Showing papers in "Physics-Uspekhi in 1982"

The Jahn-Teller effect and magnetism: transition metal compounds

Abstract: The properties of magnetic insulators containing orbitally degenerate transition metal ions (Jahn-Teller ions) are discussed. The Jahn-Teller effect in these insulators causes structural phase transitions, lowers the lattice symmetry, and gives rise to an orbital ordering. Various interactions responsible for these effects are discussed: the electron-lattice, quadrupole-quadrupole, and exchange interactions. The mutual effects of the orbital ordering and the magnetic properties of corresponding compounds are discussed. The exchange interaction in the cases of twofold and threefold orbital degeneracy is discussed. The effect of a magnetic field on the orbital and magnetic structure and the temperature dependence of the exchange interaction are studied. The properties of several representative compounds containing Jahn-Teller ions are discussed.

ABC of instantons

Abstract: An attempt is made to present an instanton "calculus" in a relatively simple form. The physical meaning of instantons is explained by the example of the quantum-mechanical problem of energy levels in a two-humped potential. The nonstandard solution to this problem based on instantons is analyzed, and the reader is acquainted with the main technical elements used in this approach. Instantons in quantum chromodynamics are then considered. The Euclidean formulation of the theory is described. Classical solutions of the field equations (the Belavin–Polyakov–Shvarts–Tyapkin instantons) are obtained explicitly and their properties are studied. The calculation of the instanton density is described and the complete result is given for an arbitrary number of colors. The effects associated with fermion fields are briefly described.

Effects of double passage of waves in randomly inhomogeneous media

Abstract: In this review, the effects stemming from double passage of reflected and backscattered waves through the same inhomogeneities of the medium, are discussed. The enhancement of phase and intensity fluctuations and of the average intensity and other effects, characteristic of double passage, are described in detail. Multichannel coherent effects, arising due to mutual coherence of different waves passing in opposite directions along identical channels, are investigated. The experimental work on double passage effects is reviewed. Possible practical applications of these effects are analyzed.

Solitons in quasi-one-dimensional molecular structures

Abstract: Theoretical investigations of solitons arising in quasi-one-dimensional molecular structures such as alphahelical protein molecules are described. The properties of solitons and excitons are discussed. The reasons for the great stability of solitons are investigated. It is noted that the vibrational energy of Amide I vibrations of peptide groups can be transported in the form of solitons along a protein molecule without losses due to creation of phonons. Deceleration of solitons by dissipative forces and the effect of thermal motion and external fields are investigated. A new mechanism for muscle contraction in animals, based on the idea of solitons, is discussed on a molecular level.

The energy problem in Einstein's theory of gravitation (Dedicated to the memory of V. A. Fock)

Abstract: The review is devoted to a discussion of the definition and properties of energy in Einstein's theory of gravitation. Asymptotically flat space-time is defined in terms of admissible asymptotically Cartesian coordinates and a corresponding group of coordinate transformations. A Lagrange function is introduced on such a space-time, and a generalized Hamiltonian formulation of the theory of gravitation is constructed in accordance with Dirac's method. The energy is defined as the generator of displacement with respect to the asymptotic time. It is shown that the total energy of the gravitational field and the matter fields with normal energy-momentum tensor is positive and vanishes only in the absence of matter fields and gravitational waves. The proof follows Witten's proof but contains a number of corrections and improvements. Various standard criticisms of the energy concept in general relativity are discussed and shown to be without substance.

Spectrum and polarization of hot-electron photoluminescence in semiconductors

Abstract: Experimental and theoretical research on the recombination photoluminescence of free hot electrons in semiconductors (primarily GaAs) is reviewed. The polarization characteristics are discussed. These characteristics reflect, in particular, a momentum alignment of the electrons by linearly polarized light and an effect of a ripple in the constant-energy surfaces in the valence band. The dependence of the linear polarization on the spectrum is discussed in connection with various mechanisms for the energy relaxation of the hot electrons. The depolarization of the hot-electron photoluminescence in a magnetic field is discussed. A procedure is discussed for determining the energy relaxation times and the scale times for intervalley transitions through an analysis of depolarization curves. The energy distribution of the hot electrons is found from the hot-electron photoluminescence spectrum. The recombination luminescence of hot holes is discussed. These holes appear when the semiconductor is illuminated in the spin-split-off subband.

