scattering mechanism, and the experimental and theoretical data relating to this form a large part of the review.
Abstract: The properties of hot electrons in systems where electrons and phonons experience quantum confinement are reviewed. The modifications to the behaviour of electrons and phonons brought about by confinement are described, particularly with reference to the principal scattering mechanism. The latter include the interaction with longitudinal optical phonons and plasmons, along with carrier-carrier effects. Some conflict in the literature concerning Fuchs-Kliewer polaritons is discussed. Low-temperature interactions with acoustic phonons are described. A central topic is that of energy relaxation, and the experimental and theoretical data relating to this form a large part of the review. Energy relaxation mechanisms in the femtosecond to nanosecond regimes, including intersubband and well-capture processes, are eventually summarized. An equally large section deals with hot-electron transport; in which negative differential resistance and other instabilities associated with parallel transport are discussed before turning to ballistic transport and impact ionization.
197 citations
TL;DR: In this article, the importance of observational data taken over a wide range of frequencies, from radio and infrared through optical and ultraviolet to X-rays and gamma-rays, is emphasized.
Abstract: Our current knowledge of active galactic nuclei is reviewed. The importance of observational data taken over a wide range of frequencies, from radio and infrared through optical and ultraviolet to X-rays and gamma -rays, is emphasized. Important overall principles include the continuity from quasars and QSOs through Seyfert and radiogalaxies to low-luminosity LINERs, the importance of considering roughly cylindrically symmetric (rather than spherically symmetric) structures, and that the various regions generally have different axes and planes of symmetry, and are often warped.
163 citations
TL;DR: In this paper, the inverse problem of how to proceed from the desired wavefield in space to the design of a diffractive element that is able to form this field is treated by considering various approximations that define digital holography as a subset of diffractive optics.
Abstract: An essential aspect of digital diffractive optics is the inverse problem of how to proceed from the desired wavefield in space to the design of a diffractive element that is able to form this field. This subject is treated by considering various approximations that define digital holography as a subset of diffractive optics. Analysis and models of inverse wave propagation are presented for boundary conditions determined by the type of material, the fabrication technique and the application. Coding theory is a central topic in the examination of this situation. Coding methods and the prediction of the efficiency of the diffraction are discussed. Nonlinear methods are used to treat the optimization problems. It is shown how diffractive elements can be designed by applying iterative and non-iterative methods. Diffractive elements of different kinds are presented and illustrated. The model examined seems to be of value in treating various ingredients of digital diffractive optics.
143 citations
TL;DR: In this article, the evaporation cascade of fissile excited compound systems is compared with the expectations of the statistical model, and experimental evidence is presented that the hindrance to fusion is strongly influenced by the nuclear structure of the reaction partners.
Abstract: In parallel to the attempts to synthesize the heaviest nuclei, systematic studies have been made to obtain better understanding of the reaction aspects. The most comprehensive data have been taken for nearly mass-symmetric massive systems. They combine high Coulomb forces in the entrance channel with evaporation-residue cross sections which are high enough to be easily detectable. With these systems, rather cold compound nuclei can be produced, and even radiative fusion was observed. While one of the most salient features of the thoroughly studied fusion of light and medium-heavy systems is the enhanced sub-barrier fusion, the massive systems exhibit a considerable deficit of fusion above the expected potential barrier. This hindrance to fusion may be attributed to the dynamical evolution of the composite system which may lead to immediate reseparation. The experimental data reveal that the hindrance to fusion is strongly influenced by the nuclear structure of the reaction partners. The high fission competition in the evaporation cascade of fissile excited compound systems as deduced from measured evaporation-residue cross sections is compared with the expectations of the statistical model. Experimental evidence is presented.
130 citations
TL;DR: In this article, the authors review the description of the electromagnetic properties of baryons with explicit reference to the quark degrees of freedom and present a theoretical framework for interpreting the experimental baryon data, including photon-proton scattering and pion photoproduction.
