Showing papers in "Reports on Progress in Physics in 1995"
TL;DR: In this article, the authors review the progress in the theory of one-dimensional (ID) Fermi liquids which has occurred over the past decade and present a description of the low-energy properties of gapless 1D quantum systems can be based on the exactly solvable Luttinger model which incorporates these features, and whose correlation functions can be calculated.
Abstract: We review the progress in the theory of one-dimensional (ID) Fermi liquids which has occurred over the past decade. The usual Fermi liquid theory, based on a quasi-particle picture, breaks down in one dimension because of the Peierls divergence in the particle-hole bubble, producing anomalous dimensions of operators, and because of charge-spin separation. Both are related to the importance of scattering processes transferring finite momentum. A description of the low-energy properties of gapless 1D quantum systems can be based on the exactly solvable Luttinger model which incorporates these features, and whose correlation functions can be calculated. Special properties of the eigenvalue spectrum, parameterized by one renormalized velocity and one effective coupling constant per degree of freedom, fully describe the physics of this model. Other gapless 1D models share these properties in a low-energy subspace. The concept of a Luttinger liquid implies that their low-energy properties are described by an effective Luttinger model, and constitutes the universality class of these quantum systems. Once the mapping on the Luttinger model is achieved, one has an asymptotically exact solution of the 1D many-body problem. Lattice models identified as Luttinger liquids include the 1D Hubbard model off half-filling, and variants such as the t-J- or the extended Hubbard model. In addition, 1D electron-phonon systems or metals with impurities can be Luttinger liquids, as well as the edge states in the quantum Hall effect.
946 citations
TL;DR: In this article, the authors describe the properties of the triangular flux-line lattice (FLL), which is more or less perturbed by material inhomogeneities that pin the flux lines, and also by thermal fluctuations.
Abstract: Magnetic flux can penetrate a type-II superconductor in the form of Abrikosov vortices (also called flux lines, flux tubes, or fluxons) each carrying a quantum of magnetic flux phi 0=h/2e. These tiny vortices of supercurrent tend to arrange themselves in a triangular flux-line lattice (FLL), which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-Tc superconductors (HTSCs) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSCs the FLL is very soft mainly because of the large magnetic penetration depth lambda . The shear modulus of the FLL is c66~1/ lambda 2, and the tilt modulus c44(k)~(1+k2 lambda 2)-1 is dispersive and becomes very small for short distortion wavelengths 2 pi /k<< lambda . This softness is enhanced further by the pronounced anisotropy and layered structure of HTSCs, which strongly increases the penetration depth for currents along the c axis of these (nearly uniaxial) crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may `melt` the FLL and cause thermally activated depinning of the flux lines or ofthe two-dimensional `pancake vortices` in the layers. Various phase transitions are predicted for the FLL in layered HTSCs. Although large pinning forces and high critical currents have been achieved, the small depinning energy so far prevents the application of HTSCs as conductors at high temperatures except in cases when the applied current and the surrounding magnetic field are small.
866 citations
TL;DR: A review of the basic mechanisms of premelting and their roles in the environmental phenomena can be found in this article, where the principal results of numerous studies are reviewed, and trends in current research are outlined.
Abstract: Several mechanisms can extend the equilibrium domain of a liquid phase into the solid region of the normal phase diagram. The causes of premelting, which include surface melting, interface curvature and substrate disorder, occur in all types of substances, including H2O. In the case of H2O, premelting can have important environmental consequences, among which are the heaving of frozen ground, breakdown of rock and concrete, sintering of snow, flow of glaciers, scavenging of atmospheric trace gases by snow and ice, and the electrification of thunderclouds. The article reviews the basic mechanisms of premelting and discusses their roles in the environmental phenomena. The principal results of numerous studies are reviewed, and trends in current research are outlined.
590 citations
TL;DR: In this article, the authors present an exposition of the various theoretical models currently in use for describing the dynamics of molecular dissociation at surfaces and the methods that have been developed for treating the dissipative motion as the molecule nears the surface are presented.
Abstract: We present an exposition of the various theoretical models currently in use for describing the dynamics of molecular dissociation at surfaces. We begin by outlining the representations of the nuclear and electronic dynamics and how these define the potential energy surfaces for the interactions. Strategies for solving the nuclear motion follow with particular emphasis being paid to a quantum description on the electronic ground state which is in line with experiments employing hyperthermal molecular beams. These can be performed in either a time-dependent or time-independent fashion and both approaches are considered. Following this, the methods that have been developed for treating the dissipative motion as the molecule nears the surface are presented. This is divided into energy loss to the electronic subsystem and to the substrate atomic vibrations. The final part of the review shows how the results of theoretical simulations have been usefully applied to rationalize data obtained from molecular beam scattering experiments.
