Metal-insulator transition in a two-layer electron-hole system

This paper discusses a quantum-mechanical metal-insulator transition that occurs in an anisotropic electron-hole system with the electrons and holes separated and confined to a double quantum well. The critical concentration n _c of carriers in the system above which the excitonic (insulating) phase becomes an electron-hole (metallic) phase is investigated, along with its dependence on the distance between wells D. Fiz. Tverd. Tela (St. Petersburg) 39, 1654–1656 (September 1997)     

Nonequilibrium properties of electron-hole plasma in direct-gap semiconductors

The gain spectra of the electron-hole plasma recombination in CdS are investigated as a function of the excitation conditions and of the lattice temperature. From a lineshape analysis which includes such many-body effects as collision broadening, single-particle energy renormalization and excitonic enhancement, average plasma parameters are obtained. In contrast to the predictions of quasi-equilibrium theory, one finds that the electron-hole plasma does not reach a full thermal quasi-equilibrium in direct-gap materials because of the short lifetimes of the carriers. The nonequilibrium effects are shown to lead to the formation of electron-hole plasma density fluctuations. No well-defined coexistence region exists. The experimental results in the phase transition region can consistently be explained by theoretical treatments of this nonequilibrium phase transition.     

1/f Noise in a nonequilibrium phase transition: Experiment and mathematical model

The results of an experimental investigation of a high-power source of broad-band 1/f noise, which can be generated in a system of two interacting nonequilibrium phase transitions, are presented. This process takes place when a normal conductor-superconductor phase transition is superposed on the critical liquid-vapor transition in a boiling coolant. A mathematical model describing a nonequilibrium phase transition in a complicated nonlinear system with two interacting order parameters, which involves the conversion of white noise into stochastic fluctuations of the order parameters with 1/f and 1/f ² spectra, is proposed. The properties of the model fluctuations with a 1/f spectrum agree qualitatively with the experimentally observed properties. A characteristic difference between the model fluctuations with a 1/f ² spectrum and random walks is also noted. Zh. éksp. Teor. Fiz. 113, 1748–1757 (May 1998)     

Photoinduced instability and semiconductor-metal phase transition in a Peierls system

A microscopic theory of the appearance of electron-phonon instability and a semiconductor-metal phase transition away from thermodynamic equilibrium in a Peierls system upon the optical excitation of electron-hole pairs is devised. An equation which specifies the dependence of the order parameter of the phase transition and the uniquely related gap width in the electron spectrum, on the concentration n of conduction-band electrons is obtained. The critical concentration n=n _c, above which the semiconductor phase of the system is unstable toward the transition to the metallic state, is calculated. A comparison with an experiment on the irradiation of a substrate-supported vanadium dioxide film by a laser pulse is made. Fiz. Tverd. Tela (St. Petersburg) 40, 2113–2118 (November 1998)     

New magnetic phase transition between electron-hole drops and free-excitons in pure Ge

We have discovered a new magnetic phase transition between free-excitons and electron-hole drops in high purity Ge near the critical point of the liquid-gas phase diagram. The critical magnetic field is found to be H_c ≈ 0.4 T. For H?H_c the electron-hole drops are stable to higher temperatures by about 1 K with respect to zero field.     

Photovoltaic effect in unipolar multivalley semiconductors

Abstract

The theory of the nonequilibrium charge carrier transport in unipolar multivalley semiconductors is developed. It is shown that the diffusion of photoexcited nonequilibrium heavy and light electrons in multivalley semiconductors is a correlated process, like the ambipolar diffusion in the case of the electron-hole plasma. The light-induced intervalley transitions, resulting in the imbalance between the subsystems of the light and heavy electrons, give rise to the electromotive force (emf) through the mechanism of the Dember photovoltaic effect. The value of the emf occurring in the ‘metal-semiconductor-metal’ structure is calculated in the linear approximation in terms of the light intensity as a small parameter. It is shown that the emf is determined by the conductivity of heavy and light electron subsystems, as well as by the surface conductivity of the metal-semiconductor interface.     

Multiphoton avalanche generation of free carriers in a multiband crystal

A new mechanism for generating nonequilibrium electron-hole pairs in transparent wide-gap crystals under high-power picosecond light pulses of prebreakdown intensity is considered. The kinetics of free carrier generation is investigated by the numerical solution of the system of balance equations for particle concentrations within the multiband model of electron energy spectrum of the crystal. It is shown that in the intensity range of 10¹⁰–10¹² W/cm² the proposed way of nonequilibrium carrier generation is more effective in the case of picosecond pulses than the conventional multiphoton absorption. It is shown that the nonequilibrium carrier generation considered here may initiate a multiphoton avalanche.     

