Showing papers in "Reviews of Modern Physics in 1988"
TL;DR: In this article, the authors review the theoretical models that have been developed to describe the physics of polyacetylene and related conducting polymers and summarize the relevant experimental results obtained for these materials.
Abstract: Self-localized nonlinear excitations (solitons, polarons, and bipolarons) are fundamental and inherent features of quasi-one-dimensional conducting polymers. Their signatures are evident in many aspects of the physical and chemical properties of this growing class of novel materials. As a result, these polymers represent an opportunity for exploring the novel phenomena associated with topological solitons and their linear confinement which results from weakly lifting the ground-state degeneracy. The authors review the theoretical models that have been developed to describe the physics of polyacetylene and related conducting polymers and summarize the relevant experimental results obtained for these materials. An attempt is made to assess the validity of the soliton model of polyacetylene and its generalization to related systems in which the ground-state degeneracy has been lifted.
2,907 citations
TL;DR: In this article, a review of advances in the study of solid surfaces and thin films using variable-energy positron beams is presented, with more emphasis on the most recent measurements and interpretations than on the chronology of various developments.
Abstract: Recent advances in the study of solid surfaces and thin films using variable-energy positron beams are reviewed. In the first part the authors discuss the process of positron moderation and technical aspects of positron beam production and application. The second part is (roughly) organized in sections that apply to increasing time scales appropriate to the positron-solid interaction. These are (a) first encounter and scattering effects, (b) energy loss and stopping profiles, (c) diffusion of thermalized positrons, (d) positron-surface interactions, and (e) studies of defects near surfaces and interfaces. The review is written with more emphasis on the most recent measurements and interpretations than on the chronology of various developments.
1,339 citations
TL;DR: In many materials with a highly anisotropic band structure, electron-phonon interactions lead to a novel type of ground state called the charge-density wave as mentioned in this paper, which can, even for small electric fields, carry current in a fashion originally envisioned by Frohlich.
Abstract: In many materials with a highly anisotropic band structure, electron-phonon interactions lead to a novel type of ground state called the charge-density wave. The condensate is pinned to the underlying lattice by impurities and by boundary effects, but can, even for small electric fields, carry current in a fashion originally envisioned by Fr\"ohlich. This review discusses some of the underlying theories and the main experimental observations on this new collective transport phenomenon. The frequency- and electric-field-dependent conductivity, current oscillations, electric-field-dependent transport coefficients and elastic properties, together with nuclear-magnetic-resonance experiments, provide clear evidence for a translational motion of the condensate. Various theories, involving classical and quantum-mechanical concepts, are able to account for a broad variety of experimental findings, which were also made in the presence of combined dc and ac fields.
1,308 citations
TL;DR: In this paper, a review of the detailed mechanisms for resistivity with an approximately 1/f 1 spectrum is presented, and several new techniques are explained. And connections with other widespread phenomena, such as highly nonexponential dielectric relaxations, are briefly investigated.
Abstract: Fluctuations in resistivity with an approximately $\frac{1}{f}$ spectrum are found in most conducting materials. Some of the detailed mechanisms, which turn out to be quite diverse, are explored in this review. Several new techniques are explained. Various theories are critically examined. Some connections with other widespread phenomena, such as highly nonexponential dielectric relaxations, are briefly investigated.
1,177 citations
TL;DR: A comprehensive account of the macroscopic and microscopic physical properties of chromium (and where appropriate those of its dilute alloys) that relate to its antiferromagnetism is given in this paper.
