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)     

Phase transitions in a system of spatially separated electrons and holes

The formation of a superfluid exciton liquid in a system of spatially separated electrons and holes in a system of two coupled quantum wells is predicted and its properties are investigated. The ground-state energy and the equilibrium density of the exciton liquid are calculated as functions of distance D between the quantum wells. The properties of a rarefied exciton gas with dipole-dipole repulsions are considered, where this gas is the metastable phase for D<1.9a* and the stable phase for D<1.9a* (a* is the radius of the two-dimensional exciton). The gas-liquid quantum transition is examined for increasing D. The Berezinskii-Kosterlitz-Thouless transition temperatures, at which superfluidity arises in the system, are found for different values of D. Possible experimental manifestations of the predicted effects are discussed. Zh. éksp. Teor. Fiz. 111, 1879–1895 (May 1997)     

Possible pairing of spatially separated electrons and holes in the presence of peierls transition

We investigate a model of two quasi-one-dimensional systems with electron and hole conductivity. We find that in general, Peierls ordering suppresses the possible excitonic pairing of the spatially separated electrons and holes.     

Coulomb drag of conduction electrons in spatially separated two-dimensional layers: An effective-parameter approximation

The Coulomb drag of electrons in spatially separated two-dimensional layers is considered under conditions of electron heating when the nonequilibrium distribution of electrons in the first layer can be described by macroparameters, such as the effective temperature. The nonequilibrium response is calculated using a projection operator that is an obvious generalization of the Mori operator to the case of nonequilibrium systems.     

Electron beam excitation of nonequilibrium effects in spatially separated superconductors

A superconductor driven into a nonequilibrium state under electron beam irradiation emits phonons. They can be detected by a spatially separated superconductor of the same material. This phenomenon, which is typical only to be superconducting state, is used to map a superconductor in a scanning electron microscope.     

Spontaneous decay in a system of two spatially separated atoms (One-dimensional case)

The problem of the dynamics and the spectrum of spontaneous radiation is solved for a system of two atoms in one-dimensional space. In order to single out, to the maximum possible degree, phenomena associated with the influence of spatially separated atoms on each other via the radiation field, the present analysis is performed precisely for the one-dimensional case. As a result, two effects are revealed and considered in detail: (i) the existence of stable (metastable) entangled superposition states at specific distances between the atoms and (ii) a considerable distinction between the spectra of photons emitted in two opposite directions from the system where only one of the atoms is initially excited. The possibilities of observing these effects are discussed.     

Coulomb effects in spatially separated electron and hole layers in coupled quantum wells

We report on the (magneto-) optical study of many-body effects in spatially separated electron and hole layers in GaAs/Al_xGa_1?x As coupled quantum wells (CQWs) at low temperatures (T = 1.4 K) for a broad range of electron-hole (e-h) densities. Coulomb effects were found to result in an enhancement of the indirect (interwell) photoluminescence (PL) energy with increasing the e-h density both for a zero magnetic field and at high fields for all Landau level transitions; this is in contrast to the electron-hole systems in single QWs where the main features are explained by the band-gap renormalization resulting in a reduction of the PL energy. The observed enhancement of the ground state energy of the system of the spatially separated electron and hole layers with increasing the e-h density indicates that the real space condensation to droplets is energetically unfavorable. At high densities of separated electrons and holes, a new direct (intrawell) PL line has been observed: its relative intensity increased both in PL and in absorption (measured by indirect PL excitation) with increasing density; its energy separation from the direct exciton line fits well to the X ^? and X ⁺ binding energies previously measured in single QWs. The line is therefore attributed to direct multiparticle complexes.     

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.     

Fractional Aharonov-Bohm oscillation of a two-layer ring with two electrons

When a circular ring suffers a special topological transformation, it becomes a two-layer ring. Due to the special topology of the two-layer ring, orbital angular momenta are allowed to be a half-integer. This would affect the traditional Aharonov-Bohm oscillation (ABO). In this paper, the fractional ABO (FABO) of the ground state energy, persistent current and dipole transition of a two-layer ring with two electrons has been studied. Collective and internal coordinates (θ_C, ϕ) have been introduced. Based on them, a very simple formula for the current has been obtained, the symmetry constraint imposed on the dipole transition has been clarified and a strict relation between the photon energies of the dipole radiation and the persistent current of the ground state has been found. Comparing with the one-layer ring, the period of the fractional ABO of the two-layer ring becomes much shorter.     