Phase transitions of cellular automata

Cellular automata (CA) are simple mathematical models of the dynamics of discrete variables in discrete space and time, with applications in nonequilibrium physics, chemical reactions, population dynamics and parallel computers. Phase transitions of stochastic CA with absorbing states are investigated. Using transfermatrix scaling the phase diagrams, critical properties and the entropy of one-dimensional CA are calculated. The corners of the phase diagrams reduce to deterministic CA discussed by Wolfram (Rev. Mod. Phys.55, 601 (1983)). Three-state models are introduced and, for special cases, exactly mapped onto two-state CA. The critical behaviour of other threestate models with one or two absorbing states and with immunization is investigated. Finally CA with competing reactions and/or with disorder are studied.     

Effect of external noise on the optical thermal breakdown of semiconductor wafers

The optical thermal breakdown of a semiconductor wafer with a small Biot number under conditions of fluctuating incident light intensity is discussed. It is shown that external noise shifts the breakdown region toward somewhat higher values of the control parameter, and as the fluctuations grow this process leads to suppression of the critical point. Light intensities below the threshold noise can induce a nonequilibrium phase transition. Numerical estimates are given for germanium illuminated by a CO₂ laser. Zh. Tekh. Fiz. 67, 117–119 (September 1997)     

Dynamics of the phase transitions in the system of nonequilibrium charge carriers in quantum-dimensional Si1 − x Ge x /Si structures

The dynamics of the phase transition from an electron-hole plasma to an exciton gas is studied during pulsed excitation of heterostructures with Si_{1 ? x} Ge _x /Si quantum wells. The scenario of the phase transition is shown to depend radically on the germanium content in the Si_{1 ? x} Ge _x layer. The electron-hole system decomposes into a rarefied exciton and a dense plasma phases for quantum wells with a germanium content x = 3.5% in the time range 100–500 ns after an excitation pulse. In this case, the electron-hole plasma existing in quantum wells has all signs of an electron-hole liquid. A qualitatively different picture of the phase transition is observed for quantum wells with x = 9.5%, where no separation into phases with different electronic spectra is detected. The carrier recombination in the electron-hole plasma leads a gradual weakening of screening and the appearance of exciton states. For a germanium content of 5–7%, the scenario of the phase transition is complex: 20–250 ns after an excitation pulse, the properties of the electron-hole system are described in terms of a homogeneous electron-hole plasma, whereas its separation into an electron-hole liquid and an exciton gas is detected after 350 ns. It is shown that, for the electron-hole liquid to exist in quantum wells with x = 5–7% Ge, the exciton gas should have a substantially higher density than in quantum wells with x = 3.5% Ge. This finding agrees with a decrease in the depth of the local minimum of the electron-hole plasma energy with increasing germanium concentration in the SiGe layer. An increase in the density of the exciton gas coexisting with the electron-hole liquid is shown to enhance the role of multiparticle states, which are likely to be represented by trions T ⁺ and biexcitons, in the exciton gas.     

Transition states in the melting of germanium in dynamic and quasistatic regimes

The conditions under which transition states arise and exist during the melting of germanium are identified by digital differential-thermal analysis. The pre-and postmelting cooperative effects arising under real conditions in dynamic and quasistatic regimes are nonequilibrium phase transitions and can be characterized by a system of thermodynamic parameters. Instability of the thermodynamic parameters of the transition states is observed in quasistatic regimes as a result of dynamic instability. The system of parameters introduced characterizes the formation of a special phase state — pre-and postmelting mesophases. Zh. Tekh. Fiz. 69, 57–61 (December 1999)     

Nonequilibrium quantum criticality in open electronic systems

A theory is presented of quantum criticality in open (coupled to reservoirs) itinerant-electron magnets, with nonequilibrium drive provided by current flow across the system. Both departures from equilibrium at conventional (equilibrium) quantum critical points and the physics of phase transitions induced by the nonequilibrium drive are treated. Nonequilibrium-induced phase transitions are found to have the same leading critical behavior as conventional thermal phase transitions.     

