Relaxation time of nonequilibrium nuclear system

In the framework of the exciton model an expression for the relaxation time of nonequilibrium nuclear system has been obtained. It was demonstrated that one can determine this time by the values of transition rates at equilibrium.     

The non-uniform level density effect on exciton state densities

The equidistant spacing model has been widely used in the exciton model. To test the accuracy, we used the harmonic oscillator potential and the Fermi gas spacing model to account the non-uniform level effect based on the method of exact Pauli exclusion principle correction. The results indicate that the non-uniform effects increase with the increasing of excitation energies. The harmonic oscillator potential overestimated the non-uniform effects. The Fermi gas spacing model is more realistic which gives small correction to the equidistant spacing model. So that the equidistant spacing model used in the exciton model is a good approximation.     

Modeling of the interrelation of mechanical and electrical parameters of the surface of solids

A mathematical model for the determination of physical constants in equations of state at the interface between a metal and an inert gas medium has been developed in terms of the surface physics and thermodynamics of nonequilibrium processes with due regard for internal stresses induced by a redistribution of conduction electrons. Physical characteristics of a surface layer, in particular, at the copper-inert gas medium interface and the silicon-inert gas medium interface, have been determined using experimental values of surface tensions and energies for contacting media.     

Strongly driven exciton resonances in quantum wells: light-induced dressing versus coulomb scattering

The nonequilibrium dynamics of a two-dimensional electron-hole gas is studied in the regime of strong and resonant pumping of the exciton resonance. The Coulomb collision rates are consistently determined by taking into account the light-induced coherence of the two-band system that leads to a dressing of the carrier spectral functions. The light dressing dramatically reduces the Coulomb scattering efficiency. Results are presented for Rabi oscillations in the time domain and dynamical Stark splitting in the pump-probe absorption spectra.     

Heating of two-dimensional excitons by nonequilibrium acoustic phonons

The energy distribution functions of two-dimensional excitons in the presence of nonequilibrium acoustic phonons have been calculated for the geometry used in heat-pulse experiments. The results were obtained by solving numerically the kinetic equation for the case where the exciton gas equilibrates with phonons during its lifetime. The cases of the low and high exciton-gas density limits are considered. It is shown that at low exciton-gas densities the distribution does not follow the Boltzmann function and depends on the quantum-well width. A comparison with earlier experimental data is made. Fiz. Tverd. Tela (St. Petersburg) 41, 1707–1711 (September 1999)     

Response of a Bose-Einstein condensate of dipole excitons to static and dynamic perturbations

Studies of the interaction of a Bose-Einstein condensate of two-dimensional spatially indirect excitons with the static fields of impurities, surface acoustic waves, and elementary excitations of a degenerate electron gas have been reviewed. The effects of screening of charged impurities and absorption of a Bleustein-Gulyaev surface acoustic wave by an exciton condensate have been considered. Friedel oscillations of the exciton density in a hybrid electron-exciton system, which consists of spatially separated layers of condensed exciton gas and degenerate electron gas, have been studied. The lifetimes of quasiparticle excitations (electrons, plasmons, and bogolons) in the hybrid system have been calculated. The contributions to the effects under study from condensate and above-condensate particles have been determined. The properties of an excitonic insulator have been analyzed within the Bardeen-Cooper-Schrieffer model with a built-in dissipation-free current.     

High-order shock-fitting methods for direct numerical simulation of hypersonic flow with chemical and thermal nonequilibrium

