Showing papers in "Physics-Uspekhi in 1978"

The cathode spot of a vacuum arc

Abstract: In this review we analyze the current state of experiments and theory relating to the cathode spot of a vacuum arc. We discuss technical features of the experimental study of the near-cathode region of a vacuum arc with high time and space resolution. We discuss in detail the experimental data on such basic parameters characterizing the cathode spot as velocity, nature of motion, and lifetime of the spot, the current to the spot, current density, cathode fall, charge-transfer coefficient, vapor-jet velocity, charged-particle concentration, and electron temperature. We evaluate the reliability of the results obtained by various authors. A classification of cathode spots is proposed, and conditions for the existence of various types of spots and the behavior in the transition from one type to another are described. On the basis of the experimental data considered, we propose the explosive and diffusion models which provide qualitative and sometimes quantitative descriptions of cathode spots of various types.

Electron cooling and new possibilities in elementary particle physics

Abstract: This review is devoted to a new method in experimental physics: electron cooling, which opens the possibility of storing intense and highly monochromatic beams of heavy particles and carrying out a wide range of experiments with high luminosity and resolution. The method is based on the cooling of beams by an accompanying electron flux as the result of Coulomb collisions of the particles. In the first part of the review the theoretical basis of the method is briefly considered. The apparatus NAP-M is described with its electron cooling, and the results of successful experiments on cooling a proton beam are presented. In the second part the new possibilities opened by the use of electron cooling are discussed: storage of intense beams of antiprotons and achievement of proton-antiproton colliding beams, performance of experiments at the ultimate in low energies (with participation of antiparticles), storage of polarized antiprotons and other particles, production of antiatoms, storage of antideuterons, and experiments with ion beams.

Anomalous photovoltaic effect in ferroelectrics

Abstract: It is well known that the photovoltage in semiconductors, whatever its nature, cannot exceed the energy gap, which is several volts. For instance, in a homogeneous semiconductor the Dember (diffusion) photovoltage does not exceed the following value [3.6] for an infinitely large intensity of the exciting light: $$ V = \frac{{kT}}{q}\ell n\frac{{{n_1}}}{{{n_0}}} < < \frac{{kT}}{q}\ell n\frac{{{N_c}}}{{{n_0}}} = {E_g}.$$ (6.1)

Thermodynamic states and symmetry of liquid crystals

Abstract: All the diverse types of liquid-crystalline states can be described in a unified manner by analogy with the group-theoretical description of the symmetry and properties of crystals. The symmetry properties of liquid crystals with respect to the orientation of the molecules, and the conditions for the appearance of a modulated orientational structure, are considered. The phase diagrams in liquid crystals are described, and a general scheme of the phase transitions between liquid-crystalline phases is given. The problem of the ferroelectric state in liquids is considered, and the classes of anisotropic liquids that possess ferroelectric and piezoelectric properties are demonstrated. A thermodynamic description is given of the ferroelectric phenomena observed experimentally in smectic liquid crystals. The question of the physical nature of the low-symmetry smectic phases is discussed and the character of the phase transitions in liquid crystals is considered.

Stochastic self-oscillations and turbulence

Abstract: Until quite recently, it was thought that turbulence, i.e., stochastic self-oscillations of a continuous medium, was related exclusively to the excitation of an exceedingly large number of degrees of freedom. This review is devoted to the discussion of new ideas on the nature of the unpredictable turbulent motions in dissipative media connected with the discovery of strange attractors, i.e., attractive regions in phase space within which all paths are unstable and behave in a very complex fashion (motions on an attractor of this kind are characterized by a continuous spectrum). Turbulence represented by a strange attractor is described by a finite number of degrees of freedom, i.e., modes whose physical nature may be different. The example of a simple electronic noise generator is used to illustrate how the instability (divergence) of such paths leads to stochastic behavior. The analysis is based on the introduction of a nonreciprocally single-valued Poincare mapping onto itself, which is then used to describe the strange attractors encountered in different physical problems. An example of this mapping is used to demonstrate the discrete, symbolic, description of dynamic systems. The properties of such systems which indicate their stochastic nature, for example, positive topologic entropy and hyperbolicity are discussed. Specific physical mechanisms leading to the appearance of stochastic behavior and characterized by a continuous time spectrum are discussed. Strange attractors that appear in the case of parametric instability of waves in plasmas, laser locking by an external field, and so on, are demonstrated. "Attractor models" of hydrodynamic turbulence are reviewed, and in particular, finite-dimensional hydrodynamic models of convection in a layer and of Couette flow between rotating cylinders are constructed and found to exhibit stochastic behavior.

