Exciton-exciton collisions and conversion of interwell excitons in GaAs/AlGaAs superlattices

The ratio of the densities of intra-and interwell excitons in a symmetric system of coupled quantum wells — a superlattice based on a GaAs/AlGaAs heterostructure — is investigated over a wide range of optical excitation power densities. Conversion of interwell excitons into intrawell excitons as a result of exciton-exciton collisions is observed at high exciton densities. Direct evidence for such a conversion mechanism is the square-root dependence of the interwell exciton density on the optical excitation level. The decrease in the lifetime of interwell excitons with increasing excitation density, as measured directly by time-resolved spectroscopy methods, confirms the explanation proposed for the effect. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 623–628 (25 April 1997)     

Resonance fluorescence in an atom + dielectric microsphere system excited by a single photon

The resonance interaction of a two-level atom with a continuum of free-space modes modified by the presence of a dielectric microsphere (modified free-space modes — MFSMs) is studied. In the case that quantized MFSMs are initially excited within the contour of one of the resonance modes of the microsphere, the spectrum of emitted photons depends strongly on the excitation method. Under optimal excitation conditions efficient excitation of the atom accompanied by the formation of a Rabi doublet in the fluorescence spectrum occur. As the excitation conditions depart from optimality, the spectrum becomes a triplet. If the departure from optimality of excitation is large, the atom remains essentially unexcited, and the fluorescence spectrum has a singlet character. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 2, 115–120 (25 July 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)     

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)     

229m Th(3/2+, 3.5 eV) and a check of the exponentiality of the decay law

A new object is proposed for checking the exponentiality of the decay law of an isolated metastable state at long times — the anomalously low-lying level 3/2⁺ (3.5±0.5 eV) in ²²⁹Th. Highly efficient excitation of this level by laser radiation in combination with optimal collection of optical photons emitted in an isomeric transition make it possible to achieve measurement times longer than 50T _1/2. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 8, 521–525 (25 April 1998)     

Additional microwave modes in systems with conductivity of metals and superconductors

Semiclassical model, which takes into consideration the spatial dispersion effects in conductivity and permittivity, demonstrates the possible appearance of additional waves in conducting media, which are known to exist in transparent dielectrics near a narrow absorption band. The dispersion law of additional waves for modelling media (one-dimensional conductance) with parameters of Cu and Nb is obtained, which predicts the novel phenomenon—the possibility for additional microwave modes to propagate in metals with low enough attenuation at cryogenic temperatures.     

On the electric activity of superfluid helium at the excitation of first and second sound waves

We show that the electric activity of superfluid helium (HeII) observed in the experiments [3] during the excitation of standing second sound waves in an acoustic resonator can be described in terms of the phenomenological mechanism of the inertial polarization of atoms in a dielectric, in particular, in HeII, when the polarization field induced in the medium is proportional to the mechanical acceleration, by analogy with the Stewart-Tolman effect. The variable relative velocity w = v _n − v _s of the normal and superfluid HeII components that emerges in the second sound wave determines the mean group velocity of rotons, V _g ≈ w, with the density of the normal component related to their equilibrium number density in the temperature range 1.3 K ≤ T ≤ 2 K. Therefore, the acceleration of the 4He atoms involved in the formation of a roton excitation is proportional to the time derivative of the relative velocity.w. In this case, the linear local relations between the variable values of the electric induction, electric field strength, and polarization vector should be taken into account. As a result, the variable displacement current induced in the bulk of HeII and the corresponding potential difference do not depend on the anomalously low polarizability of liquid helium. This allows the ratio of the amplitudes of the temperature and potential oscillations in the second sound wave, which is almost independent of T in the above temperature range, consistent with experimental data to be obtained. At the same time, the absence of an electric response during the excitation of first sound waves in the linear regime is related to an insufficient power of the sound oscillations. Based on the experimental data on the excitation of first and second sounds, we have obtained estimates for the phenomenological coefficient of proportionality between the polarization vector and acceleration and for the drag coefficient of helium atoms by rotons in the second sound wave. We also show that the presence of a steady heat flow in HeII with nonzero longitudinal velocity and temperature gradients due to finite viscosity and thermal conductivity of the normal component leads to a change in the phase velocities of the first and second sound waves and to an exponential growth of their amplitudes with time, which should cause the amplitudes of the electric signals at the first and second sound frequencies to grow. This instability is analogous to the growth of the amplitude of long gravity waves on a shallow-water surface that propagate in the direction of decreasing basin depth.     