Abstract: A comprehensive account is given of the macroscopic and microscopic physical properties of chromium (and where appropriate those of its dilute alloys) that relate to its antiferromagnetism. Neutron scattering is treated in great detail, first in the historical introduction, then as an experimental probe of both the magnetic structure and the excitations of the incommensurate spin-density-wave state and (with the assistance of x rays) of the concomitant charge-density wave and strain wave. Neutron scattering is considered as a tool to explore not only the disappearance of long-range order with increasing temperature through the growth of excitations as the weak first-order N\'eel transition is approached, but also the persistence of these spin fluctuations well into the paramagnetic state---processes that are still little understood. The article surveys, without mathematical details, model systems designed to reproduce the magnetic and thermodynamic properties of Cr. The energy-band structure calculations are given a more comprehensive review. Special attention is paid to calculations of the wave-vector-dependent susceptibility that reproduce the observed wave vector of the spin-density wave, and to a recent finite-temperature calculation that gives almost the right N\'eel temperature. The review of Fermi-surface studies emphasizes those designed to relate the spin-density wave vector (and its pressure dependence) to the nesting vector of the Fermi surface. An account is given of the spectroscopic determination of the energy gap(s), whose theoretical analysis is still unclear, and of experiments aimed at determining physical properties that throw light on the origin of the weak first-order N\'eel transition. The article describes the use of magnetic anomalies in the elastic moduli to determine the volume dependence of the exchange interaction responsible for antiferromagnetism in Cr. The experimental features of the spin-flip transition are reviewed, although a theory of this phenomenon is wanting. The experimental study of microscopic structure by the use of hyperfine-interaction properties is surveyed. An account is given of both experimental and theoretical studies of the surface of Cr and of Cr films and sandwiches. Finally, "technical antiferromagnetism" is discussed: the effect of severe internal strain in producing a commensurate antiferromagnetic state, wave-vector Q domains, polarization S domains (for which the experimental evidence is scanty), and ultrasonic attenuation as a tool to study them.
866 citations
TL;DR: In this paper, recent progress made towards the understanding of closed bosonic and fermionic string perturbation theory, formulated in a Lorentz-covariant way on Euclidean space-time, is devoted to recent progress.
Abstract: This paper is devoted to recent progress made towards the understanding of closed bosonic and fermionic string perturbation theory, formulated in a Lorentz-covariant way on Euclidean space-time. Special emphasis is put on the fundamental role of Riemann surfaces and supersurfaces. The differential and complex geometry of their moduli space is developed as needed. New results for the superstring presented here include the supergeometric construction of amplitudes, their chiral and superholomorphic splitting and a global formulation of supermoduli space and amplitudes.
721 citations
TL;DR: In this article, a theoretical approach to surface structure and reactivity is given, which is within the framework of solid-state theory, yet strives for chemical ways of interpretation. But it is quite feasible to construct interaction diagrams for surfaces, and to make frontier orbital arguments, just as for molecules.
Abstract: An account is given of a theoretical approach to surface structure and reactivity that is within the framework of solid-state theory, yet strives for chemical ways of interpretation. One begins from highly delocalized band structures, but introduces interpretational tools (density-of-states decompositions, crystal orbital overlap populations) that allow a tracing of local, chemical acts. It is quite feasible to construct interaction diagrams for surfaces, and to make frontier orbital arguments, just as for molecules. There are some interesting ways in which the surface-adsorbate interaction differs from simple molecular binding---in particular, in the way that two-orbital four-electron and zero-electron interactions can turn into bonding. The surface and bulk acting as a reservoir of electrons or holes at the Fermi level are important in this context. Chemisorption emerges as a compromise in a continuum of bonding whose extremes are dissociative adsorption and surface reconstruction.
700 citations
TL;DR: A review of the current status of research into the problem of two-dimensional melting, with an emphasis on computer simulations, is given in this article, where an attempt is made to point out unresolved issues pertaining to this fascinating and still open question.
Abstract: For a decade now the subject of the nature of the two-dimensional melting transition has remained controversial. An elegant theory based on the unbinding of pairs of crystal defects suggested that two-dimensional solids might melt by a transition sequence involving two continuous transitions separated by a novel, nearest-neighbor-bond-orientationally ordered fluid---the hexatic phase. Competing theories predict that the transition is of the usual first-order type observed in three-dimensional systems. This paper is a critical review of the current status of research into the problem of two-dimensional melting, with an emphasis on computer simulations. An attempt is made to point out unresolved issues pertaining to this fascinating and still open question.
