Showing papers in "Physics-Uspekhi in 1979"

Dynamic self-diffraction of coherent light beams

Abstract: A review is given of the present state of theoretical and experimental work on the interaction of two coherent light beams of equal frequency in a nonlinear medium. It is shown that dynamic self-diffraction is substantially different for media with different types of response. Stationary energy transfer between interacting beams is possible in the case of nonlocal nonsymmetric response, whereas this type of transfer is forbidden in the case of local response. Different mechanisms for self-induced changes in the refractive index are examined together with the corresponding processes of energy transfer between the writing beams. The more important applications of dynamic self-diffraction are discussed.

Optics of cholesteric liquid crystals

Abstract: A review is presented of the theory of optical properties of cholesteric liquid crystals (CLC). The diffraction nature of the unusual optical properties of CLC is exhibited in the simplest and most easily visualizable kinematic approximation of diffraction theory. A quantitative description of the optical properties is given based on the exact solution of Maxwell equations for light propagated along the optical axis of CLC and within the framework of the dynamic diffraction theory for an arbitrary direction of propagation. Considerable attention is devoted to the manifestation of the connection between the structural and optical properties of the CLC observed when light is propagated at an angle to the optical axis of the CLC and when the CLC structure is distorted by an external field. Investigations of the optical properties of absorbing CLC reveal that suppression of light absorption in the selective reflection band is analogous in its nature to the well-known Borrmann effect in x-ray diffraction. Different approaches to the optics of imperfect CLC which frequently most nearly correspond to the conditions of actual experiments are described. A theory of the optical properties of chiral smectic crystals is presented. This theory, on the whole, is analogous to that for CLC, but it predicts a number of qualitative differences in the optics of chiral smectic crystals. The theory and special features of the Vavilov-Cherenkov radiation in CLC is briefly presented. A comparison is made of the theoretical results with the experimental data and the most promising directions for theoretical and experimental research are indicated.

Spinodal decomposition (phase transitions via unstable states)

Abstract: Considerable deviations from equilibrium conditions are observed for nonstatic, first-order phase transitions. In some cases, unstable (labile) phase states may precede the onset of the phase transition. The relaxation of the system is then accompanied by an enhancement of random inhomogeneities and the appearance of a modulated intermediate structure. This mechanism of the initial stage of a phase transition is called spinodal decomposition (SD). Theoretical and experimental studies of SD in two-component systems are reviewed in this paper. Thermodynamic stability and the possibility of SD in one-component liquid-vapor systems and an alternative nucleation mechanism are discussed. The phenomenological theory of SD is based on the Ginzburg-Landau expression for the free energy of an inhomogeneous system. A linearized diffusion equation is derived, for which thermodynamically unstable states have exponentially increasing solutions for the Fourier components of composition. Subsequent refinements of SD theory take into account thermal fluctuations and involve the derivation of the kinetic equation for the distribution functional. Diffraction methods are the most effective in the experimental study of SD in alloys, glasses, and binary liquid mixtures. So far, the agreement between theory and experiment must be regarded as only qualitative.

Electric fields and collective oscillations in superconductors

Abstract: Effects associated with the penetration of an electric field E into a superconductor with deviations from thermodynamic equilibrium are considered. The penetration of a static field E incident to the passage of a current across the boundary between the superconductor and the normal metal (S-N boundary) is analyzed. At temperatures close to the critical temperature the penetration depth lE of a field E into the S region may be much greater than the correlation length or the London depth and may reach macroscopic dimensions in sufficiently pure specimens. In isotropic superconductors the magnitude of lE is determined by the branch imbalance relaxation processes. The change in the gap width at the S-N boundary leads to an additional branch imbalance relaxation mechanism which, in pure specimens, is due to Andreev reflection of quasiparticles. The resistance of a superconductor in the intermediate state is calculated. Weakly damped collective oscillations with an acoustic spectrum, which exist in superconductors near the critical temperature, are considered. This collective mode is characterized by oscillations of both the field E and the branch imbalance. The propagation velocity of the oscillations is somewhat lower than the Fermi velocity. Effects associated with the penetration of the field E to great depths in Josephson bridges are analyzed. The theory of the phenomena considered is presented, using the kinetic equation and the equations for the Green's functions. Experiments are described for measuring effects associated with the penetration of a static field E into a superconductor and for detecting the collective oscillations.