Abstract: The description of the electromagnetic properties of baryons with explicit reference to the quark degrees of freedom is reviewed. After a short discussion of the available phenomenological information, such as magnetic moments, form factors and photoexcitation amplitudes, the constituent quark model in the potential approach is introduced and the consequent description of the baryon spectrum is briefly discussed. The model is then systematically used as a theoretical framework for the interpretation of the experimental baryon data, including photon-proton scattering and pion photoproduction, and for the discussion of recent studies of the electromagnetic properties of few-nucleon systems. In particular, the reformulation of meson exchange currents in terms of quark degrees of freedom is reported and the results of various quark approaches to the form factors are presented. Finally, some recent improvements and future developments are briefly discussed.
TL;DR: In this paper, the problem of determining the phases of a crystal when only the magnitudes mod EH mod are known is considered. But the phase problem is solvable in principle, since the available intensities usually exceed the number of parameters needed to describe the structure.
Abstract: The electron density function of rho (r) in a crystal determines its diffraction pattern, that is, both the magnitudes and phases of its X-ray diffraction maxima, and conversely. If, however, as is always the case, only magnitudes are available from the diffraction experiment, then the density function rho (r) cannot be recovered. If one invokes prior structural knowledge, usually that the crystal is composed of discrete atoms of known atomic numbers, then the observed magnitudes are, in general, sufficient to determine the positions of the atoms, that is, the crystal structure. The intensities of a sufficient number of X-ray diffraction maxima determine the structure of a crystal. The available intensities usually exceed the number of parameters needed to describe the structure. From these intensities a set of members mod EH mod can be derived, one corresponding to each intensity. However, the elucidation of the crystal structure also requires a knowledge of the complex numbers EH= mod EH mod exp(i phi H), the normalized structure factors, of which only the magnitudes mod EH mod can be determined from experiment. Thus, a 'phase' phi H, unobtainable from the diffraction experiment, must be assigned to each mod EH mod , and the problem of determining the phases when only the magnitudes mod EH mod are known is called 'the phase problem'. Owing to the known atomicity of crystal structures and the redundancy of observed magnitudes mod EH mod , the phase problem is solvable in principle.
TL;DR: In this article, the theory of low-emittance optics is presented, including some important examples of magnet lattices, and a significant increase of the photon density is achieved by inserting special wiggler and undulator magnets into a storage ring, providing very high intensities of coherent synchrotron radiation.
Abstract: A relativistic electron passing through a bending magnet emits electromagnetic radiation in the forward direction with an extremely narrow opening angle. The photon density of this 'so-called synchrotron' radiation is correspondingly high and has, in addition, a very broad radiation spectrum. Because of its outstanding properties, synchrotron radiation has become a very powerful tool in basic research and technical applications. The required relativistic electron beam is provided by utilizing modern particle accelerator techniques. Presently the most successful accelerator type used as a dedicated synchrotron radiation source is the storage ring. Very important for a high quality of radiation is strong focusing of the electron beam circulating in the accelerator. The theory of this low-emittance optics is presented, including some important examples of magnet lattices. Inserting special wiggler and undulator magnets into a storage ring gives a significant increase of the photon density. In particular, the undulator magnets provide very high intensities of coherent synchrotron radiation. A logical consequence of the coherent undulator radiation was the development of the free electron laser (FEL), which is described in the last section of this review.
TL;DR: In this paper, the complexity of the nuclear spectrum remains as a stumbling block for a simple interpretation of the large mass of experimental data, which is a limitation of the sum rule technique.
Abstract: The interaction of electrons with nuclei is well understood and sufficiently weak to allow the separation of nuclear structure aspects from those connected with the reaction itself. Nevertheless the complexity of the nuclear spectrum remains as a stumbling block for a simple interpretation of the large mass of experimental data. Sum rules, which wash out the details of nuclear excited states, allow one to remain with the basic features of the nucleus. The rather long history of electronuclear sum rules has proved the power of the method, while the progress made in the many-body problem, together with the high accuracy of electron scattering data have transformed the sum rule techniques into stringent tests for nuclear theories and experiments.
TL;DR: A review of the physical processes involved in the greenhouse effect and current progress, theoretical and experimental, towards an understanding of the effect on the climate, especially the mean surface temperature, of recent and expected changes in atmospheric composition can be found in this article.
