Electron-hole System Research Articles

Multiparticle states and the factors that complicate an experimental observation of the quantum coherence in the exciton gas of SiGe/Si quantum wells

The measured stationary and time-resolved photoluminescence is used to study the properties of the exciton gas in a second-order 5-nm-thick Si0.905Ge0.095/Si quantum well. It is shown that, despite the presence of an electron barrier in the Si0.905Ge0.095 layer, a spatially indirect biexciton is the most favorable energy state of the electron–hole system at low temperatures. This biexciton is characterized by a lifetime of 1100 ns and a binding energy of 2.0–2.5 meV and consists of two holes localized in the SiGe layer and two electrons mainly localized in silicon. The formation of biexcitons is shown to cause low-temperature (5 K) luminescence spectra over a wide excitation density range and to suppress the formation of an exciton gas, in which quantum statistics effects are significant. The Bose statistics can only be experimentally observed for a biexciton gas at a temperature of 1 K or below because of the high degree of degeneracy of biexciton states (28) and a comparatively large effective mass (about 1.3m e ). The heat energy at such temperatures is much lower than the measured energy of localization at potential fluctuations (about 1 meV). This feature leads to biexciton localization and fundamentally limits the possibility of observation of quantum coherence in the biexciton gas.

Wake potential with exchange-correlation effects in semiconductor quantum plasmas

Using the non-relativistic quantum hydrodynamic model, wake potential has been studied in a magnetized semiconductor quantum plasma in the presence of upper hybrid wave which is excited via externally injected electron beam. The quantum effect contains electron exchange and correlation potential, Fermi degenerate pressure, and Bohm potential. It is found that the contribution of quantum mechanical electron exchange and correlation potential significantly modifies the amplitude and the effective length of the oscillatory wake potential. In the electron-hole plasma systems, electron exchange-correlation effects tend to increase the magnitude of the wake potential and are much effective at the distances of the order of Debye-length. The application of the work in context of the semiconductor plasmas have also been analyzed graphically.

The role of asymmetry in formation of the superfluid state in electron–hole systems

We consider a system of two coaxial nanotubes separated by a dielectric with the spatially separated electrons and holes confined on each nanotube. The pairing conditions in such systems are studied in the weak coupling limit. It is found that the electron–hole asymmetry caused by electron and hole mass difference and the difference in the radii of the nanotubes can adversely affect the pairing in the system. It is also found that under certain conditions these two asymmetries can effectively compensate each other, therefore significantly increasing the range of the system's parameters for which the superfluid state can exist.

The formation of indirect excitons in atomic layer doped systems

The investigation on the formation of indirect exciton states in atomic layer doped structures is presented. A GaAs-based system with n-type and p-type delta-doping layers is proposed and the separately confined electron and hole states are calculated in the effective mass approximation, with the exciton states being determined trough a diagonalization of the Hamiltonian for the electrostatically coupled electron–hole system. The binding energy of the s-type electron-heavy-hole-like exciton state is calculated as a function of the inter-doping-layer distance and the optical absorption coefficient related with inter-exciton-state transitions is reported.

Charge transport in gapless electron-hole systems with arbitrary band dispersion

Using the semiclassical Boltzmann transport theory, we analytically consider dc charge transport in gapless electron-hole (both chiral and non-chiral) systems in the presence of resistive scattering due to static disorder arising from random quenched impurities in the background. We obtain the dependence of the Boltzmann conductivity on carrier density and temperature for arbitrary band dispersion in arbitrary dimensionality assuming long-range ($\sim 1/r$) Coulomb disorder and zero-range white noise disorder [$\sim \delta(r)$]. We establish that the temperature and the density dependence of the Boltzmann conductivity manifests scaling behaviors determining respectively the intrinsic semimetallic or the extrinsic metallic property of the gapless system. Our results apply equally well to both chiral and non-chiral gapless systems, and provide a qualitative understanding of the dependence of the Boltzmann conductivity on the band dispersion in arbitrary dimensionality.

On the phase diagram of a two-dimensional electron–hole system

The phase diagram for a system of spatially separated electrons and holes in coupled quantum wells or graphene double layers is studied in the framework of a BCS-like mean-field approach and a Landau expansion in terms of the pairing order parameter. We find a second order transition between an electron–hole plasma and a BCS phase, as well as a first-order transition between the BCS phase and a bosonic Mott phase of tightly bound electron–hole pairs without phase coherence. The electron–hole plasma exists at low and at high densities for weak interaction, the BCS phase at moderate density and the Mott phase at high density and strong interaction.

Kinetics of accumulation of excess holes under photoexcitation and their relaxation in GaAs/AlGaAs shallow quantum wells

The kinetics of accumulation of long-lived excess holes that appear in GaAs/Al0.05Ga0.95As shallow quantum wells under above-barrier photoexcitation and their relaxation is studied by time-resolved photoluminescence spectroscopy. The establishment of a steady state in the nonequilibrium electron-hole system under various combinations of above-barrier and intrawell excitation is also investigated. It is found that the temperature dependence of the excess-hole relaxation time (their lifetime in the quantum wells) exhibits activation behavior with two activation energies. It is established that excitons produced by nonresonant intrawell excitation undergo efficient cooling as they scatter off accumulated long-lived holes.