364 citations
TL;DR: An introduction to the basic ideas and methods of chiral perturbation theory is presented in this article, and several phenomenological applications of the effective Lagrangian technique to strong, electromagnetic and weak interactions are discussed.
Abstract: An introduction to the basic ideas and methods of chiral perturbation theory is presented. Several phenomenological applications of the effective Lagrangian technique to strong, electromagnetic and weak interactions are discussed.
363 citations
TL;DR: In this paper, a review of wave-packet dynamics with a focus on "femto-chemistry" is presented, where basic theory and simple properties of wavepackets, the application of two-state models to wave packet dynamics, and wavepacket processes in the presence of light.
Abstract: We present a review of wave-packet dynamics, with a focus on 'femto-chemistry'. We review basic theory and simple properties of wave-packets, the application of two-state models to wave-packet dynamics, and wave-packet processes in the presence of light. We also discuss the treatment of wave-packets in dissipative problems.
237 citations
TL;DR: In the field of low energy electron diffraction (LEED), the last decade has seen remarkable new developments as mentioned in this paper which are reviewed in the present paper, and the basics and the development of both TLEED and DLEED is reviewed.
Abstract: The last decade has seen remarkable new developments in the field of low energy electron diffraction (LEED) which are reviewed in the present paper. The arrival of sophisticated and fast techniques for the measurement of diffraction intensities in the early 1980s was a challenge to theory. Its answer was the development of tensor LEED (TLEED) in 1985. It allows the fast calculation of intensities for structures not too far from a certain reference. This made quantitative surface structure analysis approach new frontiers both with respect to structural complexity and-with the help of the routine use of reliability factors-to precision. Simultaneously, the new experimental techniques allowed access to two-dimensional intensity maps, i.e. the measurement of diffuse intensity distributions. Theory could be modified to calculate such distributions and so gave birth to the diffuse LEED technique (DLEED) which allows the retrieval of the local structure in case of disordered adsorption. The basics and the development of both TLEED and DLEED is reviewed. TLEED is the basis for direct methods and is used in effective search procedures for surface structures structurally close to a certain reference. It can also be used to simulate the substitution of surface atoms by different chemical species as well as to account for surface vibrations. In addition to carrying the information about the local structure, DLEED patterns very recently could be successfully interpreted in a holographic sense yielding real space images directly.
158 citations
TL;DR: An overview of the possibilities and limitations of secondary ion mass spectrometry as an analytical tool in the investigation of near-perfect, i.e., almost atomically sharp, dopant and impurity distributions is given in this paper.
Abstract: An overview is given of the possibilities and limitations of secondary ion mass spectrometry as an analytical tool in the investigation of near-perfect, i.e. almost atomically sharp, dopant and impurity distributions. The operating principles of the technique and the various quantification schemes are briefly presented. The most elaborate discussion pertains to the factors that determine the attainable depth resolution and what can be done to improve things, both from an experimental and from a theoretical point of view. Emphasis is placed on semiconductors and other brittle target materials, but the implications for metals are indicated.
113 citations
TL;DR: In this paper, a review of the H3+ molecular ion and its deuterated isotopomers is presented, focusing on its fundamental nature, astrophysical significance and dynamical richness.
Abstract: This review discusses the H3+ molecular ion and its deuterated isotopomers. The ion is important because of its fundamental nature, astrophysical significance and dynamical richness. The following topics are discussed: the discovery of H3+, its unusual bonding and the important role played by ab initio electronic-structure calculations; the formation of H3+ and its importance in models of the interstellar medium; the unusual spectroscopy of H3+ and the accurate quantum calculations which led to laboratory observations; the failure to detect H3+ in the interstellar medium and its accidental observation in Jupiter; work on H3+ in the giant planets and other astronomical emission spectra; the very unusual infrared photodissociation spectrum of H3+; and the classical and quantal behaviour of the molecule at its dissociation limit.
105 citations
TL;DR: In this paper, the authors provide an introduction to ultrafast laser development for scientists new to the field and provide a snapshot of the state-of-the-art of ultrafast lasers and to indicate how this state evolved over the last thirty years.
