Structural transformations of the cumulene form of amorphous carbyne at high pressure

Structural transformations of the cumulene form of amorphous carbyne which are induced by heating at high pressure (7.7 GPa) are investigated. These can be described by the sequence amorphous phase — crystal — amorphous phase — disordered graphite. Raman scattering shows that predominately the chain structure of carbyne remains at the first three stages. It was found that the intermediate crystalline phase is an unknown modification of carbon whose structure is identified as cubic (a=3.145 Å). A mechanism of structural transformations in carbyne that involves the formation of new covalent bonds between chains is discussed. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 4, 237–242 (25 August 1997)     

Fabry-Perot cavity in the field of a gravitational wave

The structure of the electromagnetic field inside a laser cavity—a gravitational-wave detector—is studied. The properties of the spatial and temporal phases of the standing electromagnetic wave are discussed in detail and the corrections appearing in the electric field of the wave as a result of the action of gravitational radiation on the optical system are determined. Zh. éksp. Teor. Fiz. 113, 398–408 (February 1998)     

Antipolarization effect in quantum wells in a strong external alternating field

It is shown that when a strong ac electric field acts on an electron in a double quantum well, the dipole moment is an almost periodic function of the dc voltage applied to the structure. An antipolarization effect — the structure is polarized in a direction opposite to the external field — appears during one half of the period. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 6, 386–391 (25 September 1999)     

Electronic spectrum of a two-dimensional Fibonacci lattice

The electronic spectrum and wave functions of a new quasicrystal structure—a two-dimensional Fibonacci lattice—are investigated in the tight-binding approximation using the method of the level statistics. This is a self-similar structure consisting of three elementary structural units. The “central” and “nodal” decoration of this structure are examined. It is shown that the electronic energy spectrum of a two-dimensional Fibonacci lattice contains a singular part, but in contrast to a one-dimensional Fibonacci lattice the spectrum does not contain a hierarchical gap structure. The measure of allowed states (Lebesgue measure) of the spectrum is different from zero, and for “central” decoration it is close to 1. The character of the localization of the wave functions is investigated, and it is found that the wave functions are “critical.” Zh. éksp. Teor. Fiz. 116, 1834–1842 (November 1999)     

On the possibility of investigating the dispersion of the energy bands of a crystal using resonant x-ray emission spectra

It is shown that, when the dispersion of the energy bands of crystals is investigated by resonant x-ray emission (RXE), an ambiguity can arise in the determination of k _tr — the transition point in k space — as a result of the characteristic features of the band structure and the value of the matrix elements. It is proposed that calculations of the distribution of partial contributions to interband optical transitions in the Brillouin zone (BZ) be used as a means of obtaining a simplified estimate of k _tr. The basic properties of this distribution are examined for lithium hydride, which is a convenient model object having a simple band structure and interband transitions which have been studied in detail. It is shown that the k points of the general position, which form a complex figure in the BZ, make the main contribution to the interband transitions. The contribution of high-symmetry points is much smaller mainly because of their small representativeness. This make it easier to understand the reason for the possible nonmonotonic variation of k _tr in RXE measurements of the dispersion of the energy bands of crystals or the sharp changes in the resonance-fluorescence intensity. Fiz. Tverd. Tela (St. Petersburg) 41, 1597–1601 (September 1999)     

Application of symmetry operations for construction of quasiperiodic structures in a plane

A method is proposed for constructing a quasiperiodic structure of symmetry elements — regular pentagons and five-pointed stars — in a plane. The growth of the structure is determined by the action of the symmetry operations, whose effect is not completely identical to that of similar operations in classical crystallography. The tiling (“flower of pentagons”), consisting of a central pentagon and five side pentagons joined along the edges, is studied. The growth of this tiling is accompanied by the appearance of a “flower of stars” and by the formation of isolated pores in the form of rhombi. The relation between the obtained structure and Penrose tiling is examined, and it is noted that some vertices of the Penrose tiling coincide with all vertices of the polygons of the packing obtained. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 635–640 (25 April 1997)     

Critical experiments in the search for fermion condensation

The specific features of fermion condensation — a phase transition associated with the rearrangement of the one-particle degrees of freedom in strongly correlated Fermi systems — by which this phenomenon can be detected experimentally are discussed. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 11, 828–833 (10 June 1997)     