Parity breaking in the interaction of neutrons with heavy nuclei

Abstract: Mechanisms for the following parity-breaking effects are examined: a) asymmetry in the emission direction of the light fission fragment; b) rotation of the neutron spin around the direction of its momentum in a medium and the difference between the cross sections for the capture of right-hand- and left-hand-polarized neutrons; c) circular polarization and asymmetry in the angular distribution of γ rays. There is particular emphasis on the reasons for the enhancement of these effects in heavy nuclei. Theoretical results are compared with experimental data.

Thermochemical action of laser radiation

Abstract: The processes induced by laser radiation in chemically active media are examined. Special attention is given to the analysis of different mechanisms of formation of feedback between chemical and thermal degrees of freedom of the system and the resulting complex dynamics of thermochemical processes (such as instabilities, self-oscillation, stochastic regimes, etc.).

Properties of high pressure phases in metal-hydrogen systems

Abstract: The development of high-pressure technology has in recent years permitted obtaining extensive new information on the properties of hydrides of group VI–VIII transition metals. In this review, the experimental procedures for compressing macroscopic quantities of hydrogen to record high pressures, phase transitions, and structures of new high-pressure phases in Me–H systems are briefly described. Special attention is devoted to the magnetic properties of solid solutions of hydrogen in 3d metals and their alloys, whose study has yielded definite conclusions concerning the effect of hydrogen on the band structure and exchange interaction in these materials. The role of structural instabilities in the formation of superconducting properties of hydrogen solutions in 4d metal alloys based on palladium is examined.

Dissociative recombination of electrons and molecular ions

Abstract: The mechanisms of dissociative recombination of electrons and molecular ions are described. Methods for experimental study of this process are discussed. The results of measurements of the dissociative recombination cross section and coefficient for various systems are reported. Theoretical models of the process are analyzed. The role of dissociative recombination in the low-temperature plasma of the upper atmosphere, gas lasers, and the glow discharge is demonstrated.

Anomalous magnetic structures and phase transitions in non-Heisenberg magnetic materials

Abstract: This review is primarily devoted to a discussion of isotropic magnetic materials with localized magnetic moments between which the exchange interaction is more complex than in the usual Heisenberg model. They include insulators for which the biquadratic or multispin exchange mechanisms are comparable with the bilinear exchange, conductors which do not satisfy the conditions of validity of the RKKY indirect exchange theory, and the nuclear magnetic material solid helium. Additionally, an analysis is made of high-anisotropy magnetic materials behaving similarly to isotropic non-Heisenberg materials. These anisotropic materials have anomalous properties, compared with the Heisenberg case. Experimental data on these materials and their theoretical interpretation are given. The following topics are discussed: 1) the conditions for a strong non-Heisenberg exchange and non-Heisenberg Hamiltonians; 2) quadrupole ordering and order-proper disorder phase transitions; 3) order-order and order-improper disorder phase transitions; 4) metamagnetism of isotropic materials; 5) normal sequences and "devil's ladders" of phase transitions between commensurable structures; 6) canted antiferromagnetism of high-symmetry crystals which do not obey the condition for the existence of relativistic Dzyaloshinskiĩ-type interactions.

Phase conjugation in stimulated scattering

Abstract: Various aspects of optical phase conjugation are discussed: the properties of the conjugate wave, its potential applications, the basic conjugation methods, and a brief history of the question. The theory for phase conjugation in stimulated scattering is set forth in detail. The basic experimental results on this conjugation method are also discussed in detail. Phase conjugation occurs because that configuration of the back-scattered field which has the conjugate wavefront is amplified to the greatest extent (at a doubled gain) in the intense speckle-inhomogeneous conjugate wave in a medium in which stimulated scattering occurs. Because of the large overall amplification in stimulated scattering, all the other, uncorrelated, configurations of the spontaneously scattered nucleating waves are amplified by a factor of 107 less and are discriminated against. The intervals of values of the various parameters in which the conjugate configuration (the specklon) exists are discussed theoretically, as is the effect of nonlinear selection and saturation on phase conjugation in stimulated scattering. There is a review of experimental results on the first observation of the effect, on the measurement of the angular structure of the uncorrelated waves and of the extent to which they are discriminated against, on the phase fluctuations of the conjugate wave, on the conjugation of subthreshold and depolarized radiation, and on phase conjugation in stimulated scattering in focused beams and for other scattered-wave amplification mechanisms.

