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.     

Influence of disorder on electron-hole pairing in graphene bilayer

We consider disorder effect on electron-hole pairing in the system of two graphene monolayers separated by dielectric barrier. The influence of charged impurities on temperature of phase transition is studied. In spite of large values of mobility of charge carriers in graphene disorder can considerably reduce temperature of electron-hole condensation in weak-coupling regime. The quantum hydrodynamics of the system is considered and phase stiffness of electron-hole condensate and temperature of Berezinskii-Kosterlitz-Thouless transition to the superfluid state are calculated.     

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.     

Condensation effects of excitons

The theory of the electronic excitations in a highly excited semiconductor is presented. The relaxation processes, the formation of excitons and excitonic molecules, the interaction among the various forms of electronic excitations, as well as their optical and thermodynamical properties are analyzed. At low temperatures one expects condensations into the quantum statistically degenerate phases of the excitonic molecules and of the electron-hole plasma. The physical properties of these low temperature phases are investigated. Possibilities and previous attempts to observe the Bose-Einstein condensation in excitonic systems are discussed critically. The experimental observations of the electron-hole liquid phase transition are reviewed.     

Luminescence from silicon-surface metallized by extremely high excitation

Time resolved spectra of photoluminescence in Si at 4.2K and 20K under high excitation levels by a Q-switched laser confirm that a new component exists at the higher energy side of the Haynes line. This component is ascribed to an electron-hole liquid phase extending uniformly over the crystal surface and having larger carrier concentration compared to the equilibrium electron-hole drop.     

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)     

Photoinduced superstructure in the low-temperature phase of a Peierls system

This paper is a theoretical study of the properties of the low-temperature phase of a Peierls system when nonequilibrium electron-hole pairs are excited in the phase. A microscopic theory is developed to show that at low temperatures a spatially nonuniform periodic structure with a modulated band gap forms in the thermodynamically nonequilibrium system considered. The critical temperature of formation of such a superstructure, the critical electron-hole pair concentration, the spatial period, and the percentage modulation are calculated. Zh. éksp. Teor. Fiz. 115, 1297–1314 (April 1999)     

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.     

CDW patterns of two-dimensional electrons in strong magnetic fields

The phase diagram of a two-dimensional electron system in a strong magnetic field is studied in the Hartree-Fock approximation. Special attention is paid to the consequences of the electron-hole symmetry in the system. When the ground Landau level is half-filled, the high-temperature gaseous phase undergoes a second-order transition to a square CDW with a period dependent on temperature. This transition preserves the electron-hole symmetry in the sense that the square CDW is self-dual in contrast to a triangular CDW where the dual CDW has a honeycomb pattern. If the density is slightly less than the half-filled case, the gaseous phase first undergoes a first-order transition to a triangular CDW, followed by another first-order transition to a square CDW. We discuss the role of quantum effects which are responsible for this unusual phase diagram.     

Internal-motion dependence of self-trapping of a Wannier exciton

The phase diagram of a Wannier exciton in the phonon fields is presented for 1s, 2s and 2p states of the internal (relative) motion on the basis of the adiabatic approximation. Differences in self-trapping among these states are revealed for an exciton with strong electron-hole Coulomb binding.     

Excitonic condensation in a symmetric electron-hole bilayer

Using diffusion Monte Carlo simulations we have investigated the ground state of a symmetric electron-hole bilayer and determined its phase diagram at T = 0. We find clear evidence of an excitonic condensate, whose stability however is affected by an in-layer electronic correlation. This stabilizes the electron-hole plasma at large values of the density or interlayer distance, and the Wigner crystal at low density and large distance. We have also estimated pair correlation functions and low-order density matrices to give a microscopic characterization of correlations as well as to try and estimate the condensate fraction.     

Picosecond luminescence spectroscopy of highly excited GaAs

Time resolved luminescence of highly excited GaAs is studied using a streak camera. We observe the Mott transition from the electron-hole plasma to the excitonic state. This transition is smooth and does not show a phase separation. The plasmon sideband of the electron-hole plasma emission is identified.     

