Showing papers in "Physics-Uspekhi in 2006"

Spatial dispersion and negative refraction of light

Abstract: Negative refraction occurs at interfaces as a natural consequence of the negative group velocity of waves in one of the interfacing media. The historical origin of this understanding of the phenomenon is briefly discussed. We consider several physical systems that may exhibit normal electromagnetic waves (polaritons) with negative group velocity at optical frequencies. These systems are analyzed in a unified way provided by the spatial dispersion framework. The framework utilizes the notion of the generalized dielectric tensor eij(ω, k) representing the electromagnetic response of the medium to perturbations of frequency ω and wave vector k. Polaritons with negative group velocity can exist in media (whether in natural or in artificial meta-materials) with a sufficiently strong spatial dispersion. Our examples include both gyrotropic and nongyrotropic systems, and bulk and surface polariton waves. We also discuss the relation between the spatial dispersion approach and the more familiar, but more restricted, description involving the dielectric permittivity e(ω) and the magnetic permeability μ(ω) .

Let there be white light: supercontinuum generation by ultrashort laser pulses

Abstract: Three centuries after Newton's experiments on the decomposition of white light into its spectral components and the synthesis of white light from various colors, nonlinear-optical transformations of ultrashort laser pulses have made it possible to produce an artificial white light with unique spectral properties, controlled time duration, and a high spectral brightness. Owing to its broad and continuous spectrum, such radiation is called supercontinuum. The laser generation of white light is an interesting physical phenomenon and the relevant technology is gaining in practical implications — it offers novel solutions for optical communications and control of ultrashort laser pulses, helps to achieve an unprecedented precision in optical metrology, serves to probe the atmosphere of the Earth, and suggests new strategies for the creation of compact multiplex light sources for nonlinear spectroscopy, microscopy, and laser biomedicine. Here, we provide a review of physical mechanisms behind the laser generation of white light, examine its applications, and discuss the methods of generation of broadband radiation with controlled spectral, temporal, and phase parameters.

The Nambu–Jona-Lasinio model and its development

Abstract: The historical development of the Nambu–Jona-Lasinio (NJL) model is briefly reviewed. The SU(2) × SU(2) and U(3) × U(3) local quark NJL models are considered. The mechanisms responsible for spontaneous breaking of chiral symmetry and vector dominance are exhibited. The local NJL model is adequate in describing the mass spectrum and the strong and electroweak decay modes of the four ground-state meson nonets: pseudoscalar, scalar, vector, and axial-vector. The applicability of the model to mesons in a hot dense medium is discussed. It is shown that solving problems related to the description of meson radial excitations and quark confinement requires the nonlocal extension of the NJL model. The primary emphasis of this review is on the methods that are used in various versions of the NJL model. The reader is referred to the cited works for what these models predict in low-energy hadron physics.

Giant nonlinearities in structurally inhomogeneous media and the fundamentals of nonlinear acoustic diagnostic techniques

Abstract: Giant nonlinearities in structurally inhomogeneous media and the fundamentals of nonlinear acoustic diagnostic techniques

Chaotic advection in the ocean

Abstract: The problem of chaotic advection of passive scalars in the ocean and its topological, dynamical, and fractal properties are considered from the standpoint of the theory of dynamical systems. Analytic and numerical results on Lagrangian trans- port and mixing in kinematic and dynamic chaotic advection modelsaredescribedformeanderingjetcurrents,topographical eddies in a barotropic ocean, and a two-layer baroclinic ocean. Laboratory experiments on hydrodynamic flows in rotating tanks as an imitation of geophysical chaotic advection are described. Perspectives of a dynamical system approach in physical oceanography are discussed.