In recent years, much progress has been made in the direct numerical simulation of laminar-turbulent transition of hypersonic boundary layer flow. However, most of the efforts at the direct numerical simulation of transition previously have been focused on the idealized perfect gas flow or “cold” hypersonic flows. For practical problems in hypersonic flows, high-temperature effects of thermal and chemical nonequilibrium are important and cannot be modeled by a perfect gas model. Therefore, it is necessary to include the real gas models in the numerical simulation of hypersonic boundary layer transition in order to accurately predict flow field parameters. Currently most numerical methods for hypersonic flow with thermo-chemical nonequilibrium are based on shock-capturing approach at relatively low order of accuracy. Shock capturing schemes reduce to first-order accuracy near the shock and have been shown to produce spurious oscillations behind curved strong shocks. There is a need to develop new methods capable of simulating nonequilibrium hypersonic flow fields with uniformly high-order accuracy and avoid spurious oscillations near the shock. This paper presents a fifth-order shock-fitting method for numerical simulation of thermal and chemical nonequilibrium in hypersonic flows. The method is developed based on the state-of-the-art real gas models for thermo-chemical nonequilibrium and transport phenomena. Shock-fitting approach is used because it has the advantage of capturing the entire flow field with high-order accuracy and without any oscillations near the shock. The new method has been tested and validated for a number of test cases over a wide span of free stream conditions. The developed method is applied for the study of receptivity of free stream acoustic waves over a blunt cone for hypervelocity flow. Some preliminary results of the computations of the high order shock fitting method for the above mentioned study have also been presented.     

Calculating the nonequilibrium carrier relaxation kinetics in AlGaAs

The effect of the initial density of nonequilibrium electron–hole pairs on the kinetics of their relaxation is analyzed. The influence of cooling, formation, ionization, and radiative recombination of excitons is discussed. It is shown that the exciton lifetime increases with the optical excitation density and, as a result, maximum exciton density is attained for a longer time.     

Momentum transfer in nonequilibrium steady states

When a Brownian object interacts with noninteracting gas particles under nonequilibrium conditions, energy dissipation associated with Brownian motion causes an additional force on the object as a "momentum transfer deficit." This principle is demonstrated first by a new nonequilibrium steady state model and then applied to several known models such as an adiabatic piston for which a simple explanation has been lacking.     

Acousto-exciton interaction in a gas of 2D indirect dipolar excitons in the presence of disorder

A theory for the linear and quadratic responses of a 2D gas of indirect dipolar excitons to an external surface acoustic wave perturbation in the presence of a static random potential is considered. The theory is constructed both for high temperatures, definitely greater than the exciton gas condensation temperature, and at zero temperature by taking into account the Bose–Einstein condensation effects. The particle Green functions, the density–density correlation function, and the quadratic response function are calculated by the “cross” diagram technique. The results obtained are used to calculate the absorption of Rayleigh surface waves and the acoustic exciton gas drag by a Rayleigh wave. The damping of Bogoliubov excitations in an exciton condensate due to theirs scattering by a random potential has also been determined.     

Spin fluctuations of nonequilibrium electrons and excitons in semiconductors

Effects that are related to deviations from thermodynamic equilibrium have a special place in modern physics. Among these, nonequilibrium phenomena in quantum systems attract the highest interest. The experimental technique of spin-noise spectroscopy has became quite widespread, which makes it possible to observe spin fluctuations of charge carriers in semiconductors under both equilibrium and nonequilibrium conditions. This calls for the development of a theory of spin fluctuations of electrons and electron–hole complexes for nonequilibrium conditions. In this paper, we consider a range of physical situations where a deviation from equilibrium becomes pronounced in the spin noise. A general method for the calculation of electron and exciton spin fluctuations in a nonequilibrium state is proposed. A short review of the theoretical and experimental results in this area is given.     

Calculating thermal radiation of a vibrational nonequilibrium gas flow using the method of <Emphasis Type="Italic">k</Emphasis>-distribution

The method has been developed to calculate infrared radiation of vibrational nonequilibrium gas based on k-distribution. A comparison of the data on the calculated nonequilibrium radiation with results of other authors and with experimental data has shown satisfactory agreement. It is shown that the results of calculation of radiation intensity using nonequilibrium and equilibrium methods significantly differ from each other. The discrepancy increases with increasing height (decreasing pressure) and can exceed an order of magnitude.     