Contribution of electron-nuclear interaction to the residual resistivity of metals

In nonmagnetic metals the spin-spin interaction of the electrons and nuclei makes a strongly magnetic field and temperature T dependent contribution to the residual resistivity. The nuclei act as magnetic impurities. For magnetic metals (Tb, Ho, Dy) with a high internal magnetic field, the nuclear contribution to the resistivity vanishes at low temperatures T, where the nuclear spins are ordered, and saturates at high temperatures T, where the nuclear spins are disordered—the analog of the Schottky effect for the nuclear specific heat. The electron-nuclear interaction can destroy superconductivity in metals with low critical magnetic fields under conditions of ferromagnetic ordering of the nuclear spins. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 193–197 (10 August 1996)     

Influence of the thickness of the liquid layer on the ratio of the dimensions of a convection cell

A theory is developed for a new type of transition — a change in the ratio of the longitudinal and transverse dimensions of a convection cell as the thickness of a liquid layer is varied. A sudden change in the ratio of the cell dimensions takes place because of a change in the predominant mechanism for excitation of convection. The governing influence of buoyancy forces gives way to one of thermocapillary forces, and they in turn give way to the influence of thermoelectric forces for yet thinner layers. As the layer thickness is reduced gradually at a fixed external heating, the ratio of the dimensions will take on the values 0.7, 0.65, and 1, respectively. Zh. Tekh. Fiz. 68, 7–11 (November 1998)     

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)     

Parametric excitation of spin waves in uniaxial ferrites

Parametric excitation of spin waves in uniaxial ferrites with large anisotropy is investigated theoretically. First-order processes in a sphere with arbitrary orientation of an external static magnetic field, in which case the pumping is oblique, are studied. The threshold field and the parameters of the excited spin waves are calculated numerically for two ferrites — easy-axis and easy-plane. Zh. Tekh. Fiz. 68, 65–71 (May 1998)     

Breakdown of the homogeneity of weakly conducting liquids in high electric fields

A study of the structure of electrohydrodynamic flows shows that the electric charge carriers are ions that are practically frozen into the surrounding liquid. In other words, ions in weakly conducting liquids are capable of forming more or less stable structures whose viscoelastic properties are different from those of an uncharged liquid. One method of studying this effect is to investigate the velocity dispersion of ultrasound on charged supermolecular formations. The results of theoretical and experimental investigations of acoustic dispersion in liquid dielectrics subjected to prebreakdown electric fields are presented. A model problem of sound propagation in a liquid in which supermolecular structures have formed around elementary charge carriers is studied theoretically. Approximate formulas describing the dispersion of the acoustic phase velocity as a function of the electric field parameters and the electrophysical parameters of the liquid are obtained. The frequency dependence of the sound velocity is of a resonance character, the resonance frequency being determined by the electric charge density and the mass of the charged supermolecular structures. The experiments showed that the space charge affects the velocity of acoustic waves in liquid dielectrics. Zh. Tekh. Fiz. 67, 105–111 (October 1997)     

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)     

Structures and thermoelectric convection in cholesteric liquid crystals

The possibilities for excitation of electromagnetic field structures and convection cells, i.e., temperature and velocity structures, in a thermotropic cholesteric liquid crystal in the presence of flow are studied. Estimates are made and possible experiments for observing such structures are discussed. A special thermoelectric effect is investigated as the cause of these excitations — the influence of a heating-induced change in the pitch of the cholesteric helix of the molecules on the permittivity and the electric conductivity of the material. Fiz. Tverd. Tela (St. Petersburg) 41, 165–170 (January 1999)     