Phase transitions in two-dimensional systems with continuous degeneracy

Abstract: We discuss the nature and sequence of phase transitions in two-dimensional systems with continuous degeneracy, in which other factors must be taken into account in addition to the interaction of point-like topological excitations, namely, the existence of an additional discrete degeneracy (which leads to the possibility of domain wall formation), the removal of accidental degeneracy by fluctuations, the formation of solitons, and the presence of a random potential acting on vortices. Formally, we discuss various modifications of the two-dimensional XY model, while physical objects that can be described by such models are various arrays of superconducting junctions and noncollinear planar antiferromagnets.

Simple metals at high pressures: the Fermi sphere – Brillouin zone interaction model

Abstract: High-pressure structural transformations are analyzed for simple sp-elements and some binary alloys. The crystal structure stability of these metals depends on the Fermi surface–Brillouin zone interaction. An increase in this interaction with pressure results in transitions to less symmetric and less closely packed structures. A structural similarity is shown to exist between the high-pressure phases for alkali and alkali-earth metals and for polyvalent group IV and V elements. The correlation between the behavior under compression of the structure and the physical properties (resistivity and superconductivity) of these metals is discussed in terms of the Fermi sphere–Brillouin zone interaction model.

Similarity laws for pulsed gas discharges

Abstract: The feasibility of applying the similarity law to different types of pulsed discharges is analyzed. The analysis is based on the dependence pτ = f(E/p), where τ is the charge formation time, p is the gas pressure, and E is the pulsed field strength at which the breakdown occurs. The law holds for the Townsend and streamer breakdowns for a relatively long discharge gap d (for atmospheric air, d > 1 cm). For millimeter gaps, this law applies to many gases only in the case of the multielectron breakdown initiation down to the picosecond range. In this case, the time τ is measured from the instant the voltage amplitude sets in to the onset of current buildup and of the drop in voltage across the gap during the simultaneous development of a large number of electron avalanches. In the initiation by a small number of electrons, the time τ is longer than in the multielectron initiation by nearly an order of magnitude; this is due to the relatively low rate of free-electron accumulation in the gap, with runaway electrons (REs) playing an important role in this process. But the time θ of the fast voltage drop and current buildup obeys the similarity law pθ = F(E/p) in both cases. It is hypothesized that the source of REs is the field emission from cathodic micropoints, which terminates at the onset of explosive electron emission to limit the RE current pulse duration to 10−10 s. The similarity law pτ = f(E/p) is shown to hold for a pulsed microwave breakdown.

Bose – Einstein condensates in a laser radiation field

Abstract: The properties of Bose–Einstein condensates in an external potential produced by a laser light field are discussed. Considered are the condensates embedded in periodic lattices produced by standing laser waves and the condensates confined within 'optical traps' near the focus of a traveling wave. Observations of the Mott transition in a periodic lattice are described, as are experiments on atoms near the Feshbach resonance and experiments on the condensates in a double potential well, which permit investigating the tunnel dynamics of condensate phases.

Thomas precession: correct and incorrect solutions

Abstract: A wealth of different expressions for the frequency of the Thomas precession (TP) can be found in the literature, with the consequence that this issue has been discussed over a long period of time. It is shown that the correct result was obtained in the works of several authors, which were published more than forty years ago but remained unnoticed against the background of numerous erroneous works. Several TP-related physical paradoxes formulated primarily to disprove the special relativity theory are shown to be fallacious. Different techniques for deriving the correct expression are considered and the reasons for the emergence of the main incorrect expressions for the TP frequency are analyzed.

Cladding modes of optical fibers: properties and applications

Abstract: One of the new methods of fiber optics uses cladding modes for controlling propagation of radiation in optical fibers. This paper reviews the results of studies on the propagation, excitation, and interaction of cladding modes in optical fibers. The resonance between core and cladding modes excited by means of fiber Bragg gratings, including tilted ones, is analyzed. Propagation of cladding modes in microstructured fibers is considered. The most frequently used method of exciting cladding modes is described, based on the application of long-period fiber gratings. Examples are presented of long-period gratings used as sensors and gain equalizers for fiber amplifiers, as well as devices for coupling light into and out of optical fibers.