Autowave processes in distributed kinetic systems

Abstract: The basic experimental data and the theory for autowave processes in active kinetic systems are reviewed. Each volume element in such a system is in a state far from thermodynamic equilibrium, and the different volume elements are coupled by transport processes. Some examples of these systems are certain chemical and biological objects in which various types of waves and stable structures can be produced. Mathematically, autowave processes are described by quasilinear and nonlinear parabolic equations. These autowave processes are quite different from processes which occur in conservative systems, e.g., solitons. A classification of autowave processes is offered, and the experimental data are summarized. In accordance with this classification, the review itself is organized in sections on the physics of the basic models for autowave systems in a one-dimensional space and qualitative methods for studying them. The basic cases are wave propagation, autonomous wave sources, spontaneous oscillations and quasistochastic waves which are synchronized over the entire space, and the formation of dissipative structures. At present, the primary fields of application of the theory of autowave processes are neural conductivity, combustion, self-organization in living systems, etc. The necessary conditions for these autowave situations are listed.

Determination of electron-phonon coupling from structured optical spectra of impurity centers

Abstract: Recent developments in the theory of impurity centers are reviewed. Particular attention is devoted to the way this theory can be exploited in the quantitative analysis of structured optical spectra through the use of formulas relating the absorption band shape to the shape of the fluorescence band. Analysis of departures from mirror-image relation between these two spectra can be used to deduce the parameters of the electron-phonon coupling from the shape of the structured bands. Topics discussed include zerophonon lines, the dependence of their shape and position on temperature, departures from mirror-image relation for both phonon wings and the vibronic absorption and fluorescence spectra, the energy gap between the absorption and fluorescence spectra of certain molecules, and the nature of excimer emission in solids.

Low-temperature calorimetry of biological macromolecules

Abstract: This review shows the modern methods of calorimetry to be highly informative in studying the conformational properties of biopolymers when applied over a broad range of temperatures including the liquid-helium region, i.e., at temperatures far from the region of functioning of the macromolecules. Such an approach is typical in solving problems of the physics of the condensed state, since it allows one to reveal the characteristic features of a material by analyzing its physical properties under extreme external conditions of the environment (low and high temperatures, high pressures, etc.). This article collects and classifies the experimental data on the heat capacity of amino acids, peptides, polypeptides, proteins, and nucleic acids, and correlates them with the existing theories of the heat capacity of highly anisotropic structures at low temperatures. The observed low-temperature phase transitions in biopolymer-solvent systems are described, and the thermal properties of biopolymer chains are discussed in the light of the effect of the solvent and of dissolved ions of salts.

Nonequilibrium shells of neutron stars and their role in sustaining x-ray emission and nucleosynthesis

Abstract: A review is presented of theoretical concepts of properties of matter and of processes in neutron star shells and of their relation to observations. The formation of a hot neutron star and its subsequent cooling leads to the appearance of a nonequilibrium layer in which energy of up to 1048 ergs is stored. The lack of equilibrium consists of the presence of superheavy nuclei with a large neutron excess near the limit of neutron evaporation Qn = 0 and of free neutrons. The slow diffusion of neutrons into the interior of the star maintains the x-ray luminosity Lx 1034 ergs/sec during t?104 years. Nonstationary processes are discussed which are associated with the existence of the nonequilibrium layer. Outward transport of nonequilibrium matter and nuclear explosions may be associated with the observed x-ray bursts. Breaking up of the crust accompanying the diffusion of neutrons into the interior of the star and the expansion of the shell may explain abrupt changes in the period of the Crab pulsar. Expulsion of neutrons and superheavy nuclei into interstellar space accompanying explosions and subsequent electrodynamic acceleration of particles may explain the origin of heavy elements with A>150 and also of deuterium.

The problem of intermediate valency

Abstract: The situation is found in a number of rare-earth compounds in which the narrow f level lies near the Fermi level, and configurations of the rare-earth ions differing in the number of f electrons (different valency) have close-lying energies. In this situation many properties of the corresponding substances, both thermodynamic (heat capacity, magnetic susceptibility, compressibility) and kinetic (optical properties, electric conductivity, etc.) are anomalous. Isostructural electronic phase transitions occur in these systems upon changing the external conditions. These are often simultaneously dielectric-metal and magnetic-nonmagnetic transitions. A phase having a narrow resonance level near EF has been termed an intermediate-valence (IV) phase. This article reviews generally the problem of intermediate valency. The fundamental experimental facts are summarized. The conditions for appearance and the qualitative picture of IV states are discussed. A table is given of the currently know IV substances. The fundamental theoretical approaches to describing their properties and the electronic phase transitions in these systems are treated. The connection is established with other problems (the problem of dielectric-metal transitions, the Kondo effect, etc.). Major attention is paid to the fundamental unsolved problems.