Abstract: Summary form only given. This article is intended to provide an introduction to ultrafast laser development for scientists new to the field. It also aims to provide a snapshot of the state-of-the-art of ultrafast lasers and to indicate how this state evolved over the last thirty years. In the first section, the main issues concerning ultrashort pulse generation are discussed and the most important ultrafast laser media are briefly reviewed, An extensive historical survey of mode-locking and pulse compression from 1964 until 1994 is then presented which covers the most important developments and aims to put recent advances and state-of-the-art femtosecond lasers in context. This review also anticipates future developments in practical ultrafast lasers for real-world applications, The basic techniques of mode-locking are then reviewed at a tutorial level. These include active mode-locking, passive mode-locking with real, resonant saturable absorbers and passive mode-locking with the optical Kerr effect. Emphasis is placed on ultrafast solid-state lasers, dye lasers and fibre lasers. Group velocity dispersion and self-phase modulation are introduced and their interaction discussed in some detail, Fibre-optic pulse compression is described and the significance of soliton shaping and solitary lasers is highlighted. Some of the phenomena limiting the minimum achievable duration of laser pulses are identified. The final section describes techniques for measuring ultrashort pulses including electrooptic streak cameras and second harmonic generation autocorrelation.
103 citations
TL;DR: A review of the observations that have detected or placed upper limits on the emission of X-rays from supernovae can be found in this article, where the authors briefly review the proposed emission mechanisms.
Abstract: Supernovae have been expected to be X-ray sources for many years. This paper reviews the observations that have detected or placed upper limits on the emission of X-rays from supernovae. A firm upper limit exists on X-ray emission from Type Ia supernovae based upon an observation of SN 1992A (parent galaxy: NGC 1380) made with ROSAT. The type II supernovae detected include SN 1978K (NGC 1313), SN 1980K (NGC 6946), SN 1986J (NGC 891), SN 1987A (Large Magellanic Cloud), and, most recently, SN 1993J (NGC 3031). These supernovae emit X-rays by at least two different processes. The author briefly reviews the proposed emission mechanisms.
TL;DR: In this article, the scope of measurements which can be made on fission gamma-rays and the techniques used in the experiments is reviewed, as well as some results recently obtained on nuclear structure of fragments and on beta-decays.
Abstract: High-resolution measurements on gamma -rays from fission fragments have given a wealth of information on the structure of neutron-rich nuclei and on the mechanism of the fission process. Experiments to study prompt gamma -rays which deexcite secondary fragments have been greatly facilitated by the introduction of large arrays of Compton suppressed gamma -ray detectors. These have been used recently to give important data on new neutron-rich nuclei with up to ten more neutrons than the nearest stable isotope. High-resolution experiments have also been performed to study gamma -rays which follow the beta - decays of the ground states of secondary fragments, and extensive data now exist on decay properties of nuclei with up to twelve more neutrons than the nearest stable isotope. This paper reviews the scope of measurements which can be made on fission gamma -rays and the techniques used in the experiments. It also discusses some results recently obtained on nuclear structure of fragments and on beta - decays, and reviews information on fragment spins and other data relevant to fission dynamics.
TL;DR: A critical review of the various observables that have been proposed to signal the change from dense hadronic matter to a quark-gluon plasma at high temperature or baryon density is given in this article.
Abstract: This is a critical review of the various observables that have been proposed to signal the change from dense hadronic matter to a quark-gluon plasma at high temperature or baryon density. I discuss current models of quark-gluon plasma formation in relativistic heavy-ion collisions and analyse the virtues and ambiguities of various signatures.
TL;DR: In this paper, the authors review the status of and recent developments in dynamical electroweak symmetry breaking, concentrating on the ideas of technicolour and top quark condensates.
Abstract: We review the status of and recent developments in dynamical electroweak symmetry breaking, concentrating on the ideas of technicolour and top quark condensates. The emphasis is on the essential physical ideas and experimental implications rather than on detailed mathematical formalism. After a general overview of the subject, we give a first introduction to technicolour, and extended technicolour, illustrating the ideas with a simple (unrealistic) model. Then we review the progress that has been made with enhancing the technicolour condensate, using the Schwinger-Dyson gap equation. The discussion includes the so-called walking technicolour and strong extended technicolour approaches. We then turn to the experimental prospects of technicolour models, including longitudinal gauge boson scattering experiments at the LHC, the detection of pseudo-Goldstone bosons and the hints about electroweak symmetry breaking which comes from precision measurements at LEP. We also discuss a low-scale technicolour model, which has experimental signatures at LEP and the Tevatron. Finally we turn to the idea of the top quark condensate. After reviewing the basic ideas of this approach, we turn to some extensions of these ideas involving fourth-family condensates and the role of irrelevant operators.
TL;DR: In this article, a new generation of ion storage-cooler rings dedicated to atomic and molecular physics is proposed. But the storage rings are not suitable for the study of very slow processes occurring in charged (positive and negative) atoms, molecules and clusters.