Radiation of relativistic particles in single crystals

Abstract: The review treats the problem of radiation of relativistic charged particles in matter. The radiation of fast particles in an external field is considered from a unified point of view for an amorphous medium and for a single crystal. The basic attention is paid to the process of radiation in a single crystal where an enhancement of the radiation occurs as compared with an amorphous medium. This effect is shown to be due to the coherent and the interference mechanisms of radiation of relativistic particles in single crystals. First we outline the Born theory (quantum and classical) of coherent radiation of fast particles and show that this theory is valid if the particle propagates through the crystal far from the directions of channeling and if the scattering angle of the particle is small compared with the typical radiation angle of a relativistic particle. Then we show that a violation of these conditions leads to new effects in radiation such as, e.g., the effect of intense radiation of superbarrier and channeled particles, and the effect of suppression of coherent radiation. The comparison of theoretical and experimental results confirms the existence of new effects in the radiation. In conclusion, we review briefly new physical effects that must take place in single crystals at high energies in several other electrodynamical processes.

Glow discharge in a gas flow

Abstract: The status of research on glow discharges in a gas flow, used in pumping fast-flow lasers, is reviewed. Systematic study of this discharge began about 10 years ago. This discharge form, according to a number of properties (current flow mechanism, nature of the development of instabilities, etc.), differs considerably from the well-studied glow discharge in tubes. One of the distinguishing features of such a discharge is the negligibly small role of ionization in a large part of the positive column, so that narrow regions near the electrodes contribute most of the positive ions and electrons. Negative ions, which compensate the charge of the positive column, are generated in its volume. The discharge turns out to be weakly inhomogeneous in the direction from the cathode to the anode and the ion currents can form an appreciable part of the total current. These facts have not yet been sufficiently widely discussed in reviews and monographs. In this review, results of experimental investigations of the mechanism of current flow, energy balance, plasmochemical processes, and discharge instabilities are presented, the mathematical models used in their analysis are analyzed, and the possibilities for increasing the stability and efficiency of the discharge in fast-flow lasers are examined.

Transition temperature of strong-coupling superconductors

Abstract: The possible existence of superconductors with high transition temperatures is analyzed for both the ordinary electron-phonon interaction and nonphonon superconductivity mechanisms. The existence of such systems depends on a negative static dielectric permittivity. The possible occurrence of this situation for an electron-ion system is analyzed. It is shown that negative values of the static dielectric permittivity are possible in the case of strong local-field effects, i. e., in the case of exchange-correlation interactions in the electron subsystem or the localization of point ions for an electron-phonon system. Equations for the transition temperature of strong-coupling superconductors are analyzed. Restrictions imposed on the transition temperature by the equations themselves and by the conditions for stability of the material are examined.

Intense electromagnetic radiation from relativistic particles

Abstract: Several mechanisms for emission of electromagnetic radiation by relativistic electrons in various media which are of interest for generating intense ultraviolet, x-ray, and γ radiation are examined. Theoretical and experimental results on ultraviolet and x-ray Cherenkov radiation, quasi-Cherenkov radiation in artificial periodic structures, radiation from crystals which results from the diffraction of virtual photons, and radiation accompanying channeling are discussed.