Low-temperature plasmas with nonequilibrium ionization

Abstract: Low-temperature plasmas are frequently not in thermodynamic equilibrium. External fields, the emission of radiation, gradients of various physical properties, and the finite rates of various processes can all prevent the attainment of equilibrium. In nonequilibrium conditions the ionization state, the distribution of atomic excited states, and the electron energy distribution all become complicated functions of the factors responsible for the deviation from equilibrium. Since the components of the plasma—the electrons, atoms, and ions—are strongly coupled, a departure from equilibrium in one component causes departure in the others. The criteria for a deviation from local thermodyamic equilibrium are given. A study is made of plasmas far from equilibrium, in which the electron density is not described by the Saha equation, the atoms do not have a Boltzmann energy-level distribution, and the electrons do not have a Maxwellian energy distribution. A steady-state nonequilibrium plasma and time-dependent relaxation phenomena are studied. The theory is compared with the extensive experimental data available.

Micropinch in a high-current diode

Abstract: The basic data on the micropinch in a high-current, low-inductance spark are reviewed. This phenomenon was discovered by Cohen et al. in 1968. A theory for the equilibrium and collisionless emission of a plasma compressed to a degenerate state by the magnetic field of the high current yields a natural explanation for several properties of the micropinch which are unexpected at first glance: the hard x-ray burst, the time evolution and directional pattern of this burst, the power-law spectrum above 150 keV, the confinement of the emission sources to certain spatial regions, and the highly ionized atoms, among others. (Similar effects are observed in z pinches, the plasma focus, and the explosion of wires.) At maximum compression, the micropinch seems to be similar to the Thomas-Fermi "linear atom," which consists of ions which are at rest on the whole and drifting electrons which carry the current. The micropinch has potential applications (in the fusion problem and in developing sources of induced synchrotron radiation for the x-ray range), but it is also of much physical interest in itself, since it allows laboratory research on a supercontracted hot plasma, i.e., matter in a state approaching that in stars. In order to prove unmistakably that supercontracted matter is present in a micropinch, however, it would be necessary to carry out direct experiments to observe the strong electromagnetic fields and the high density of matter in regions with dimensions measured in angstroms.

Volume interactions in the statistical physics of a polymer macromolecule

Abstract: Theoretical approaches to the spatial structure of linear macromolecular chains are reviewed. The associated problems are found to be essentially different when repulsion and attraction between chain elements are respectively dominant. In the case of attraction, the chain condenses on itself and forms a globule with a definite structure which can be analyzed in the self-consistent field approximation. In the case of repulsion, the result is a coil with a fluctuating structure and, in this situation, one must use the methods of scaling theory. The coil-globule transition is also analyzed.

The renormalization group and ultraviolet asymptotics

Abstract: This article describes in detail the method of the renormalization group and outlines the possibilities of using it for the analysis of high-energy asymptotics in the framework of quantum field theory. The renormalization group formalism is constructed for an arbitrary scheme of renormalization. The exposition is based on the concepts of effective charge and effective mass. Principal attention is given to the problem of deriving reliable information about the ultraviolet properties of quantized-field models on the basis of perturbation theory calculations. A summary of the results of such calculations for a wide variety of models is given.

Electron theory of metals and geometry

Abstract: Modern electron theory of metals abounds in geometric terminology and, by using geometric ideas, provides clear descriptions of many complicated phenomena. Some problems in the theory of normal metals that can be interpreted geometrically are examined in this paper in a language accessible to many physicists. Particular attention is devoted to those phenomena and properties that are connected with qualitative (topological) changes in geometric figures such as Fermi surfaces, plane sections though such surfaces, "belts" on the Fermi surface, and so on. Graphical illustrations of these ideas are provided.

Electrons and positrons in dense gases

Abstract: The appearance of electrons or positrons in dense gaseous media produces a whole series of striking phenomena governed by the properties of the states of the light particles in the dense medium. The present review considers these phenomena systematically. The results of measurements of electron mobility and positron annihilation rates in moderately dense gases are discussed. Effects that arise as the density of the gas increases are considered. Self-trapped states become possible at high densities: these are bound states of the light particles with the medium, "clusters," "bubbles," or mixed-type formations. The fluctuon theory of these states is set forth, along with the results of experiments in which the light particles make transitions to new states. The possible existence of "orientation clusters" in gases consisting of polar molecules is investigated. Certain applications of the results to the theory of discharges in dense gases and liquids and to the theory of the weakly ionized nonideal plasma are discussed.