699 citations
TL;DR: In this paper, the event rates and their recognized uncertainties are calculated for 11 solar neutrino experiments using accurate solar models and the models are also used to evaluate the frequency spectrum of the p and g oscillations modes of the sun.
Abstract: The event rates and their recognized uncertainties are calculated for 11 solar neutrino experiments using accurate solar models These models are also used to evaluate the frequency spectrum of the p and g oscillations modes of the sun It is shown that the discrepancy between the predicted and observed event rates in the Cl-37 and Kamiokande II experiments cannot be explained by a 'likely' fluctuation in input parameters with the best estimates and uncertainties given in the present study It is suggested that, whatever the correct solution to the solar neutrino problem, it is unlikely to be a 'trival' error
642 citations
IBM1
TL;DR: In this article, a review on the problems of high-temperature superconductivity is presented, and ideas encouraging the search for high-time superconductivities are elucidated.
Abstract: The review dwells on the problems of high-temperature superconductivity. Ideas encouraging the search for high-temperature superconductivity are elucidated. The way from cubical alloys, containing niobium, to laminated copper-containing oxides with the perovskite-type structure is shown. Properties of new laminated oxide superconductors are described.
492 citations
TL;DR: In this paper, the authors used the convolution method to calculate straggling functions for thin silicon absorbers and compared the results with those obtained from other theories (Landau) for relativistic particles (e.g., electrons, mesons, protons, and their antiparticles).
Abstract: Experimental and theoretical data for dielectric functions, x-ray absorption coefficients, and generalized oscillator strengths needed for a description of the energy-loss spectrum of fast charged particles in solid silicon are given. Theories used to calculate spectra of total energy loss ("straggling spectra") are described. The convolution method is used to calculate straggling functions for thin silicon absorbers. They are compared with those obtained from other theories (Landau). For relativistic particles ($\ensuremath{\gamma}g100$), the Vavilov-Shulek theories give incorrect functions for absorbers of thicknesses $tl1$ mm. The conversion of energy-loss spectra into ionization spectra is discussed, and the latter are compared with experimental functions. Good agreement is found between calculated and observed functions for electrons, mesons, protons, and their antiparticles and for $\ensuremath{\alpha}$ particles. From this agreement, the error ($1\ensuremath{\sigma}$) of the theoretical values of the most probable energy loss ${\ensuremath{\Delta}}_{p}$ and the full width at half maximum, $w$, is estimated to be less than \ifmmode\pm\else\textpm\fi{}1%.
TL;DR: In contrast to the case of hydrogen, which is not known to form a paramagnetic state in semiconductors, two coexisting types of muonium states are seen as discussed by the authors.
Abstract: Positive muons implanted into diamond and zincblende-structured semiconductors often form hydrogen-like paramagnetic muonium (${\ensuremath{\mu}}^{+}\ensuremath{-}{e}^{\ensuremath{-}}$) states whose characteristics can be investigated with the "muon spin rotation" ($\ensuremath{\mu}\mathrm{SR}$) technique. In contrast to the case of hydrogen, which is not known to form a paramagnetic state in semiconductors, two coexisting types of muonium states are seen. "Mu" with a large isotropic hyperfine interaction, and "${\mathrm{Mu}}^{*}$" with a small [111]-axially symmetric hyperfine interaction. Both "spectroscopic" properties of these states, such as the electronic $g$ factors and the nuclear hyperfine interactions, and "dynamic" properties, such as their diffusion rates and their rates of interconversion, are accessible with $\ensuremath{\mu}\mathrm{SR}$. Direct information about the site of the muonium states is available using the channeling effect of the positron from muon decay in a crystalline host. The techniques for probing semiconductors with positive muons are described in this review, and the results they have provided to date are critically discussed. The considerable amount of theoretical work that has been invested in microscopic models of Mu and ${\mathrm{Mu}}^{*}$ is also summarized.