Multiple temperature model for the information preservation method and its application to nonequilibrium gas flows

The information preservation (IP) method has been successfully applied to various nonequilibrium gas flows. Comparing with the direct simulation Monte Carlo (DSMC) method, the IP method dramatically reduces the statistical scatter by preserving collective information of simulation molecules. In this paper, a multiple temperature model is proposed to extend the IP method to strongly translational nonequilibrium gas flows. The governing equations for the IP quantities have been derived from the Boltzmann equation based on an assumption that each simulation molecule represents a Gaussian distribution function with a second-order temperature tensor. According to the governing equations, the implementation of IP method is divided into three steps: molecular movement, molecular collision, and update step. With a reasonable multiple temperature collision model and the flux splitting method in the update step, the transport of IP quantities can be accurately modeled. We apply the IP method with the multiple temperature model to shear-driven Couette flow, external force-driven Poiseuille flow and thermal creep flow, respectively. In the former two cases, the separation of different temperature components is clearly observed in the transition regime, and the velocity, temperature and pressure distributions are also well captured. The thermal creep flow, resulting from the presence of temperature gradients along boundary walls, is properly simulated. All of the IP results compare well with the corresponding DSMC results, whereas the IP method uses much smaller sampling sizes than the DSMC method. This paper shows that the IP method with the multiple temperature model is an accurate and efficient tool to simulate strongly translational nonequilibrium gas flows.     

Manifestation of the gas translational nonequilibrium in a resonance electromagnetic field

The translational nonequilibrium of the resonance impure gas in a field of a traveling electromagnetic wave produced by both the radiation pressure and the difference in cross-sections of elastic collisions between the excited- and ground-state atoms and a buffer gas has been studied.     

Kinetic properties of a system of spatially separated excitons and electrons in the presence of a Bose condensate of excitons

A system of interacting, spatially separated excitons and electrons is examined in the presence of a Bose condensate of excitons. The kinetic properties of the system that are governed by the interaction of excitations in the exciton subsystem with electrons are investigated. It is shown that a nonequilibrium distribution of excitations in the exciton subsystem gives rise to an induced electron current. Experimental observation of the kinetic phenomena described can provide new information on the exciton phase state. Zh. éksp. Teor. Fiz. 116, 1440–1449 (October 1999)     

Nonequilibrium state of the two-dimensional electron gas in the integer quantum Hall effect regime

We have compared the properties of the nonequilibrium state of the two-dimensional electron gas observed in the samples of different types by means of magnetotransport and magnetization measurements in the quantum Hall effect regime at integer filling factors n. It has been found that the range of filling factors corresponding to the nonequilibrium state is universal for the samples of different types and different measurement techniques and varies from 0.1 to 0.3 for n changing from 1 to 4. The comparison indicates that the observed nonequilibrium state is not directly caused by the appearance of eddy currents and the dielectric phase in the two-dimensional electron gas but is probably associated with the magnetic field-induced phase transition.     

Exciton condensation in semiconductor quantum wells with a periodical modulation of potential

The appearance and properties of a structure in the density distribution of indirect excitons in coupled quantum wells in semiconductor alloys upon periodical field modulation are studied. Calculations showed that besides periodical dependence of density distribution, caused by the potential modulation, a stratification of the exciton density shafts into separate islands in the cross direction arises. Appearance of islands is the result of exciton condensed phase formation and a nonequilibrium state of the system due to the finite exciton lifetime and pumping presence. The dependence of the structure on system parameters (the pumping value, the modulated field depth and period) is investigated. Also the influence of the exciton–exciton annihilation is taken into account.     

Modulation of the resonant Rayleigh light scattering spectrum of GaAs/AlGaAs structures with quantum wells under above-barrier illumination

It is found that additional illumination by photons with energies above the band gap width in barrier layers leads to a strong (up to 40% in depth at the values of the illumination power used in this work) modulation of the light intensity elastically scattered upon resonant excitation of exciton states in quantum wells of GaAs/AlGaAs structures. Evidently, the effect observed is associated with the redistribution of oscillator strengths of exciton transitions due to the formation of three-particle exciton complexes (trions). These complexes arise through preferred capture of nonequilibrium like charge carriers (in our case, holes).