Structure of granular and homogeneous Re-Co alloys and the mechanism of its formation

Films of granular and homogeneous Re-Co alloys are obtained by electrolytic deposition. The results of x-ray diffraction and electron microscope studies are analyzed. A mechanism is proposed to explain the formation of the structure of Re-Co films, taking into account the effect of the concentration of rhenium and other components of the electrolyte on the nucleation and growth of the film. It is shown that a "polycrystalline — amorphous" structural transformation is realized in accordance with the following scheme:columnar → ultradisperse → aggregate → amorphous film structure. Institute of Solid-State Physics and Semiconductors, Academy of Sciences of Belarus. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 12–17, July, 1998.     

Evolution of large-scale correlations in a strongly nonequilibrium Bose condensation process

The evolution of off-diagonal correlation functions (for the example of a single-particle density matrix) in the process of Bose condensation of an initially nonequilibrium interacting gas is discussed. Special attention is given to the character of the decay of the density matrix at distances much greater than the size of the quasicondensate region. Specifically, it is shown that the exponential decay of the density matrix necessarily presupposes the presence of a chaotic vortex structure — a tangle of vortex lines — in the system. When topological order is established but there is no off-diagonal long-range order, the density matrix decays with distance according to a power law. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 7, 495–501 (10 April 1998)     

Possibility of the appearance of spin autowaves in a model of a ferromagnet with nonuniform dissipation

A model of a ferromagnet with nonuniform dissipation is introduced for the Landau-Lifshitz equations. It is shown that in this model a ferromagnet can be regarded as an oscillating active medium where the formation of autowave structures — spin autowaves, pacemakers, and spiral waves — is possible. Their wave characteristics, expressed in terms of the parameters of the medium, are found for a special case. Fiz. Tverd. Tela (St. Petersburg) 39, 513–515 (March 1997)     

Possibility of the levitation of droplets in the atmosphere when they are charged by induction in an electric field under nonuniform evaporation conditions

The behavior of droplets in the atmosphere in an electric field has important bearing on the theory of thunderstorms. One of the possible mechanisms by which droplets become charged—the induction mechanism in the presence of nonuniform evaporation—is investigated in greater detail on the basis of recent experimental results (V. A. Saranin, Zh. Tekh. Fiz. 65(6), 21 (1995) [Tech. Phys. 40, 332 (1995)]). Zh. Tekh. Fiz. 68, 16–21 (February 1998)     

Observation of a single laser-excited center at the point of a crystalline needle

The points of lithium fluoride needles are investigated by laser photoelectronic projection microscopy. A situation in which a single atomic-size source of electrons — an F ₂ color center — is observed in the region near the point is realized. As a result of the good fluorescence properties of these centers, these needles can be used as the active element of a scanning fluorescence microscope employing resonance transfer of electronic excitation energy. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 6, 441–444 (25 March 1997)     

Nonelectrical method of pumping solid-state lasers

An alternative, nonelectrical method for obtaining a dense radiating plasma and the possibilities of using this method to pump solid-state lasers are investigated. The plasma was obtained experimentally by heating the working gas in a two-stage ballistic plasmatron. A new device — a vortex chamber — is proposed for transferring energy into the plasmatron-laser system. Zh. Tekh. Fiz. 68, 67–70 (September 1998)     

Effect of focusing of primary electrons on their reflection from a crystal and on the associated Auger emission

The orientational dependence for different groups of secondary electrons — quasi-elastically scattered, inelastically reflected with excitation of a plasmon and with ionization of the core level M _4.5, and the Auger electrons M _4.5 VV — are measured in the primary electron energy range 0.6–1.5 keV. The data are obtained for a Nb (100) single crystal by varying the azimuthal angle of incidence of the primary beam, with complete collection of secondaries. A relationship is established between the processes of focusing and defocusing of the electrons that have penetrated into the crystal in the 〈110〉 and 〈133〉 directions, which differ substantially in the atomic packing density. Specific details of the Auger orientation effect, due to the focusinginduced variation of the flux density of the reflected electrons, are identified and explained. The contributions, both of anisotropy of ionization of the core level and of variation of the backscattering intensity, to the angular dependence of Auger emission and reflection with ionization loss are estimated. The possibilities of using such orientational dependences for an element-sensitive analysis of the local atomic structure of surfaces are assessed. Zh. Tekh. Fiz. 67, 117–123 (August 1997)     