TL;DR: In this article, the authors report detailed experiments and comparison with first-principle theoretical calculation of the diffraction of cold neutrons at single and double-slit assemblies of dimensions in the 20-100 µm range.
Abstract: The authors report detailed experiments and comparison with first-principle theoretical calculation of the diffraction of cold neutrons ($\ensuremath{\lambda}\ensuremath{\approx}2$ nm) at single- and double-slit assemblies of dimensions in the 20-100 \ensuremath{\mu}m range. Their experimental results show all predicted features of the diffraction patterns in great detail. Particularly, their double-slit diffraction experiment is its most precise realization hitherto for matter waves.
TL;DR: The IBA-1 is reviewed in this article with particular emphasis on the symmetry structure that arises naturally from its inherently algebraic approach, and the limitations of the model, and efforts to ameliorate these by appropriate extensions, are also reviewed in some detail.
Abstract: The IBA-1 is reviewed with particular emphasis on the symmetry structure that arises naturally from its inherently algebraic approach. The formulation of the model, in both its algebraic and its numerical aspects, is presented and the basic character of its predictions is discussed and compared with nuclear data. The limitations of the model, and efforts to ameliorate these by appropriate extensions, are also reviewed in some detail. An effort is made to provide a simple, transparent understanding of the basic physics by clarifying the relation of the mathematical structure to underlying physical ideas, and to provide guidance in understanding and carrying out practical calculations.
TL;DR: In this paper, the authors review the ideas and manifestations of QCD coherence in high-energy reactions producing jets and suggest experiments involving two-jet production, high-${p}_{\ensuremath{\perp}}\ensuredmath{\gamma}$ production and high-$ {p}$ W$ production at hadronic colliders.
Abstract: The authors review the ideas and manifestations of QCD coherence in high-energy reactions producing jets. They suggest experiments involving two-jet production, high-${p}_{\ensuremath{\perp}}\ensuremath{\gamma}$ production and high-${p}_{\ensuremath{\perp}}W$ production at hadronic colliders as ways to see QCD coherence in stringlike effects. They also suggest a procedure for finding the dip in the inclusive hadron spectrum of jets for those jets produced at hadron colliders. Simple explanations of the various coherence effects in QCD are discussed, as is the idea of local parton-hadron duality.
TL;DR: In this article, the adiabatic quantum transport in multiply-connected systems is examined, where the main theme is the differential-geometric interpretation of Kubo's formulas as curvatures.
Abstract: The adiabatic quantum transport in multiply connected systems is examined. The systems considered have several holes, usually three or more, threaded by independent flux tubes, the transport properties of which are described by matrix-valued functions of the fluxes. The main theme is the differential-geometric interpretation of Kubo's formulas as curvatures. Because of this interpretation, and because flux space can be identified with the multitorus, the adiabatic conductances have topological significance, related to the first Chern character. In particular, they have quantized averages. The authors describe various classes of quantum Hamiltonians that describe multiply connected systems and investigate their basic properties. They concentrate on models that reduce to the study of finite-dimensional matrices. In particular, the reduction of the "free-electron" Schr\"odinger operator, on a network of thin wires, to a matrix problem is described in detail. The authors define "loop currents" and investigate their properties and their dependence on the choice of flux tubes. They introduce a method of topological classification of networks according to their transport. This leads to the analysis of level crossings and to the association of "charges" with crossing points. Networks made with three equilateral triangles are investigated and classified, both numerically and analytically. Many of these networks turn out to have nontrivial topological transport properties for both the free-electron and the tight-binding models. The authors conclude with some open problems and questions.