Dynamic polarization of nuclei in solid dielectrics

Abstract: A review is given of the present state of the theory of dynamic nuclear polarization (DNP) by the dipole electron-nucleus interaction in solids. The two-particle (solid effect), three-particle (electron-nucleus cross relaxation), and many-particle (dynamic cooling) mechanisms of DNP are discussed both for idealized conditions and under inhomogeneous broadening of the ESR line. The effect of foreign impurities, spin diffusion, phonon bottlenecks, and very low temperatures is analyzed. The treatment is based on a unified approach based on the concepts of spin temperature and thermal mixing in a rotating coordinate frame. The review covers experimental data illustrating the DNP mechanisms, possible applications of this phenomenon in the development of highly polarized proton targets, and so on. A brief discussion is also given of other DNP methods and mechanisms (the Overhauser effect and optical nuclear polarization).

Liquids in ultrathin channels (Filament and cluster crystals)

Abstract: CONTENTS 1. Introduction 77 2. Channel Matrices—Zeolites and Asbestos Minerals 78 3. Filling of Channels of Atomic Diameters with Materials 79 4. Behavior of Non-Wetting Liquids in Capillaries of Atomic Diameters. Surface Tension and Heat of Vaporization 80 5. Physical Properties of Systems of Clusters and Filaments 81 References 82

"Jarring" of a quantum system and the corresponding stimulated transitions

Abstract: A systematic theory of sudden perturbations is derived for quantum systems whose states are described both by wave functions (a pure ensemble) and by a quantum density operator (a mixed ensemble). A perturbation series is written in powers of the parameter ωτ, which is small when the perturbation is "sudden"; ω is the typical eigenvalue of the unperturbed system; and τ is the characteristic collision time. When the perturbation (t), taken at different times, commutes with itself, the theory yields a compact analytic expression for the probabilities for stimulated transitions for any value of Vτ /. The results of many cross-section calculations for atomic collision processes are discussed from a common standpoint: the processes are treated as "jarring" processes which stimulate transitions in the quantum system. If a momentum δp is rapidly transferred to the system in a collision, regardless of the physical nature of the "jarring," the probabilities for the stimulated transitions are governed by the parameter N ~δpδR/ where δR is a measure of the uncertainty in the coordinates which is due to the relatively slow motions in the unperturbed system.

Resonance light pressure

Abstract: The forces acting on a resonance particle in the fields of laser radiation sources can be large. Acceleration, heating, and cooling of atoms by using these forces are discussed. Since the forces are of a resonance type, one can employ them to separate isotopes and excited atoms from unexcited atoms. Light pressure can play an important role in the spectroscopy of narrow atomic and molecular resonances. The quantum features of the motion of particles in a standing light wave are manifested in a fine structure of the absorption coefficient. This review is concerned with analyzing the resonance light-pressure forces and discussing the application of them in optics and quantum electronics.

On the nature of turbulence

Abstract: The definition of turbulence and the differences between turbulence and random wave motions of liquids or gases are discussed. The Landau scheme of the generation of turbulence with increasing Reynolds number as a result of a sequence of normal bifurcations that creates a quasiperiodic motion is considered; several examples are discussed, including flow between rotating cylinders, convection at small Prandtl numbers, and the boundary layer at a flat plate. Results obtained in recent years from the ergodic theory and associated with the discovery of strange attractors in the phase spaces of typical dynamic systems are described. Flows with inverse bifurcations are considered, including the plane Poiseuille flow and Lorenz's example with idealized three-mode convection at large Prandtl numbers. In the latter case, the results of numerical calculations are analyzed and point to the existence of a strange attractor with the structure of a Cantor discontinuum; other examples of systems with strange attractors are also considered. It becomes clear that strange attractors in the phase spaces of systems with few modes may explain their nonperiodic behavior, but cannot explain why turbulence has a continuous spatial spectrum.

Ferromagnetism of disordered systems

Abstract: This review concerns disordered ferrotnagnets in which the magnetic atoms are randomly located on sites of a crystal lattice. Attention is given principally to magnets with strong spatial fluctuations of the exchange interaction. Naturally the magnetism of such systems differs substantially from the properties of ordinary ferromagnets; and for them, as for strongly disordered nonmagnetic systems, the methods of perturbation theory are inapplicable. The general concepts of the theory of disordered magnetic systems are presented with application to two models: disordered ferromagnets with interaction between nearest neighbors in the lattice, and ferromagnets in which the exchange interaction depends exponentially on the distance between the magnetic atoms. An example of ferromagnets of the second type is provided by dilute alloys of palladium with iron, cobalt, and manganese. The principal experimental results for these alloys are presented, and it is shown that the theory of disordered magnetic systems describes their properties well.