Abstract: Advances in ion-source, accelerator and beam-cooling technology have made it possible to produce high-quality beams of atomic ions in arbitrary charged states as well as molecular and cluster ions that are internally cold Ion beams of low emittance and narrow momentum spread are obtained in a new generation of ion storage-cooler rings dedicated to atomic and molecular physics The long storage times ( approximately 5
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TL;DR: In this paper, a review of modern theoretical approaches to turbulence can be seen as a branch of statistical field theory, and where the treatment has been strongly influenced by analogies with the quantum many-body problem.
Abstract: This review is concerned with modern theoretical approaches to turbulence, in which the problem can be seen as a branch of statistical field theory, and where the treatment has been strongly influenced by analogies with the quantum many-body problem. The dominant themes treated are the development (since the 1950s) of renormalized perturbation theories (RPT) and, more recently, of renormalization group (RG) methods. As fluid dynamics is rarely part of the physics curriculum, in section 1 we introduce some background concepts in fluid dynamics, followed by a skeleton treatment of the phenomenology of turbulence in section 2, taking flow through a straight pipe or a plane channel as a representative example. In section 3, the general statistical formulation of the problem is given, leading to a moment closure problem, which is analogous to the well known BBGKY hierarchy, and to the Kolmogorov -5/3 power law, which is a consequence of dimensional analysis. In section 4, we show how RPT have been used to tackle the moment closure problem, distinguishing between those which are compatible with the Kolmogorov spectrum and those which are not. In section 5, we discuss the use of RG to reduce the number of degrees of freedom in the numerical simulation of the turbulent equations of motion, while giving a clear statement of the technical problems which lie in the way of doing this. Lastly, the theories are discussed in section 6, in terms of their ability to meet the stated goals, as assessed by numerical computation and comparison with experiment.
TL;DR: A survey of work using resonanceenhanced multiphoton ionization and double-resonance techniques to study excited-state photoionization dynamics in molecules can be found in this article.
Abstract: This review presents a survey of work using resonance-enhanced multiphoton ionization and double-resonance techniques to study excited-state photoionization dynamics in molecules These techniques routinely provide detail and precision that are difficult to achieve in single-photon ionization from the ground state The review not only emphasizes new aspects of photoionization revealed in the excited-state experiments but also shows how the excited-state techniques can provide textbook illustrations of some fundamental mechanisms in molecular photoionization dynamics Most of the examples are confined to diatomic molecules
TL;DR: In this paper, the BC8 and ST12 structures of tetrahedrally coordinated semiconductors are reviewed. But the authors do not discuss the properties of these structures.
Abstract: Recent experimental and theoretical studies of exotic forms of tetrahedrally coordinated semiconductors are reviewed. These unusual phases are synthesized as long-lived metastable forms of the elemental semiconductors silicon and germanium by the application and subsequent removal of high pressure. Rather than being simply crystallographic oddities, the bonding arrangements in these phases show many similarities to those found in amorphous semiconductors. As a result, these dense structures have been used as so-called 'complex crystal' models for the amorphous state. Advances in experimental and computational techniques have recently allowed for detailed study of the structural, electronic and vibrational properties of these phases to be made under variable temperature and pressure conditions. In view of the considerable difficulties associated with performing theoretical studies of non-crystalline solids, the BC8 and ST12 structures are useful in that an understanding of their properties provides insight into the essential physics of amorphous tetrahedral semiconductors.
TL;DR: In this article, the authors proposed neutrinoless double beta decay (NB) as a probe of the Majorana nature of the neutrino, which is considered as one of the most powerful probes to search for physics beyond the Standard Model.
Abstract: It is certainly fair to say that understanding the nature of the neutrino is one of the most critical issues in particle physics, nowadays. In fact, a lot of experimental and theoretical work is being devoted to this problem. To study the properties of neutrinos there are essentially two different approaches. The first one makes use of neutrino (antineutrino) beams from high-energy accelerators and nuclear reactors, or takes advantage of the scarce neutrinos provided to us by nature (solar or cosmic neutrinos). The other approach consists in searching for processes where neutrinos do not appear: either they annihilate each other or they are replaced by some (more or less exotic) particle. For instance, neutrinoless double beta decay was proposed a long time ago as a probe of the Majorana nature of the neutrino. We now recognize the utmost importance of neutrinoless double beta decay in the context of the modern gauge theories and the quest for physics beyond the Standard Model: some fundamental issues are the problem of neutrino masses, the presence of right-handed charged weak currents, the existence of Goldstone bosons (majorons). Another well-known example is the neutrinoless process μ → eγ, which historically has led to the “two neutrino hypothesis”, and is nowadays considered as one of the most powerful probes to search for physics beyond the Standard Model, as we shall see below.