Self-organized criticality and 1/f fluctuation under conditions of nonequilibrium phase transitions

The results are given of an experimental investigation of fluctuation phenomena under conditions of electric arc discharge. Fluctuations are observed whose spectral density is inversely proportional to frequency (1/f noise). Power dependences are revealed of the fluctuation distribution functions. The behavior of spectral density and of distribution functions is associated with the simultaneous occurrence of various nonequilibrium phase transitions. Within the framework of the mean field theory, a mathematical model is suggested of interacting nonequilibrium phase transitions in a distributed system, which predicts the self-organization of the critical state and the generation of fluctuations with diverging spectral characteristics. An adequate agreement is observed between the suggested model and experimental data.     

Nonlinear absorption of CO2 laser radiation by nonequilibrium carriers in germanium

Nonlinear absorption of CO₂ laser pulses at the multi-MW level has been studied in n-type germanium at room temperature. This nonlinear absorption limits the level of CO₂ laser intensity that can be transmitted through an optical grade germanium crystal. The observed nonlinearity may be interpreted in terms of nonequilibrium electron-hole pairs generated by hot electrons created by the intense laser field. The number of nonequilibrium carriers generated is measured by photoconductivity experiments, and the results are correlated to the absorption measurements.     

A characteristic critical quantity in nonequilibrium phase transitions

The bit-number variance, a generalization of specific heat, which was already introduced in earlier papers [7-10] is discussed, with respect to the critical behaviour in equilibrium-and nonequilibrium phase transitions. In the considered mean field examples it shows a uniform behaviour dependent on to which of two classes the system belongs. With it a new characteristic critical quantity is found appropriate for the comparison of different nonequilibrium phase transitions. New arguments are given with respect to the connection between critical correlations and the bit-number cumulants.     

Photoinduced semiconductor-metal phase transition in a peierls system

The dynamical equation for the order parameter of the metal-semiconductor phase transition, as well as the kinetic equation for the density of nonequilibrium electron-hole pairs of a Peierls system in a light field, has been derived. An expression for the time τ of the nonthermal photoinduced semiconductor-metal phase transition has been obtained from these equations for the case of an ultrashort light pulse. It has been shown that, to initiate the phase transition, the energy density W of the light pulse must be higher than the critical value W _c. The W _c, τ, and optical absorption coefficient γ₀ that are calculated in the framework of the proposed model are in agreement with the experimental data (W _c ≈ 12 mJ/cm², τ ≈ 75 fs, and γ₀ ≈ 10⁵ cm^?1) on the irradiation of a vanadium dioxide film by a laser pulse with a duration of τ_p ≈ 15 fs, a photon energy of ?θ₀ = 1.6 eV, and an energy density of W = 50 mJ/cm².     

Effect of interparticle interactions on radiative lifetime of photoexcited electron-hole system in GaAs quantum wells

The paper reports on an investigation of changes in the photoluminescence linewidth and lifetime of excitons and electron-hole plasma over a wide range of densities between 3×10⁷ and 3×10¹² cm⁻² at a temperature of 77 K in GaAs/AlGaAs quantum wells. The roles played by thermal ionization of excitons at low densities of nonequilibrium carriers, exciton-exciton and exciton-electron collisions, and ionization of excitons at high pumping power densities have been studied. Zh. éksp. Teor. Fiz. 112, 353–361 (July 1997)     

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 noise-induced phase transitions in simple systems

The theory and the results of an investigation of nonequilibrium noise-induced phase transitions in the simple example of a physical pendulum with a randomly oscillating pivot are presented. It is shown that such transitions lead to the appearance of a more ordered state of the system. The possibility of a difference between noise-induced oscillations and chaotic oscillations of dynamic origin is discussed. Zh. éksp. Teor. Fiz. 111, 358–378 (January 1997)     

Prebreakdown generation of nonequilibrium electron-hole pairs: The multiphoton avalanche effect

A new mechanism of prebreakdown generation of electron-hole pairs is considered. It includes a cascade of interband multiphoton transitions as well as Auger-type processes involving two or three photons. A combination of these processes leads to the multiphoton avalanche effect. The threshold pumping radiation intensities required for triggering the avalanche mechanism lie in the range of 10¹¹–10¹²W/cm². The band population balance equations describing the kinetics of pair production are obtained and solved numerically. It is shown that the proposed mechanism of production of nonequilibrium electron-hole pairs is more effective than “conventional” multiphoton absorption for intensities exceeding the threshold values.