L. I. Mandel'shtam and the modern theory of nonlinear oscillations and waves

Abstract: The principal models and phenomena of the modem theory of nonlinear oscillations and waves are briefly reviewed in observance of L. I. Mandel'shtam's centennial. Each part of the paper, broken down into sections on Oscillators, Self-oscillations, and Modulation, begins with a discussion of the classical models and effects of the role they play in the modern theory. Attention is concentrated on an analysis of complex behavior of simple systems, collective phenomena and ensembles of nonlinear oscillators, the initiation of self-oscillations in space, stochastic self-oscillations and waves, and the excitation, transfer, and origins of modulation in nonlinear systems and media, etc. Oscillation-theory and wave-theory models are discussed in parallel.

Electronic transition probabilities and lifetimes of electronically excited states of diatomic molecules

Abstract: Recent results on the probabilities of electronic transitions of diatomic molecules are reviewed. Experimental methods for determining the absolute (Se,Remn2, femn, τnv') and relative probabilities, the systematic classification and analysis of the data, and criteria of the reliability of the recommended values are discussed. The absolute and relative probabilities published up to and including 1977 have been collected and used to obtain recommended (most reliable) values of the band strengths Se(rv'v'') and oscillator strengths femn for more than 200 molecular systems. The experimental lifetimes τnv' are analyzed for more than 150 electronically excited states. Most reliable values are recommended for these lifetimes.

On the history of the Einstein–de Haas effect

Abstract: This article contains information on Einstein's experimental work. It pays main attention to three of his studies (1915–1916) aimed at experimental demonstration of Ampere molecular currents. It treats the content of these studies, their prehistory, and the peculiar revaluation of their results after the discovery of the spin of the electron. It traces the effect of these studies on Einstein's attitude toward the Stern-Gerlach experiments and the studies of Uhlenbeck and Goudsmit, and also points out their genetic tie with the well-known work of Einstein and Ehrenfest.

Self-consistent electron theory of a metallic surface

Abstract: This is a review of researches on the calculation of the electronic characteristics of a free or perturbed metal surface; these include the electron density, the potential, the spectrum of surface states, the work function, the surface energy, response of the surface to an electric field, the chemisorption of atoms, and the adhesion of two metals. Preference is given to papers that employ the theory of the ground state of an inhomogeneous electron gas, the density-functional method. The various approximations to the density-functional used in the calculations, and the various models of the lattice (jelly, pseudopotential), are analyzed as to their applicability. Various sorts of experimental data are discussed on the basis of the theoretical results.

Tests of fundamental physical theories from measurements on free charged leptons

Abstract: After a brief introduction in which the concepts of the magnetic and electric dipole moments of particles are introduced and questions are discussed associated with the discrete P and T transformations, the main part of the paper follows which consists of two sections. In the first of these the validity of quantum electrodynamics is analyzed on the basis of measurements of the anomalous magnetic moments of electrons and muons. Special emphasis is placed on experimental methods of measurement and on their historical development. The most detailed description is provided of three recent experiments: 1) measurement of (g – 2) for muons at CERN; 2) the spin resonance experiment on single electrons at the University of Washington; 3) the precise comparison of electron and positron magnetic moments carried out using the VEPP-2M storage ring at Novosibirsk. In the next section a review is given of the tests of CPT, CP, and T invariance from muon decay correlations and lifetime measurements. Finally the present status of experimental searches for an electric dipole moment (the existence of which would violate both P and T invariance) of electrons and muons is discussed.

From the equivalence principle to the equations of gravitation

Abstract: The main stages in the discovery by Einstein of the equations of the general theory of relativity in November 1915 are recounted. The evolution of his ideas from the first formulation of the equivalence principle in 1907 is followed. The part played by Hubert in the finding of the final equations of the theory is discussed.

Thermophoresis in gases

Abstract: We analyze the mechanisms that cause the motion of objects suspended in inhomogeneously heated gases. We treat two limiting cases: a) a highly rarefied gas in which the mean free path λ of the gas molecules is large in comparison with the characteristic dimension R of the object, and b) a weakly rarefied gas that satisfies the condition λ R. In both cases we assume that the characteristic scale of the temperature inhomogeneities in the gas obeys Lλ. The case of a weakly rarefied gas is of very great interest, primarily from the standpoint of studying the state of a gas near a gas-solid phase boundary in the Knudsen layer. The greater part of the review is devoted to this problem: we treat the methodology of obtaining the boundary conditions for hydrogasdynamics with slip, and present in detail a scheme for calculating the kinetic coefficients that generalizes the Chapman-Enskog method to the case in which the state of the gas inside the Knudsen layers plays a substantial role, and we discuss the problem of the applicability of the thermodynamics of irreversible processes to problems of this type and demonstrate the efficacy of its methods. In addition, we modify the well-known method of half-range expansions on the basis of some physical assumptions and express some ideas on the principles of construction of the system of moment equations in the kinetic theory of gases. We also propose a scheme of experiment for testing the validity of the presented concepts.