Influence of the absorbed optical quanta energy on GHz ultrasound generation in GaAs

Experimental results of the investigation of the optoacoustic processes taetang place in GaAs semiconductor at ultrashort time scales are reported. Femtosecond laser has been used both for the generation (through the deformation potential mechanism by the interband absorption of laser radiation) and detection of GHz ultrasound waves. First experimental observation of an abrupt change in the phase of the photoexcited GHz ultrasound, when with increase of the energy of optical quanta direct generation of the electron-hole pairs in the side-valleys of GaAs becomes allowed by the energy and momentum conservation laws, is reported. We relate this observation, at least partially, to abrupt change in the ultrafast dynamics of photogenerated electron-hole plasma, in particular to deceleration of plasma diffusion when heavy carriers in the high energy side valley are photogenerated instead of light carriers in the lowest energy valley.     

Classification of unconventional electron-hole condensates

Heavy Fermion metals with their very anisotropic quasiparticle states may support unconventional electron-hole (Peierls) pairing in addition to unconventional two electron (Cooper) pairs in the superconducting phase. For two different nesting Fermi surface models the possible types of electron hole condensates are classified according to the symmetries of their order parameters. This is performed within a continuum representation for the electronic states near the van Hove saddle point singularities. The quasiparticle bands and the unitary transformation to Bloch states in the condensed phase are derived for the two Fermi surface models with one and two independent nesting vectors respectively. Emphasis is put on the investigation of electron-hole condensed phases with 2Q-modulated structure. It is shown that in the continuum approximation the gap equations are all equivalent and the critical field curve is calculated in the rigid band model.     

Phase diagram of electron-hole systems: Interplay between exciton Mott transition and quantum pair condensation

Quasi-thermal-equilibrium states of electron-hole (e-h) systems in photoexcited insulators are studied from a theoretical viewpoint, stressing the exciton Bose-Einstein condensation (BEC), the e-h BCS-type pair-condensed state, and the exciton Mott transition between an insulating exciton/biexciton gas phase and a metallic e-h plasma phase. We determine the quasi-equilibrium phase diagram of the e-h system at zero and finite temperatures with applying the dynamical mean-field theory (DMFT) to the e-h Hubbard model with both repulsive and attractive on-site interactions. Effects of inter-site interactions on the exciton Mott transition are also clarified with applying the extended DMFT to the extended e-h Hubbard model.     

Phase fixation,excitonic and spin superfluidity of electron-hole pairs and antiferromagnetic chromium

The problem of the excitonic and spin superflow in the Bose-condensated electron-hole fluid is considered taking into account the phase fixation due to non-conservation of the electron-hole pair number and the total spin. The ways to observe the superflows in Cr are discussed. There is the analogy between the spin superfluidity in the exitonic state adn in the A-phase of the superfluid He³. The spin superflow is possible not only in the Cr (the itinerant antiferromagnetic) but in other planar antiferromagnetics too.     

High-field transport in an electron-hole plasma: transition from ballistic to drift motion

The time evolution of high-field carrier transport in bulk GaAs is studied with intense femtosecond THz pulses. While ballistic transport of electrons occurs in an n-type sample, a transition from ballistic to driftlike motion is observed in an electron-hole plasma. This onset of friction is due to the holes, which are heated by THz absorption. Theoretical calculations, which reproduce the data quantitatively, show that both electron-hole scattering and local-field effects in the electron-hole plasma are essential for the time-dependent friction.     

Comment on the electron-hole droplet condensation in direct gap semiconductors

The electron-hole droplet nucleation in highly excited direct gap semiconductors is a non-equilibrium phase transition of second order. Within the framework of a Fokker - Planck approximation modifications of the thermodynamic phase diagram and the cluster distribution function are calculated. Due to the short lifetime of the electronic excitations only very small electron - hole clusters can be formed.     

Electron-hole droplets in germanium in the high stress limit

We have observed electron-hole droplets in germanium stressed uniformly in the <111> direction to the high stress limit. The measured values of the density, binding energy and phase diagram are in good agreement with theoretical calculations.