The nature, kinetics, and ultimate storage capacity of hydrogen sorption by carbon nanostructures

Abstract: The review covers the present state of studies of the urgent open questions concerning the nature, kinetics, and limit values of hydrogen sorption by carbon nanostructures. These questions are related to the key issues in the problem of building a hydrogen electrical automobile. Considered are the thermodynamic and diffusion characteristics and the micromechanisms of the processes of chemical and physical sorption of hydrogen by graphite and related carbon nanomaterials, and also the various methodological aspects of studying and optimizing such hydrogen adsorbents. The experimental and theoretical prerequisites and prospects for developing a 'superadsorbent' (?10 mass %) for storing hydrogen onboard an automobile are also discussed.

Critical size in ferroelectric nanostructures

Abstract: Recently, several attempts have been made to determine the critical size in ferroelectricity. Due to the development of ferroelectric nanostructure technology, this fundamental problem had also become crucial for applied research. It is shown that although the theory predicts the existence of a finite critical size, ferroelectric polarization and its switching can be observed in monolayer films, at least in the case of ferroelectric vinylidene fluoride-trifluoroethylene copolymer P[VDF–TrFE] films prepared by the Langmuir–Blodgett method. The experimental search for the critical size in perovskite ferroelectrics is briefly reviewed. It is shown that the Landau–Ginzburg theory predicts the critical size to be infinitely small if the extrinsic effect of the film–electrode strain mismatch is taken into account. Special features of the switching dynamics of ultrathin ferroelectric films are also considered.

Electronics of thin layers of wide-band polymers

Abstract: The history of the creation of conducting polymers is reviewed and their properties are considered. Some special charge-transport features of phthalide-containing polymers—the injection mechanism, high conductivity, and the dependence on boundary conditions—are described. Conditions for a polymer film transition to highly conducting and superconducting states are examined. Prospects for the application of wide-band polymer materials in electronic devices are discussed.

Properties of backward electromagnetic waves and negative reflection in ferrite films

Abstract: For a backward electromagnetic wave (magnetostatic wave) in a ferrite film, reflection from a perfect mirror formed by the straight edge of the film is investigated experimentally and theoretically. It is found that when the incident wave is collinear (the group velocity vector and the wave vector have opposite directions), negative reflection occurs at any angle of incidence, i.e., the incident and reflected beams are on the same side of the normal to the boundary. It is discovered that a noncollinear backward wave is nonreciprocal in the sense that its energy can be localized both near the surface and in the middle of the film. This property, previously observed only for surface magnetostatic waves, provides both the efficiency of generating and receiving the wave and the possibility of observing the reflected beam. A situation is realized where wave reflection results in two reflected beams. The properties of backward electromagnetic waves propagating in ferrite films are briefly analyzed.

Near-electrode and transient processes in liquid dielectrics

Abstract: Electron transfer kinetics between an electrode and an electron acceptor in electrolytes, liquid dielectrics, and dense gases is comparatively analyzed for the case of a weak electric field. Near-electrode charge distribution patterns in a liquid dielectric (or a dense weekly conducting gas) exposed to an external high-voltage field are examined. It is shown that the quadratic dependence of current–voltage characteristics of the system is determined by the character of electron transitions from surface states. The transient processes involved are discussed, and characteristic formation times for steady-state near-electrode patterns are calculated.

Theory of gauge-invariant response of superconductors to an external electromagnetic field

Abstract: A consistent method of calculating the linear response to an external magnetic field that allows obtaining the result in a manifestly gauge-invariant form is proposed. Within the diagram technique for nonequilibrium processes, the self-consistency equations for the order parameter in an arbitrary-gauge field allow deriving an equation that determines the phase of the order parameter as a function of the external field. Such a method automatically accounts for the existence of collective excitations in superconductors, which must be taken into consideration in accordance with the continuity equation. The possible types of collective excitations in pure superconductors at different temperatures are considered. The authors present a microscopic theory that explains the possibility of observing collective modes in superconducting tunnel junctions.