Quasiresonant processes in slow collisions

Abstract: Collisions between slow atomic particles involving a small change in electron energy in the transition are examined. Asymptotic methods are presented for calculating the parameters of interaction potentials for atomic particles at large separations. Different models describing electron transitions during collisions between atomic particles are analyzed. The simultaneous use of the asymptotic method for the calculation of the interaction potentials between colliding particles and of the generalized two-level approximation for the quasiresonant particle collision results in reliable calculations of transition cross sections for a broad range of processes. This is demonstrated by considering examples of resonant charge transfer, spin exchange, and also some quasiresonant processes.

Molecular dynamics method in statistical physics

Abstract: An analysis is made of the results obtained in investigations of dense media by the molecular dynamics method. This method is based on mathematical simulation of the motion of a sufficiently large number of particles with a given interparticle interaction law. The attention is concentrated on new physical ideas about the nature of simple liquids and dense gases which have made their first appearance, have been derived, or confirmed in studies carried out by the molecular dynamics method. The principal laws of particle motion and their influence on the form of the temporal velocity correlation function are considered. Spatial and temporal correlations appearing in dense systems are studied and their role in the propagation of longitudinal and shear waves is discussed. An analysis is made of the results of molecular dynamics investigations of thermodynamic and transport properties of simple liquids and dense gases. The dynamics of a light classical particle in a dense medium of disordered heavy scatterers is discussed. Consideration is given to the close relationship between the behavior of the temporal velocity correlation function of a particle, its spatial velocity correlation function, and "percolation" in a random field of heavy scatterers.

Thermoelectric effects in superconductors

Abstract: It is a pleasure and an honour for me to speak before this highly esteemed audience. The subject of my talk is devoted to thermoelectric effects in superconductors. The question of whether thermoelectric phenomena can be observed in metals in the superconducting state was raised more than 50 years ago, but first attempts to observe such effects as Seebeck electromotive force, or Peltier heat, or Thomson heat, failed to reveal any trace of the existence of these effects in superconductors. In any case, it was definitely shown that these effects, if they exists are weaker by many orders of magnitude, than the analogous effects in normal metals. This was especially clearly demonstrated by the experiment of Steiner and Grassman in 1935,1 who tried to observe a Seebeck electromotive force in superconductors. These authors suspended a superconducting bimetallic ring on a torsion wire placed in a weak external magnetic field and heated the ring. If a Seebeck thermo-electromotive force would arise, as it arises in a case of normal metals, then this force would induce acceleration of superconducting electrons and the current in the ring would increase with time, leading to a torque on the wire, which it might be possible to observe.

Semiconductor-metal phase transition in rare-earth semiconductors (samarium monochalcogenides)

Abstract: A review is given of the theoretical and experimental data on the isostructural first-order semiconductor-metal phase transition in samarium monosulfides and Sm1-xLnx3+S solid solutions (Ln is a rare-earth metal). Phase transitions occurring under hydrostatic pressures and as a result of change in temperature are considered. Electrical, magnetic, galvanomagnetic, thermoelectric, thermal, optical and mechanical properties are discussed, as well as the energy band structure of the semiconductor and metal modifications of the investigated materials. Special attention is given to the experiments confirming the fractional valence of the samarium ion in the metal phase of these materials (the experimental evidence for this valence is provided by the Mossbauer effect, magnetic susceptibility, shifts of x-ray K lines, x-ray emission spectra, and specific heat).

Pulse discharges in gases under conditions of strong ionization by electrons

Abstract: A review is given of investigations of pulsed high-pressure volume discharges excited by fast-electron beams. The following topics are discussed: classification of discharges; methods for calculating the current-voltage characteristics; analysis of the optimal ways of depositing energy in the gas in the volume stage of the discharge; discharge instability mechanisms and the corresponding experimental observations; applications of discharges. The results are given of calculations of the electric field in the cathode and anode regions, and also in the discharge column in the case of a spatially inhomogeneous ionication of the gap. It is shown that a stable volume flow of the current in molecular gases in which the specific deposited energy is 0.1–1 J/cm3 may be attained in a nonself-sustaining discharge and in a discharge with ionization multiplication. In both cases a spark channel appears in two stages: formation of sparkinitiating centers in the form of plasma regions with a higher density near the electrodes is followed by growth of the spark channel from such initiating centers. In some cases the spark channel growth can be described by the available mathematical models. Discharges in mixtures of rare gases with halogencontaining compounds, when electrons are lost mainly by capture by complex molecules, are considered separately. Applications of volume discharges in laser pumping, switching of pulsed currents, plasma chemistry, etc., are described.

