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)     

Formation of a chaotic layer of nonlinear resonance by a two-frequency perturbation

A new effect [V. V. Vecheslavov, Zh. éksp. Teor. Fiz. 109, 2208 (1996) (JETP 82, 1190 (1996)]—the appearance of low-frequency secondary harmonics in the separatrix mapping of a system—is discussed in detail for the example of a pendulum with a two-frequency perturbation. It is shown that there exist regions of values of the perturbation parameters where these harmonics make the main contribution to the formation of the chaotic layer of the fundamental resonance. The results of analytical and numerical determinations of the amplitudes of the secondary harmonics are compared. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 12, 989–994 (25 June 1996)     

Energetics of the thermoelastic effect in solids

The dependence of the temperature on the external adiabatic deformation is determined for a one-dimensional model of a solid — chains of atoms with an anharmonic interaction. The resulting dependences of the average kinetic and potential components of the internal energy on this deformation are compared with a model of adiabatic loading of a single oscillator. Fiz. Tverd. Tela (St. Petersburg) 40, 1548–1551 (August 1998)     

Adiabatic Raman polariton in a Bose condensate

The Raman interaction of optical fields with a Bose condensate is studied in the adiabatic regime. A superposition of operators is found — one annihilating an atom in a metastable state and the other annihilating a photon in resonance with a transition from the ground state to an excited state — which is an adiabatic invariant of the problem (Raman polariton). Possible applications for Bose-condensate diagnostics and development of atomic lasers are proposed. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 7, 473–477 (10 October 1996)     

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)     

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)     

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)     

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)     

Electronic resonances in a quantum well having a periodically uneven boundary

The spectrum of the electronic states in an infinitely deep two-dimensional potential well, where one wall is periodically uneven, is investigated theoretically. It is shown that in non-Bragg type resonances — standing electron wave resonances, which are modes of different spatial harmonics of the electron wave function — arise in such a well. The resonances occur in a wide range of energies, starting at values close to the bottom in each 2D subband. The resonance interaction splits the energy spectrum and results in the appearance of gaps, giving the electron spectrum a miniband character. The properties of the electron gas vary substantially in accordance with the new characteristics of the spectrum. Fiz. Tverd. Tela (St. Petersburg) 41, 1867–1870 (October 1999)     

Spectroscopy of GaAs/AlGaAs microstructures with submicron spatial resolution using a near-field scanning optical microscope

A near-field scanning optical microscope (NSOM), which we built, is used to investigate 1–5-μm wide stripes with a 10-nm thick layer—a quantum well — on a GaAs surface. A map of the photoluminescence intensity is obtained synchronously with the topographic profile of the structures. The measured spatial distribution of the photoluminescence intensity is described satisfactorily in a model that takes into account carrier diffusion in the layer and the existence of a region with a short carrier lifetime near the side boundaries of the layer. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 523–527 (10 April 1996)     

Luminescence of short-lived color centers induced in LiF crystals by a pulsed microwave discharge

A new phenomenon — intense luminescence of noncolored lithium fluoride (LiF) crystals excited by an electrodeless pulsed microwave discharge at the prebreakdown stage of development — is observed. This luminescence consists of the luminescence of short-lived aggregate F₂ and F ₃ ⁺ color centers at room temperature. It is shown that the density of short-lived color centers induced in the surface layer of LiF crystals by a microsecond microwave discharge reaches values of ∼10¹⁹−10²⁰ cm⁻³. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 163–167 (10 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)     

Spin excitons — a new mechanism of superconducting pairing in copper oxides

The Fermi and Bose quasiparticle spectrum in copper oxides is studied in a many-band p-d model taking account of the strong electronic correlations. It is shown that hole-doped systems possess a Bose mode — a spin exciton — which is associated with the singlet-triplet excitation of the two-hole ground-state term of CuO₄ clusters. Intercluster hopping leads to fermion-boson interaction with a spin exciton as the intermediate boson. Such a mechanism does not exist for n-type systems. Pis'ma Zh. éksp. Teor. Fiz. 64, No. 1, 23–28 (10 July 1996)     

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)     

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)     

Amplification of the generation of the sum-frequency signal in multilayer periodic structures at the edges of the Bragg band gap

A new property of a one-dimensional periodic structure — amplification of the sum-frequency signal arising under the simultaneous action of two laser pulses on this structure with radiation frequencies corresponding to the edges of the fixed Bragg band gap — is experimentally observed and described. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 11, 718–721 (10 December 1999)     

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)     

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)     

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)