On vibrational energy localization at high levels of excitation. Vibrational excitons

Abstract: This review discusses the properties of highly excited vibrational states of polyatomic molecules and molecular crystals. As we know, one can describe small vibrations of molecules with the concept of normal modes. In molecules having several identical valence bonds (C6H6, H2O, etc.) the normal modes that describe the vibrational excitations of these bonds amount to vibrations whose energy is more or less uniformly distributed over all the bonds, with a degree of delocalization of the energy over the bonds that increases with increasing level of excitation. On the other hand, an extensive set of physical phenomena exists (e.g., dissociation of molecules) in which local excitations, a considerable fraction of which are spatially localized, play an important role. A localized state corresponds to a complicated superposition of normal modes. Hence the concept of normal vibrations is inadequate for describing vibrations (or, better expressed, movements) of a highly excited molecule. One can conveniently describe such movements of the molecule in the representation of local modes (LM). As a rule, one takes an LM to mean simply the coordinate of a valence bond of the molecule, e. g., O–H, C–H, etc. A large number of papers has been published recently on the experimental study of LM and infrared spectra, relaxation experiments, selective photochemistry, etc. This review casts light on these experimental data on the basis of the theory of LM.

Radiative lifetimes of excited states of atoms

Abstract: A review of experimental and theoretical studies of radiative lifetimes of excited states of the hydrogen atom, of atoms of alkali metals, and atoms of the copper subgroup is presented. A complete compilation of all published values of radiative lifetimes of all excited states with a critical analysis for every atom is given. The regularities in the variation of radiative lifetimes in spectral series of excited states are brought out and constants of the power-law dependences on the effective principal quantum number are determined. A correlation between the behavior of lifetimes and the effective orbital parameter is established and it is proposed to use this correlation for the determination of the application limits of the power-law constants for evaluation of lifetimes of uninvestigated states. On the basis of a critical analysis of published sources, of the regularities in the variation of lifetimes and of a statistical treatment of values available in the literature, a table of recommended values of radiative lifetimes of excited states of all atoms of the first group is compiled.

Features of the thermodynamics of superionic conductors

Abstract: This article reviews the theoretical and experimental studies on a number of the thermodynamic aspects of superionic conduction, the intensive study of which began in the past decade. We discuss the relation of superionic conductivity to the disordering of one of the sublattices of the crystal, disordering phase transitions caused by the interaction of point defects, and the mechanism of polymorphic transitions associated with partial disordering. We analyze the effect of jumpwise variation of the ionic conductivity induced by an external electric field, the thermodynamics of domain states in superionic conductors, and the effect of pressure on the phase transitions and the ionic conductivity.

Dynamic methods in plasma physics

Abstract: Hydrodynamic methods of generating and diagnosing dense plasmas with strong interparticle interaction are reviewed. Experiments in shock and isentropic compression of matter to produce Boltzmann and degenerate plasma with maximum pressures in the tens of megabars are discussed. A method for isentropic expansion of metals that have been compressed by strong shock waves, which makes it possible to investigate a broad range of the phase diagram, including the high-temperature boiling curve and the neighborhood of the critical point, is described. The experimental schemes and designs of compressed-air and explosion-type nonideal-plasma generators and the diagnostic tools used are discussed. The results of experiments with nonideal plasmas are reported, and the influence of the interparticle interaction on the thermodynamic, electrophysical, and optical properties of the strongly compressed plasma is analyzed on the basis of theoretical models. The properties of ultrahigh-pressure plasmas and prospects for laser electrodynamic and electroexplosive methods of generating strong shock waves in condensed media are discussed.

Shake-up processes accompanying electron transitions in atoms

Abstract: Elementary processes in many-electron atoms—radiative and Auger transitions, photoionization, ionization by electron impact, etc.—are usually accompanied by relaxation of the electron shells. The conditions under which such an inherently many-electron problem can be solved in the shake-up approximation are examined, and the shake-up processes occurring as a result of electron transitions are described from a unified point of view. The features that are common both to this form of excitation and to the shake-up of atomic electrons following nuclear transformations are pointed out, and the distinguishing features are also noted. The various electron shake-up processes considered are the radiative Auger effect, the two-electron-one-photon transition, double ionization, spectral line broadening, the post-collision interaction, Auger decay stimulated by collision with a fast electron, and three-electron Auger transitions (the double and half Auger effects). These processes are classified according to the type of electron transition causing the shake-up, and the experimental data and methods of theoretical description are reviewed. Other effects of a similar nature that could accompany transitions in the electron shells of atoms are also mentioned. A derivation of the shake-up approximation is given, and it is pointed out that this approximation is to a large extent analogous to the distorted-waves approximation in scattering theory. It is shown that the shake-up approximation is rather effective for obtaining estimates of the probabilities of the various effects.