Raman scattering of light by polaritons

Abstract: The main results are given of the experimental and theoretical investigations demonstrating the capabilities and characteristics of the spectroscopy utilizing spontaneous Raman scattering of light by polaritons. The following topics are considered: polariton dispersion; frequency-angular scattering spectra; experimental methods; intensity and line shape of scattered light; relationship of the Raman scattering of light by polaritons to the Raman scattering by optical phonons, to second harmonic generation, and to the linear electrooptic effect; characteristics of polariton spectra due to free carriers, localized modes, energy bands of two-particle states, and gyrotropy of crystals; coherent anti-Stokes Raman scattering of light by polaritons.

Signal velocity in quantum electrodynamics

Abstract: The fulfillment in quantum electrodynamics of the principle of relativistic causality—the signal velocity does not exceed the velocity of light—is discussed. In Sec. 1 it is argued that this principle is not guaranteed automatically merely by the local commutativity of the theory. Section 2 is a critical review of the signal transmission problems which have been formulated and solved in the literature. In Sec. 3, this problem is considered in the framework of a simple but fairly realistic model of quantum electrodynamics. The signal source is an external current localized in some region S. The arrival of a signal in a region D at a distance R is established by a change in the coordinate, momentum, or energy of a charged particle. It is shown that proof of relativistic causality of the theory requires one to take into account appropriately quantum-mechanical and, in particular, quantum-electrodynamical features of the problem. In the final fourth section, a general formulation and exact solution of the signal transmission problem are given. The treatment is of sufficient generality for one to be able to assert that in the framework of quantum electrodynamics none of the possible methods of signal transmission violate relativistic causality.

Diffraction of light by sound in solids

Abstract: Papers on acousto-optical phenomena in solids and their most important applications to research in solid state physics and to modern technology are surveyed. The theory of the diffraction of electromagnetic waves by sound in isotropic and anisotropic solids is discussed. In addition to such classical special cases of diffraction as Bragg and Raman-Nath diffraction, we also consider some cases of the diffraction of light by sound in which the reflection of light at the faces of the crystal and modulation of the light by the sound wave have an important effect on the nature of the acousto-optical interaction. The contribution of the electron density wave accompanying a sound wave in piezoactive semiconductors is considered and features of this acousto-optical interaction mechanism are discussed. The quantum theory of the diffraction of light by sound is examined. Microscopic expressions for the photoelasticity constants are obtained and the resonance characteristics of these constants at photon energies close to the width of the forbidden gap in the crystal are investigated. In the experimental part of the paper we review various methods of investigating the diffraction of light by sound in solids and present the basic results in this field together with examples of the use of acousto-optical methods in various areas of solid state physics. In the concluding section we examine some important applications of the diffraction of light by sound to modern optical and electronic systems.

Diffusive mass transfer in island films

Abstract: A systematic review is made of the experimental and theoretical investigations of the mechanism and kinetics of mass transfer in a system of islands of substance A deposited on a solid substrate of substance B in which A is insoluble. The analysis is carried out on the assumption that the total amount of matter present jointly in the islands and in a two-dimensional gas of adsorbed atoms is constant. Behavior of systems of islands at rest and in motion is discussed. Coalescence of islands is considered in the case of diffusive mass transfer between islands at rest and also in the case of direct collisions between moving islands. "Dispersion hardening" of the surface by a system of islands, which hinders evaporation and growth of a crystal, is described. Considerable attention is devoted to reporting experimental investigations carried out mainly by electron microscopy. Some new effects are predicted, including a possible phase transition during coalescence of islands, size effect in recrystallization, one-dimensional coalescence, etc.

Superconductivity and elementary particles

Abstract: The article contains a brief historical review of the mutual influences on one another of elementary-particle theory and many-body theory. The main attention is devoted to the idea of spontaneous symmetry breaking and the unified theory of particles based on it. The intimate and far reaching analogy between unified theory and the theory of superconductivity is traced. Some consequences of this analogy for particle physics and cosmology are considered.