TL;DR: The application of the finite element method in electromagnetics is reviewed in this article, where the emphasis is on formulations of 3D electromagnetic problems that are suitable for finite-element analysis.
Abstract: The application of the finite element method in electromagnetics is reviewed. The emphasis is on formulations of 3D electromagnetic problems that are suitable for finite-element analysis and difficulties that have been encountered, such as spurious modes and gauging of vector potentials. Following an introduction to scalar and vector finite elements and a discussion of techniques for handling unbounded domains, the three main areas of application ape covered: magnetic fields, electric fields, and electromagnetic waves. In the analysis of magnetic fields, the various possible potential formulations are explained and compared; magnetic material models are reviewed; and force calculation techniques are summarized. For electric fields complications arise when the relationship between electric field and charge movement is not simple: several different cases are considered. Electromagnetic wave problems are of two types: eigenvalue (resonant cavities and waveguides) and deterministic (radiation and scattering). The principle difficulty here is the occurrence of spurious (non-physical) modes, which can affect both types of problem. The reasons for spurious modes and the available remedies are outlined.
TL;DR: In this paper, a review of particle simulation of plasmas in a particular type of fusion experiment, namely the tokamak, is presented, with particle methods constituting the first, and the remaining four concern the solution via mesh-based methods of the Fokker-Planck equation, drift-wave problems, edge models and time-dependent magnetohydrodynamic problems.
Abstract: The review specializes to the modelling of plasmas in a particular type of fusion experiment, namely the tokamak. Simulation is taken to imply the use of a model which involves variation in at least two coordinate directions and is nonlinear, the nonlinearity invariably being of the advective type. Developments in the period 1976-1992 are covered under five main headings, with particle methods constituting the first. The remaining four concern the solution via mesh-based methods of (1) the Fokker-Planck equation, (2) drift-wave problems, (3) edge models and (4) time-dependent magnetohydrodynamic problems. Care is taken to outline the capabilities of the currently available software. Progress in the. Design of numerical algorithms for the mesh-based simulations is found to have been incremental rather than revolutionary. In particle simulation, gyrokinetic schemes and the ' delta f' method have been found to give dramatic gains in some circumstances. Many of the newer results obtained withstand comparison with experimental observation, although it has not always proved possible to reach the extreme conditions found in tokamaks, especially when three-dimensional effects are important.
TL;DR: In this article, the authors discuss the mechanisms based on the supposition that variations in the transparency of the atmosphere at the stratospheric level occur in association with various phenomena resulting from solar activity.
Abstract: Solar energy is radiated into space mainly in two forms: as electromagnetic radiation energy over a wide range of wavelengths, and as kinetic and thermal energy of the solar wind plasma. The former freely propagates through the interplanetary space, and only undergoes some changes and transformations in the atmospheres of the Earth and of other planets. In contrast to this, the solar wind plasma energy is continually transferred from one form to another. The most effective processes of energy conversion take place within interplanetary shocks, in magnetic barrier regions, and in magnetic field reconnection layers. A short description of these processes is presented. There has been extensive research into the possible mechanism(s) linking solar activity to the state of the lower atmosphere and the weather. The author discusses the mechanisms based on the supposition that variations in the transparency of the atmosphere at the stratospheric level occur in association with various phenomena resulting from solar activity. The author also reviews the corresponding literature.
TL;DR: In this paper, the origin and very early evolution of binary systems are discussed, as well as a discussion of some binaries that do not yet seem to fit into the scheme, and the authors suggest that the wide variety of observed binary systems can be explained as the result of the way in which these processes affect binaries of various initial properties at different stages of the evolution of the component stars.
Abstract: Stars in binary systems evolve in the way predicted by model calculations for single stars, as long as the two components of the binary are sufficiently far apart that they do not interfere with each other. An evolving star expands, however, and the components of many binaries are close enough to each other so that they may begin to interfere in the early stages of evolution. When this happens, mass may be transferred from one component to the other, or be lost (together with its associated angular momentum) from the system. Astronomers believe that the wide variety of observed binary systems can be explained as the result of the way in which these processes affect binaries of various initial properties at different stages of the evolution of the component stars. The theory is not yet, however, fully quantitative. Some discussion of the origin and very early evolution of binaries is included, as is also a discussion of some binaries that do not yet seem to fit into the scheme.