Abstract: On any planet with an atmosphere, the surface is warmed not only by the Sun directly but also by downward-propagating infrared radiation emitted by the atmosphere. On the Earth, this phenomenon, known as the greenhouse effect, keeps the mean surface temperature some 33 K warmer than it would otherwise be and is therefore essential to life. The radiative processes which are responsible for the greenhouse effect involve mainly minor atmospheric constituents, the amounts of which can change either naturally or as a by-product of human activities. The growth of the latter is definitely tending to force a general global surface warming, although because of problems in modelling complicated feedback processes, for example those involving water vapour, ozone, clouds, and the oceans, the precise rates of change and the local patterns which should be expected are not yet very well known. The author reviews the physical processes involved in the greenhouse effect and discusses current progress, theoretical and experimental, towards an understanding of the effect on the climate, especially the mean surface temperature, of recent and expected changes in atmospheric composition. It also provides an overview of recent expert forecasts of climatic changes in the next few decades, and discusses their limitations.
TL;DR: In this article, a review of the study of spatial features in diatomic liquids and gases is presented with a specific emphasis on experimental measurements using neutron and X-ray diffraction.
Abstract: The study of spatial features in diatomic liquids and gases is reviewed with a specific emphasis on experimental measurements using neutron and X-ray diffraction. The formalism for defining the pair-correlation function is described and related to the liquid structure factor which can be extracted from the diffraction measurements. Different molecular systems can be classified in terms of the intermolecular potential which depends on various factors related to geometrical conformation, electron density distribution and anisotropy. The increased complexity over monoatomic systems gives rise to new phenomena associated with the orientational correlation of molecular axes. The techniques of neutron and X-ray diffraction are critically reviewed and a full list of published work (plus some unpublished results) is presented. The experimental measurements are analysed and compared with the results of computer simulations using model potentials. It is noted that several systems (e.g. N2, O2, F2, CO) have a relatively simple behaviour but others (e.g. Cl2, Br2, I2) are only just on the point of being properly understood, while the hydrogen-bonded liquids (e.g. HF, HCl) still pose many problems that currently remain unresolved. The review ends with a brief consideration of likely developments in the foreseeable future.
TL;DR: In this paper, the relationship between the chemical and electronic structure of polyconjugated polymers, as well as the relation to surface structure, is discussed in comparison with the surfaces of inorganic semiconductors.
Abstract: Polymers as electronic materials are finding ever increasing use in the electronics industry. The study of the electronic and chemical structure of polymer surfaces and interfaces has received relatively little attention, however, compared with the surfaces of metals and inorganic semiconductors. The most interesting polymer materials today, from a point of view of electronic properties and future applications, are the polyconjugated polymers, which can be doped to a state of very high electrical conductivity, so-called 'conducting polymers', or merely conjugated polymers. The connection between the chemical (geometrical) and electronic structure of polyconjugated polymers, as well as the relation to surface structure, is discussed in comparison with the surfaces of inorganic semiconductors. It is shown that for well prepared samples, the surface electronic structure of certain ideal polymer materials can be equal to that in the bulk. In the initial stages of metal-polymer interface formation, however, certain electronic and chemical structural changes can occur. Two examples are discussed in detail: changes in the electronic structure observed at the surface of a (poly)conjugated polymer (i) upon doping, and (ii) upon the initial stages of metallization.
TL;DR: In this article, the performance of the experiments and the theory of electron cooling are described and the potential applications of cooled ion beams are presented, and the possibilities for the use of the cooling in experiments in elementary particle physics and the physics of atomic nuclei are discussed.
Abstract: Electron cooling is a method of reducing the beam phase space volume (emittance). In this review the performance of the experiments and the theory of electron cooling are described. The potential applications of cooled ion beams are presented. The possibilities for the use of the cooling in experiments in elementary particle physics and the physics of atomic nuclei are discussed.
TL;DR: In this paper, the effects of possible particle resonances X 0 in the energy range between threshold and a few MeV on electron-positron scattering were reviewed, and results of precision measurements of Bhabha scattering and e+e- annihilation in flight were also briefly surveyed.
Abstract: Electron-positron scattering provides a model-independent test for the existence of neutral particles coupling to the e+e- field. The authors review the effects of possible particle resonances X0 in the energy range between threshold and a few MeV on electron-positron scattering. The presentation focuses on theoretical aspects, but results of precision measurements of Bhabha scattering and e+e- annihilation in flight are also briefly surveyed.