Shift of K x-ray lines associated with valency change and with isomorphous phase transitions in rare earths

Abstract: Studies are reviewed that employ the effect of an anomalously strong shift of the fundamental K-series x-ray lines associated with a change in the number of 4f electrons in a given atom for investigating the phenomenon of variable valency in compounds of the rare-earth elements in general, and the so-called isomorphous phase transitions in these compounds in particular. The mechanism of the shift effect and its theory are described. The available data on the shifts of the K-lines of the different compounds of all the rare-earth elements (except promethium) are discussed. A summary table gives the valencies (including the anomalous ones), the occurrence of which has been proved by this method. Application of the K-shift method for studying isomorphous (or isostructural) transitions allows one to establish unambiguously that the mechanism of these transitions involves a change in the number of 4f electrons, and to measure the magnitude of these changes. The review concludes with a brief description of the principles and design of the required experimental apparatus-crystal diffraction spectrometers.

The four-dimensional group velocity

Abstract: The advantages of using the 4-dimensional group velocity are demonstrated. After first giving its purely kinematic definition for an arbitrary wave field, the discussion turns to packets of electromagnetic waves in electrically and magnetically anisotropic media which possess space-time dispersion and are smoothly nonuniform in space and slowly varying in time. In particular, a simple expression is obtained for the energy-momentum 4-tensor in terms of the group-velocity 4-vector. Finally, it is shown how the 4-dimensional notation simplifies the derivation of the conditions of orthogonality and conservation of the adiabatic invariant. A note by M. L. Levin which follows this paper contains a brief account of the basis results of W. R. Hamilton's investigations relating to the velocity of wave motion.

Relativistic experiments in gravitational fields

Abstract: Experimental investigations in gravitation and relativity theory are reviewed. The state of experiments that test the equivalence principle, the basis of the relativistic theory of gravitation, is discussed. The latest results of measurements of the classical effects of general relativity in the solar system are given and promising programs of similar experiments in the near future are analyzed. The new possibilities for testing gravitational theories provided by the discovery of the binary pulsar are described. Finally, the problem of searching for bursts of gravitational radiation using terrestrial Weber type antennas is discussed.

Electron localization in disordered systems (the Anderson transition)

Abstract: According to modern ideas there is a sharp boundary between the localized and extended states in the electron spectra of disordered systems (amorphous semiconductors, doped crystalline semiconductors, etc.). The mathematical description of the electron states near this boundary is a very complicated problem which is not yet completely solved. The purpose of this review is to describe, in the simplest possible way, without going into the mathematical problems, the basic physical ideas that have been put forward in this field. The Anderson and Lifshitz models are considered in detail and the criteria for localization are discussed. Examples are given of systems in which it is possible to observe experimentally the transition from localized to extended states that occurs when certain parameters are varied. The concept of minimum metallic conductivity and the properties of the wave functions near the boundary of the localized states are discussed. The results of computer experiments that attempt to model a disordered system near the mobility edge are described. The difficulties that arise in attempts to give a mathematical description of such a system, and the basic ideas of such a description, are discussed in the form of a conversation with an unsophisticated partner.

On discussions concerning the problem of ponderomotive forces

Abstract: A discussion is presented in this note from the point of view of modern concepts of the reasons for some controversies which occurred in the past concerning some problems involving ponderomotive forces In particular it is pointed out that the reason for the different expressions for the force consists of the nonuniqueness of the separation into components of the total force acting on a body situated in an electromagnetic field

Dielectric permittivity in the x-ray region

Abstract: The article reviews the current state of the study of the dielectric properties of a medium in the x-ray and γ-ray frequency regions. Particular attention is devoted to the systematic introduction of the dielectric permittivity tensor in the x-ray region, starting from the most fundamental property of crystals–spatial periodicity. The properties of the permittivity tensor are treated both in terms of the phenomenological theory and on the basis of specific microscopic models. The review includes theoretical and experimental investigations of the contributions of the electronic and nuclear subsystems to the permittivity of crystals. The polarizability of the electron subsystem is considered in the strong-coupling approximation and in terms of the Bloch formalism. Considerable space is devoted to discussion of the permittivity near isolated low-lying nuclear transitions in the presence of hyperfme interactions, where its properties are substantially more complicated and more interesting than those of the permittivity of the electron subsystem. A review is given of the principal phenomena of the polarization optics of Mossbauer radiation. The review analyzes the symmetry properties of the x-ray permittivity and establishes a number of useful relations regarding generalized symmetry operations.