Theory of parametric excitation of acoustic waves

A theory of parametric excitation of acoustic waves is constructed. It is shown that nonlinear attenuation is the main restriction mechanism for a parametrically generated sound wave. The intensity of generated waves is directly proportional to the difference ε between the value of pumping and bare attenuation. The calculated proportionality coefficient depends on the shape of the generated sound wave. Why an ordinary pattern does not form for acoustic waves is explained. The structure of the spectrum of excited waves was studied. It is shown that this structure has exponential asymptotic behavior at the frequency. The width of the intensity distribution depends on the shape of a wave. For different cases it behaves as ε ^α with α=1, 8/7, and 4/3. The results are compared with the experimental data of Ref. 5. Zh. éksp. Teor. Fiz. 112, 1630–1648 (November 1997) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Ternary eutectic growth of Ag-Cu-Sb alloy within ultrasonic field

The liquid to solid transformation of ternary Ag42.4Cu21.6Sb36 eutectic alloy was accomplished in an ultrasonic field with a frequency of 35 kHz, and the growth mechanism of this ternary eutectic was examined. Theoretical calculations predict that the sound intensity in the liquid phase at the solidification interface increases gradually as the interface moves up from the sample bottom to its top. The growth mode of (ε θ Sb) ternary eutectic exhibits a transition of "divorced eutectic- mixture of anomalous and regular structures-regular eutectic" along the sample axis due to the inhomogeneity of sound field distribution. In the top zone with the highest sound intensity, the cavitation effect promotes the three eutectic phases to nucleate independently, while the acoustic streaming efficiently suppresses the coupled growth of eutectic phases. In the meantime, the ultrasonic field accelerates the solute transportation at the solid-liquid interface, which reduces the solute solubility of eutectic phases.     

Shift of the dip in the ultralow-frequency electric excitation spectrum of the Bridgman effect

The previously predicted frequency shift of the deep dip in the ultra low-frequency (ULF) electric spectrum of the excitation threshold of the Bridgman effect in crystal hydrates has been observed. The appearance of this shift, which is caused by an increase in the temperature, is demonstrated for the example of magnesium hydroxide. The magnitude of the shift estimated qualitatively for two temperature — 20°C and 180°C — in a model with ULF-selective breakdowns of gas located in microcracks is virtually identical to the experimentally obtained value. This agreement attests to the possibility that micro breakdowns are excited in crystal hydrates under the conditions of a giant increase in their ULF permittivity in the process of strongly nonuniform quasistatic compression in relatively weak ac fields (E<2kV/cm). Pis’ma Zh. éksp. Teor. Fiz. 65, No. 12, 876–880 (25 June 1997)     

Electromagnetic generation of ultrasound in antiferromagnets

The generation of ultrasound in semi-infinite, collinear, two-sublattice antiferromagnetic metals is investigated theoretically. Two cases are considered: 1) when relaxation in the magnetic subsystem is so rapid that variations of the ferromagnetism and antiferromagnetism vectors cannot keep pace with the variation of the displacement vector and the electromagnetic field vectors; 2) when the opposite is true. It is shown for the first time that in either case only transverse sound is excited at low temperatures in zero static magnetic field, and only longitudinal sound is excited at high temperatures in the vicinity of the Néel point. It is also shown that in antiferromagnets, as opposed to ferromagnets, linear sound generation takes place in zero static magnetic field, and the efficiency of contactless sound generation is generally weaker. Fiz. Tverd. Tela (St. Petersburg) 39, 905–907 (May 1997)     

Radiation-stimulated diffusion in solids

Irradiation of solids produces a microscopic nonequilibrium state in which the vibrational energy distribution function of the atoms deviates from the thermodynamically equilibrium function. Expressions are obtained for the nonequilibrium distribution function and for the frequencies of activational transitions of atoms out of a potential well. It is shown that the radiation stimulation of diffusion processes involves a deviation of the temperature dependences of the frequencies of transitions of the atoms out of positions of equilibrium from the Arrhenius law. Under subthreshold irradiation conditions the rate of diffusion processes is higher for atoms whose vibrations thermalize over long times and depends linearly on the irradiation intensity. Under above-threshold irradiation conditions the characteristics of cascade regions in solids — their sizes and the vibrational excitation energy of the atoms — can be determined by comparing the computed and experimental temperature dependences of the diffusion coefficient. Zh. Tekh. Fiz. 68, 67–72 (August 1998)