TL;DR: In this paper, the basis properties and design criteria of superconducting accelerator magnets are reviewed with special emphasis on the following topics: field calculation and multipole expansion; layout of coils; influence of iron yoke; mechanical tolerances; magnetic forces and stresses in the coils; persistent currents in the superconductor filaments and resulting field distortions; stability of the magnets; quench origins, quench protection; performance of practical magnets; prospects for future applications.
Abstract: All new large hadron storage rings or colliders are equipped with superconducting magnets, which allow significantly higher particle energies to be achieved at much reduced operational costs. The basis properties and design criteria of superconducting accelerator magnets are reviewed with special emphasis on the following topics: field calculation and multipole expansion; layout of coils; influence of iron yoke; mechanical tolerances; magnetic forces and stresses in the coils; persistent currents in the superconductor filaments and resulting field distortions; stability of the magnets; quench origins, quench protection; performance of practical magnets; prospects for future applications.
TL;DR: In this article, the role of the pion plays when it interacts with nuclei in terms of its connection to chiral symmetry and the dynamical breaking of the latter or how can pion physics contribute to questions that are asked under the guidance of QCD.
Abstract: In the present study pion-nuclear scattering is reviewed emphasizing basic interactions and symmetries. It will be outlined that (while pions are still regarded widely as an exotic probe in nuclear physics studies) it has become clear in recent years that those investigations have reached their maturity and have become competitive with, and are complementary to, nuclear studies by means of more familiar probes like electrons, protons etc. Pion physics has improved the understanding of nuclear physics by more accurate experiments using better beams and more sophisticated techniques and superior equipment, by extremely complex theoretical calculations available promptly in response to the latest experimental results, and by discoveries which have yielded really new and surprising insight into many questions. Topics of fundamental interest in the future will certainly be how to elucidate the role that the pion plays when it interacts with nuclei in terms of its connection to chiral symmetry and the dynamical breaking of the latter or how can pion physics contribute to questions that are asked under the guidance of QCD.
TL;DR: A broad outline of the types of physical problems in which numerical simulations are used is given in this article, where various systems of equations which describe the collective behaviour of plasmas are discussed.
Abstract: In this review numerical simulations and methods, associated with the magnetic confinement of plasmas are described. A broad outline of the types of physical problems in which numerical simulations are used is given. The various systems of equations which describe the collective behaviour of plasmas are discussed. The simplest system of equations is the magneto-hydrodynamic (MHD) equations, which treat the plasma as a conducting fluid. Simulations using the MHD equations, which probably represent the largest area of numerical research in magnetic confinement, are described in some detail.
TL;DR: A review of the general principles involved in remote sensing is followed by a review of instruments designed to sense properties of the Earth's atmosphere and surface from near-Earth orbiting and geostationary satellites as discussed by the authors.
Abstract: A description of the general principles involved in remote sensing is followed by a review of instruments designed to sense properties of the Earth's atmosphere and surface from near-Earth orbiting and geostationary satellites. These include instruments for measuring atmospheric temperature, clouds, winds, precipitation and composition (water vapour and trace constituents). Emphasis is on general principles, but a few instruments are described in detail as examples. A list of space-borne atmospheric remote-sensing instruments is included. Methods for data reduction are outside the scope of this review.
TL;DR: In this article, the authors describe various non-energy-weighted sum rules for one-nucleon transfers and their applications, and they show how exact spin-dependent sum rules can be used in informative analyses of one nucleon transfer data on odd-mass nuclei, and how they can be combined with properties of nuclear structure.
Abstract: The single-particle structure of nuclei remains a subject of considerable experimental and theoretical interest. Much of the information on this subject comes from experiments in which a nucleon is transferred to or from a given nucleus to make states of adjacent nuclei. Such data can be examined critically using sum rules, and the authors describe various non-energy-weighted sum rules for one-nucleon transfers and their applications. For completeness, they include a section on the analogous two-nucleon transfer sum rules. They show how exact spin-dependent sum rules can be used in informative analyses of one-nucleon transfer data on odd-mass nuclei, and how they can be combined with properties of nuclear structure to yield approximate sum rules applicable to a subset of the data.