Coherent spin dynamics of electrons and excitons in nanostructures (a review)

The studies of spin phenomena in semiconductor low-dimensional systems have grown into the rapidly developing area of the condensed matter physics: spintronics. The most urgent problems in this area, both fundamental and applied, are the creation of charge carrier spin polarization and its detection, as well as electron spin control by nonmagnetic methods. Here, we present a review of recent achievements in the studies of spin dynamics of electrons, holes, and their complexes in the pump-probe method. The microscopic mechanisms of spin orientation of charge carriers and their complexes by short circularly polarized optical pulses and the formation processes of the spin signals of Faraday and Kerr rotation of the probe pulse polarization plane as well as induced ellipticity are discussed. A special attention is paid to the comparison of theoretical concepts with experimental data obtained on the n-type quantum well and quantum dot array samples.     

Effective scatterings between electrons,excitons and trions

The final goal of this paper is to derive the effective scattering ruling the time evolution of two semiconductor trions using the many-body formalism for composite fermions we have just proposed. However, to understand the importance of the particle composite nature, their bosonic/fermionic character and their overall charge, we also report on scatterings between free electrons, excitons and trions. This leads us to identify the form factors associated to direct processes involving excitons and trions. For transitions between ground states, these form factors reduce to zero and one respectively, in the small momentum transfer limit.     

Coulomb screening effects on the optoelectronic far-infrared properties of spatially separated few-layer graphene

We investigate the longitudinal optical conductivity of spatially separated few-layer graphene analytically and numerically. Each layer could be monolayer or bilayer graphene. The density–density correlation function has been screened by the dielectric function using the random phase approximation, which includes the inter-layer Coulomb coupling. In the presence of the potential function between the layers, the carrier densities in each layer can be tuned respectively. In these two-dimensional layered structures, the main contributions to the optical conductivity are from the intra- and inter-band transition channels in a same layer. In the infrared region, the Drude optical conductivity was observed by the unscreened intra-band transition process. But in the presence of the inter-layer Coulomb interaction, one peak structure of the optical conductivity is observed which can be modified by the dielectric environment. From the number of turning points and the turning positions, the carrier density, the Fermi wavevector, and the layered structure can be determined.     

Coupled vortex oscillations in spatially separated permalloy squares

We experimentally study the magnetization dynamics of pairs of micron-sized permalloy squares coupled via their stray fields. The trajectories of the vortex cores in the Landau-domain patterns of the squares are mapped in real space using time-resolved scanning transmission x-ray microscopy. After excitation of one of the vortex cores with a short magnetic-field pulse, the system behaves like coupled harmonic oscillators. The coupling strength depends on the separation between the squares and the configuration of the vortex-core polarizations. Considering the excitation via a rotating in-plane magnetic field, it can be understood that only a weak response of the second vortex core is observed for equal core polarizations.     

Detecting spatially localized excitons in InGaN quantum well structures with a micro-photoluminescence technique

Spatially localized excitons are observed in InGaN quantum well structures at 4 K by using a micro-photoluminescence (PL) technique. By combining PL and nano-lithographic techniques, we are able to detect PL signals with a 0.2 μm spatial resolution. A sharp PL line (linewidth of <0.4 meV) is clearly obtained, which originates from a single localized exciton induced by a quantum dot like a local potential minimum position. Sharp PL spectra detected in three QWs with different indium compositions confirm that there are exciton localization effects in quantum wells in the blue-green (about 2.60 eV, 477 nm) to purple (about 3.05 eV, 406 nm) regions.     

Interaction of two spatially separated light beams in a nonlinear Kerr medium

The evolution of two spatially separated light beams in a nonlinear Kerr medium described by a system of coupled nonlinear Schrödinger equations is studied. An analytic solution is found for the variational problem. It is shown that when two crossed beams interact, a bound state can develop in which the distance between the centers of the beams and their radii vary periodically. Here the mutual curvature of the trajectories of the centers of the beams causes the beams to bend into a helical structure whose parameters (pitch and diameter) are also periodic functions. The threshold power for mutual trapping is determined and the period of the oscillations is found.