'Nonclassical' manifestations of microstructure-induced nonlinearities: new prospects for acoustic diagnostics

Abstract: the Russian Academy of Sciences (RAS) and the Joint Physical Society of the Russian Federation was held on September 28, 2005 in the Conference Hall of the Lebedev Physics Institute, RAS under the name ``Nonlinear acoustic diagnostics.'' The following reports were presented at the session: (1) Rudenko O V (Lomonosov Moscow State University) ``Giant nonlinearities in structurally inhomogeneous media and the fundamentals of nonlinear acoustic diagnostics methods''; (2) Zaitsev V Yu, Nazarov V E, Talanov V I (Institute of Applied Physics, RAS, Nizhny Novgorod) ```Nonclassical' manifestations of microstructure-induced nonlinearities: new prospects for acoustic diagnostics''; (3) Esipov I B, Rybak S A, Serebryanyi A N (Andreev Acoustics Institute, RAS) ``Nonlinear acoustic diagnostics of the ocean and rock''; (4) Preobrazhenskii V L (Research Center for Wave Studies, Prokhorov Institute of General Physics, RAS, European Laboratory in Nonlinear Magneto-acoustics (LEMAC)) ``Parametrically phase-conjugate waves: applications in nonlinear acoustic imaging and diagnostics.'' An expanded version of the report by Rudenko is published in the `Physics of our days' section of this issue. An abridged version of reports 2 ± 4 is given below.

Extreme processes in clusters impacting on a solid surface

Abstract: A large high-energy cluster impacting on a solid surface (the number of particles in a cluster N ≥ 102 – 106, the collision energy per particle Ecol ≥ 10 – 103 eV) forms for a short time (≤ 50 – 500 fs) a medium characterized by extremely high temperature (≥ 104 – 105 K), density (up to 4 to 5 times the solid state value), and pressure (≥ 1 – 10 Mbar). As this takes place, the cluster heating rate reaches the value ≥ 1015 – 1016 E s−1. In these extreme conditions, physical and chemical processes that are impossible in thermal equilibrium can occur both in the cluster itself and the collision zone. In this paper, extreme processes induced in clusters as a result of their strong excitation at collisions with a solid surface are reviewed, including ionization, light and charged-particle emission, fragmentation, breaking and making of chemical bonds, microshock wave generation, nuclear fusion, and surface bombardment. Conditions for these processes to proceed are examined and models to describe them discussed. It is shown that the characteristics of the processes depend significantly on the velocity, size, and composition of the cluster, as well as the material and temperature of the surface. Cluster excitation by an impact with a surface and that by a superhigh-power ultrashort laser pulse are compared and practical applications of the above processes are discussed.

Structural models of water

Abstract: Various conceptions on the structure of water are reviewed and different intermolecular interaction models underlying its description are discussed. To describe the structure of water, the method of integral equations for correlation functions borrowed from the theory of liquids is applied. Some numerical simulation results obtained with the Monte Carlo and molecular dynamics methods are also discussed in the context of water structural peculiarities.

Superconductivity of repulsive particles

Abstract: Antiferromagnetic correlations in superconducting cuprates can lead to the mirror nesting of Fermi contour segments near saddle points of the electron spectrum and to a logarithmic singularity of the scattering amplitude for a large pair momentum. The Coulomb potential defined within a kinematically allowed region allows a negative eigenvalue, which provides superconducting pairing and weakly decaying, quasistationary, large-momentum pair states. The Ginzburg–Landau equations for the two-component superconducting order parameter provide pairs of coupled particles and pairs of coupled orbital current circulations, which explains the fundamental cuprate properties such as strong and weak pseudogaps, the superconducting transition temperature, the diamagnetic pseudogap state, and details of the isotope effect. A quantum critical point and a transition of two superconducting phases, one of which displays superconductivity with current circulations, are predicted.