Chaotic Rabi vacuum oscillations in cavity quantum electrodynamics

It is shown in numerical simulations with two-level atoms moving through a single-mode high-Q cavity that spontaneous emission of a new type — chaotic Rabi vacuum oscillations — arises in the strong atom-field coupling regime. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 11, 801–806 (10 June 1997)     

Critical fields and currents in a weakly nonlinear medium near the percolation threshold

For fields above a critical value the expansion of the conductivity in powers of the field ceases to be valid and the weak-nonlinearity approximation no longer works. The density behavior of the critical fields in strongly inhomogeneous media near the percolation threshold is found on the basis of two criteria—an average criterion and a local criterion. The parameter values of the medium for which crossover—a change of the critical behavior—occurs are determined. Similar calculations are performed for the critical currents. Zh. Tekh. Fiz. 68, 5–8 (June 1998)     

Predictability of ion transport properties from the structure of solid electrolytes

The concept of bond valence (BV) is widely used in crystal chemical considerations, e.g. to assess equilibrium positions of atoms in crystal structures from an empirical relationship between bond lengthR _M−X and bond valenceS _A−X =exp [(R ₀ −R _M−X ) /b] as sites where the BV sumV(A)=∑ s _M−X equals the formal valenceV _id of the cationM ⁺ . Our modified BV approach that systematically accounts for the softness of the bond may then be effectively used to study the interplay between structure and properties of solid electrolytes. This is exemplified for correlations to experimental data from IR, NMR, and impedance spectroscopy. Combining the bond valence approach with reverse Monte Carlo (RMC) modeling or molecular dynamics (MD) simulations provides a deeper understanding of ion transport mechanisms, especially in highly disordered or amorphous solids. Local structure models for crystalline electrolytes are derived by combining crystallographic structure information with simulations. A method for the prediction of the activation energy of the ionic conductivity from the bond valence analysis of the crystal structure is proposed. Taking into account the mass dependence of the conversion factor from bond valence mismatch into an activation energy scale, we could establish a correlation that holds for different types of mobile ions. The strong coupling of the H⁺ transfer to the anion motion in proton conductors requires a special treatment. For glassy solid electrolytes RMC structure models are BV-analyzed to assess the total number of equilibrium sites and to identify transport pathways for the mobile ions. Recently, we have reported a correlation between the pathway volume fraction and the transport properties that permits to predict both absolute value and activation energy of the dc ionic conductivities of disordered solids (including mixed alkali glasses) directly from their structural models. Here we discuss a corresponding BV analysis of molecular dynamics simulation trajectories that allows quantifying the evolution of pathways in time and the influence of temperature on the transport pathways. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 — 18, 2004.     

Experimental observation of cyclotron superradiance

Intense (several hundreds of kilowatts) subnanosecond coherent microwave radiation — cyclotron superradiance of an electron swarm moving in a uniform magnetic field — was recorded. The maximum power of the radiation was observed under group synchronization conditions, when the translational velocity of the swarm is equal to the group velocity of the waves in the waveguide channel. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 5, 322–325 (10 March 1996)     

Modification of porous silicon in ultrahigh vacuum and contribution of graphite nanocrystallites to photoluminescence

A replacement of the adsorbate in porous silicon is carried out in ultra-high vacuum. The photoluminescence line is shifted and quenched as the products of anodization of silicon — silicon hydrides and atomic and molecular hydrogen — undergo thermal decomposition and desorption. Adsorption of molecular chlorine restores the 560 nm photoluminescence band, which we identified as radiation from graphite nanoparticles. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 2, 106–111 (25 January 1996)     

Thermodynamic theory of the equation of state of materials

A universal derivation of the thermodynamic equations on the basis of a combined analysis of the exact relations for any material — the virial theorem, the shock adiabat, and the differential thermodynamic identity relating the thermic and caloric equations of state of matter — is given. This combination makes it possible to reduce the fundamental problem of thermodynamics to a boundary-value problem of mathematical physics. Analytic relations T _s=T(P _s,ρ _s and T _s=T(D,u) are obtained for classical systems. Zh. Tekh. Fiz. 68, 1–9 (October 1998)