Absorption of powerful resonance radiation accompanying collisional line broadening

Abstract: This review gives a systematic exposition of the qualitative problems of nonlinear laser resonance spectroscopy, taking into account the effect of the light field on the dynamics of the broadening collision and, correspondingly, on the rate of phase relaxation. The results of experiments, in which these nonlinear optocollisional effects, predicted earlier theoretically, were observed, are presented briefly.

Superflows and superfluidity

Abstract: The basic concepts of the theory of mass superflows in a superfluid are presented. The stability of such flows with respect to the creation and growth of linear defects (superfluid vortices), the relationship between superflows and the existence of long-range order, and the possible occurrence of persistent flows in one- and two-dimensional systems are discussed. Some analogs of the mass superflows in a superfluid are also examined: spin superflows in magnetically ordered systems having an easy-plane anisotropy and the current states of a Bose condensate of electron-hole pairs. The physical meaning of such ''flows'' is discussed, and a theory for their stability is derived from the calculated probability for the creation of the linear defects which are analogs of superfluid vortices. There is a discussion of the applicability of the theory of spin superflows to several experiments on the magnetic properties of the A phase of superfluid helium-3 and to a possibility which follows from this theory: that domain walls might be generated in the interior of a sample of an easy-plane magnetically ordered material and that the motion of these walls might be controlled by fields applied to the surface of the sample.

Accelerator and detector prospects of elementary particle physics

Abstract: This review treats the progressive changes in the field of physics and technology of accelerators, and in part, of detectors, which have exerted and will in the near future exert a fundamental influence on the development of elementary-particle physics. In particular, it discusses the possibilities of generation of beams of elementary particles and the prospects of performing experiments with colliding beams involving the development of methods of cooling charged-particle beams, designing superconducting systems, and developing superlinacs.

Optical problems in the electrodynamics of gyrotropic media

Abstract: The theory for the propagation of electromagnetic waves in gyrotropic media with boundaries is reviewed. Media which simultaneously exhibit a natural activity and a magneto-optic activity are discussed. The optical irreversibility effects in such media are analyzed. Magnetic crystals with a natural optical activity are discussed. The meaning of the magnetic permeability at optical frequencies is discussed. The optical properties of magnetically active ferromagnets are examined in the case of coincident roots of the dispersion relation. The propagation of light in a bigyrotropic medium is also discussed. Certain distinctive properties stemming from the existence of two types of gyrotropy, anisotropy, and absorption are discussed. The mechanisms for circular dichroism and the features of the dichroism of naturally gyrotropic media in an external magnetic field are discussed briefly.

Generation of coherent far-infrared radiation using lasers

Abstract: A review is given of methods used for generating coherent far-infrared radiation. They include the isolation of the frequency difference between two lasers, SRS by polaritons, optical pumping of gases, and the development of the electrical-discharge laser. Basic principles underlying the realization of the phenomena on which the above methods of generating far-infrared radiation are based are discussed. The properties of the working media, i.e., nonlinear properties of crystal converters (exploited in isolating the difference frequency and in SRS by polaritons) are described. Methods of producing inversion of level populations in gases used in lasers (pumped optically or by an electrical discharge) are analyzed. Specific schemes and device designs for the far infrared are described, and the possibilities and likely performance of these schemes are discussed. The review does not cover electron-beam generators (backward-wave tube, gyrotrons, etc.).

High-intensity neutron spectroscopy of nuclei

Abstract: The use of neutron spectroscopy to study the properties of nuclei is reviewed. The development of neutron spectroscopy over the years and the principal results are discussed briefly. The experimental methods of neutron spectroscopy are described. Emphasis is placed on the new directions in neutron spectroscopy of nuclei: the alpha decay of compound states excited during neutron capture by nuclei, hyperfine interactions in neutron resonances, and the properties of few-nucleon nuclei in reactions with neutrons. The most important experimental results are discussed and compared with the theory.