Metastable phases, phase transformations, and phase diagrams in physics and chemistry

Abstract: Concepts of a 'phase' and a 'phase transition' are discussed for stable and metastable states of matter. While condensed matter physics primarily considers equilibrium states and treats metastable phases as exceptions, organic chemistry overwhelmingly deals with metastable states. It is emphasized that many simple light-element compounds — including most hydrocarbons; nitrogen oxides, hydrides, and carbides; carbon monoxide CO; alcohols and glycerin — are also metastable at normal pressure in the sense that they do not correspond to a minimum Gibbs free energy for a given chemical composition. At moderate temperatures and pressures, the phase transformations for these metastable phases are reversible with the fulfilment of all laws of equilibrium thermodynamics over the entire range of experimentally accessible times. At sufficiently high pressures (> 1–10 GPa), most of the metastable molecular phases irreversibly transform to lower-energy polymer phases, stable or metastable. These transitions do not correspond to the equality of the Gibbs free energy for the involved phases before and after the transition and so they are not first-order in the 'classical' sense. At normal pressure, the resulting polymer phases can exist at temperatures above the melting point of the original metastable molecular phase, as the examples of polyethylene and polymerized CO dramatically illustrate. As pressure is increased further to 20–50 GPa, the PV contribution to Gibbs free energy gives rise to stable high-density atomic phases. Many of the intermediate-energy polymer phases can likely be synthesized by methods of 'classical' chemistry at normal pressure.

Binary and recycled pulsars: 30 years after observational discovery

Abstract: Binary radio pulsars, first discovered by Hulse and Taylor in 1974 [1], are a unique tool for experimentally testing general relativity (GR), whose validity has been confirmed with a precision unavailable in laboratory experiments. In particular, indirect evidence of the existence of gravitational waves has been obtained. Radio pulsars in binary systems (which have come to be known as recycled) have completed the accretion stage, during which neutron star spins reach millisecond periods and their magnetic fields decay 2 to 4 orders of magnitude more weakly than ordinary radio pulsars. Among about a hundred known recycled pulsars, many have turned out to be single neutron stars. The high concentration of single recycled pulsars in globular clusters suggests that close stellar encounters are highly instrumental in the loss of the companion. A system of one recycled pulsar and one 'normal' one discovered in 2004 is the most compact among binaries containing recycled pulsars [2]. Together with the presence of two pulsars in one system, this suggests new prospects for further essential improvements in testing GR. This paper considers theoretical predictions of binary pulsars, their evolutionary formation, and mechanisms by which their companions may be lost. The use of recycled pulsars in testing GR is discussed and their possible relation to the most intriguing objects in the universe — cosmic gamma-ray bursts — is examined.

Ginzburg–Landau theory: the case of two-band superconductors

Abstract: Recent studies of two-band superconductors using the Ginzburg–Landau (GL) theory are reviewed. The upper and lower critical fields [Hc2(T) and Hc1(T), respectively], thermodynamic magnetic field Hcm(T), critical current density jc(T), magnetization M(T) near the upper critical field, and the upper critical field Hc2film(T) of thin films are examined from the viewpoint of their temperature dependence at a point Tc using the two-band GL theory. The results are shown to be in good agreement with the experimental data for the bulky samples of superconducting magnesium diboride, MgB2, and nonmagnetic borocarbides LuNi2B2C and YNi2B2C. The specific heat jump turns out to be smaller than that calculated by single-band GL theory. The upper critical field of thin films of two-band superconductors is calculated and the Little–Parks effect is analyzed. It is shown that magnetic flux quantization and the relationship between the surface critical magnetic field Hc3(T) and the upper critical field Hc2(T) are the same as in the single-band GL theory. Extension of the two-band GL theory to the case of layered anisotropy is presented. The anisotropy parameter of the upper critical field Hc2 and the London penetration depth λ, calculated for MgB2 single crystals, are in good agreement with the experimental data and show opposite temperature behavior to that in single-band GL theory.