TL;DR: In this paper, a unified formulation and review of an extensive class of radiation effects and devices based on free or quasifree electrons is presented, including slow-wave radiators, periodic bremsstrahlung radiators and transverse-binding radiators.
Abstract: This article presents a unified formulation and review of an extensive class of radiation effects and devices based on free or quasifree electrons. The effects and devices reviewed include slow-wave radiators [such as \ifmmode \check{C}\else \v{C}\fi{}erenkov, Smith-Purcell, and TWT (traveling-wave tube) effects and devices], periodic bremsstrahlung radiators [such as undulator radiation, magnetic bremsstrahlung FEL's (free-electron lasers), and coherent bremsstrahlung in the crystal lattice], and transverse-binding radiators [such as the CRM (cyclotron resonance maser) and channeling radiation]. Starting from a general quantum-electrodynamic model, both quantum and classical effects and operating regimes of these radiation devices are described. The article provides a unified physical description of the interaction kinematics, and presents equations for the characterization of spontaneous and stimulated radiative emission in these various effects and devices. Universal relations between the spontaneous and stimulated emission parameters are revealed and shown to be related (in the quantum limit) to Einstein relations for atomic radiators and (in the classical limit) to the relations derived by Madey for magnetic bremsstrahlung FEL for on-axis radiative emission. Examples for the application of the formulation are given, estimating the feasibility of channeling radiation x-ray laser and optical regime Smith-Purcell FEL, and deriving the gain equations of magnetic bremsstrahlung FEL and CRM for arbitrary electron propagation direction, structure (wiggler) axis, and radiative emission angle.
TL;DR: A detailed overview of the current status of the nucleon-nucleon forces can be found in this paper, where the authors show how the small-impact-parameter nucleon nucleon scattering can be explained by the quark structure of nucleons and why this quark model is successful in reproducing the energy dependence of the "measured" $S$- and $P$- wave nucleonnuclear phase shifts.
Abstract: This review gives a detailed overview of the current status of our understanding of the nucleon-nucleon forces. The authors review the known long-range meson exchange forces and explain how these forces originate from an underlying quark model constrained by chiral symmetry, a symmetry that is very well satisfied in low-energy nuclear phenomena. These effective meson exchange forces describe the large-impact-parameter nucleon-nucleon scattering. The authors show how the small-impact-parameter nucleon-nucleon scattering can be explained by the quark structure of the nucleons and why this quark model is successful in reproducing the energy dependence of the "measured" $S$- and $P$- wave nucleon-nucleon phase shifts.
TL;DR: The detection of the neutrino burst produced at core collapse has confirmed basic ideas of Type-II supernova causality and energetics (without being able to distinguish prompt from delayed shock ejection) as discussed by the authors.
Abstract: After a period of initial confusion, we now seem to understand in some detail why a star like Sanduleak -69°202 gave rise to a supernova like 1987A. The detection of the neutrino burst produced at core collapse has confirmed basic ideas of Type-II supernova causality and energetics (without being able to distinguish prompt from delayed shock ejection). Constraints on the nature of the neutrinos themselves confirm or extend laboratory limits on, and conventional theory for, rest mass, magnetic moment, etc. The unusual early light curve and spectra turn out to be a natural result of core collapse when the star was compact and blue, after passing through a red, extended phase (confirmed by evidence for mixing and mass loss characteristic of red supergiants). Synthesis of about 0.07M of Ni56 (as was previously suspected in SNII's with exponentially tailed light curves) has been revealed by a similar light curve and by an infrared line, gamma rays, and hard x rays, all widely believed to reflect the presence and decay of Ni56 to Co56 to Fe56. A number of puzzles remain concerning unexpected variable soft-x-ray emission, a possible speckle companion, and evidence for extensive mixing, fragmentation, and filamentation. Most of the lessons still to be learned from 1987A probably pertain to the detailed (three-dimensional) structure and composition of evolved massive stars rather than to the basic physics of Type-II supernovae. © 1988 The American Physical Society.