Leonid Isaakovich Mandel'shtam

Abstract: Keynote paper at the May 30, 1979 joint scientific session of the Division of General Physics and Astronomy and the Division of Nuclear Physics of the USSR Academy of Sciences, which was devoted to the 100TH anniversary of the birth of Academician L. I. Mandel'shtam. The paper discusses the three main periods in Mandel'shtam's life and work: the Strasbourg period (1899-1914), the time from his education in Russia to his final move to Moscow (1914-1925), and the last, Moscow period (1925-1944), which was the most productive from the standpoint of both scientific and teaching activity. Mandel'shtam was (together with Academician A. F. Ioffe) the founder of Soviet physics, the creator of a physical school exceptional for breadth of its interests and the depth of its research, several of whose representatives went on to form widely known schools of their own, both theoretical (I. E. Tamm, A. A. Andronov, M. A. Leontovich) and experimental (G. S. Landsberg). His coverage of physics, the organic fusion of theoretical and experimental and mathematical and technical physics in Mandel'shtam's creativity, place him among the classics of science. The most important of Mandel'shtam's outstanding scientific and technical achievements are indicated, including the discovery of combination [Raman] and selective scattering of light, establishment of the role of fluctuations in scattering and prediction of the fine structure of Rayleigh scattering, his advancement of the theory of oscillations to the level of an independent scientific discipline and the creation of a nonlinear theory of oscillations, the invention of radiointerferometry, the discovery of the tunnel effect in quantum mechanics, etc. Also outlined are a number of general guiding concepts that Mandel'shtam developed, such as "nonlinear physical thinking," classification of phenomena on the basis of commonality (isomorphism) of the describing relationships, and application of "oscillatory mutual aid" between various fields of physics and engineering. These general ideas unified and directed all of his creative work and have by now thoroughly permeated modem physics.

Two approaches to spatial dispersion in molecular scattering of light

Abstract: A comparative discussion is given of two approaches to taking spatial dispersion into account in the electrodynamic problem of molecular scattering of light. The first, more traditional approach, may be called the "distributed dipole" approximation (DDA) and is based on the assumption that any given molecule at a given instant of time scatters light as an electric dipole. In this approach spatial dispersion, i.e., the dependence of the spectrum on the variation of the propagation vector q = k1 – k2, is determined by the correlation of the positions of a given molecule (or of different molecules) at different times. Another approach, developed in recent years by Barron and Buckingham for the problem of light scattering by molecules with right-left asymmetry, may be called the "local multipole" approximation (LMA) and is based on taking into account the magnetic dipole and the electric quadrupole as well as the electric dipole interaction of a molecule with the field. A list is given of sets of "complete experiments" for measuring all the independent constants that determine the scattering cross section in both approaches. It is shown that the DDA approach is needed to describe the relatively large (~1) effects of spatial dispersion in measurements with high spectral resolution (δωqv, where v is the velocity of sound in the medium) while the LMA approach is required to describe the small effects (~α/λ, where a is the size of the molecule and λ is the wavelength) measured with relatively low spectral resolution δωqv. It is asserted that the right-left asymmetry of the differential (with respect to frequency) cross section for scattering in a gas containing chiral molecules need not involve the smallness parameter ka if ql ~1, where a is the size of the molecule and l is the mean free path. Also new lines are predicted in the rotational Raman scattering in a gas—transitions with ΔJ =±1,±3 in the case of noncentrally-symmetric molecules with a cross section ~10−6 of the Rayleigh cross section arising in second order in α/λ due to the higher multipoles.

Selective atomic photoionization and its use in isotope separation and spectroscopy

Abstract: The development of selective photoionization of atoms is reviewed. This is a very general method for laser isotope separation, and it has extremely interesting spectroscopic applications. Processes tending to restrict the applicability of the method are discussed. Examples cited include the use of selective photoionization to study the superior structure in atomic spectra, the resonance transfer of excitation energy, and the spectroscopy of atomic Rydberg states. Questions involved in laser isotope separation by selective photoionization are discussed.

Vibrational relaxation in condensed media

Abstract: We review the literature on spectroscopic methods of studying processes of vibrational relaxation of molecules in condensed media; special attention is paid to liquids. We discuss the features and advantages of the following methods: spontaneous Raman scattering, picosecond spectroscopy, and active Raman spectroscopy. Comparison of the results obtained by the different methods yields detailed information on the course of relaxation processes.