The use of asymptotic methods for modelling of the carriers wave functions in the Si/SiGe heterostructures with quantum-confined layers

The mathematical model that allows obtaining wave functions of electrons and holes and Coulomb potential in dependence on the parameters of the layer is proposed. In contrast to the common approach widely used in considerable literature, the proposed method allows to find the wave functions for all layers of the structure, both inside and outside the barrier layer. It was shown that increase of tunnel barrier transparency entails a transition from the dipolar electron-hole system (EHS) with a double-peak wave function of electrons to the spatially direct EHS. The proposed model takes into account the effect of the correlation interaction on the wave functions and the density of distribution of the carriers crosswise the layers. These calculations are useful for the analysis of many-particle nonequilibrium states occurring in low-dimensional structures.

On the Instability of the Electron–Hole System in Coupled Quantum Wells

It is shown that the homogeneous state of the spatially separated electrons and holes in coupled quantum wells (CQW) is instable if the layer charge density is smaller than the critical value \(n_{c}\) specified by the parameters of the CQW. The effect is due to the many-body Coulomb correlations that provide the positive compressibility. The instability results in the formation of the inhomogeneous system that comprises the liquid electron–hole drops.

Influence of Boron Selective Doping on the Edge Luminescence of SiGe/Si Quantum Wells

We study low-temperature photoluminescence of undoped and boron δ-doped Si1−xGex/Si (x < 0.1) quantum wells with a thickness of 5 nm using CW and pulsed UV lasers. Many particle states in the nonequilibrium electron–hole system and the related recombination channels are revealed for both doped and undoped quantum wells. The influence of the band structure on the exciton-impurity complexes located in the quantum well (QW) centers is demonstrated. We find an anomalous quenching of the bound exciton emission line for the structure with a Ge content of 9.5% at low temperatures.

Exciton–Mott Physics in Two-Dimensional Electron–Hole Systems: Phase Diagram and Single-Particle Spectra

Exciton Mott physics in two-dimensional electron-hole (e-h) systems is studied in the quasiequilibrium, which is the crossovers or phase transitions between the insulating exciton gas and the metallic e-h plasma. By developing a self-consistent screened T-matrix approximation, we succeed in obtaining the "global" phase diagram on the plane of the e-h density and the temperature as a contour plot of the exciton ionization ratio. The detailed features of the exciton-Mott crossover at high temperature are figured out beyond the conventionally used concept of the Mott density. At low temperature, we find not only the region unstable toward the inhomogeneity but the pure Mott transition point characterized by the discontinuity in the ionization ratio. The single particle spectra also exhibit interesting features reflecting the excitonic correlations.

Exciton formation in strongly correlated electron-hole systems near the semimetal-semiconductor transition

The region surrounding the excitonic insulator phase is a three-component plasma composed of electrons, holes, and excitons. Due to the extended nature of the excitons, their presence influences the surrounding electrons and holes. We analyze this correlation. To this end, we calculate the density of bound electrons, the density of electrons in the correlated state, the momentum-resolved exciton density, and the momentum-resolved density of electron-hole pairs that are correlated but unbound. We find qualitative differences in the electron-hole correlations between the weak-coupling and the strong-coupling regime.

Exciton propagation via quantum walks based on non-Hermitian coin flip operations

We examine the coherent propagation of the one-dimensional Frenkel exciton (correlated electron-hole pair system) based on a model of a quantum walker in multi-dimensional Hilbert space. The walk is governed by a non-Hermitian coin flip operation coupled to a generalized shift process. The dissipative coin flip operation is associated with amplitude leakages at occupied sites, typical of processes which occur when an exciton is transferred along dimer sites in photosynthetic protein complexes. We analyze the characteristics probability distribution of the one-dimensional quantum walk for various system parameters, and examine the complex interplay between non-Markovian signatures and amplitude leakages within the Hilbert position subspace. The visibility of topological defects such as exceptional points, and non-Markovian signatures via quantum tomography based spectroscopic measurements are discussed.

Magnetoplasmon resonances in polycrystalline bismuth as seen via time-domain terahertz spectroscopy

We report the magnetic field-dependent far-infrared reflectivity of polycrystalline bismuth. We observe four distinct absorptions that we attribute to magnetoplasmon resonances, which are collective modes of an electron-hole liquid in magnetic field and become optical and acoustic resonances of the electron-hole system in the small-field limit. The acoustic mode is expected only when the masses of distinct components are very different, which is the case in bismuth. In a polycrystal, where the translational symmetry is broken, a big shift of spectral weight to acoustic plasmon is possible. This enables us to detect an associated plasma edge. Although the polycrystal sample has grains of randomly distributed orientations, our reflectivity results can be explained by invoking only two, clearly distinct, series of resonances. In the limit of zero field, the optical modes of these two series converge onto plasma frequencies measured in monocrystal along the main optical axes.

In-plane terahertz response of thin filmSr2RuO4

The low-energy electron dynamics of the quasi-two-dimensional electron-hole multiorbital system ${\mathrm{Sr}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$ is investigated by time-domain terahertz spectroscopy for a thin film sample. The direct observation of the real and imaginary part of the optical conductivity below 8 meV clearly demonstrates a narrowing of the coherent quasiparticle response with decreasing temperature. The optical conductivity spectra, dc conductivity, and Hall coefficient can be quantitatively well described by the multiband Drude-Lorentz model, from which band dependent scattering rates can be extracted. The resulting modest band dependent scattering rates, which are lower than 2 meV at 4 K, support a rather isotropic relaxation regime.

Many-body correlation effects in the spatially separated electron and hole layers in the coupled quantum wells

The many-body correlation effects in the spatially separated electron and hole layers in the coupled quantum wells are investigated. A special case of the many-component electron–hole system is considered. It is shown that if the hole mass is much greater than the electron mass, the negative correlation energy is mainly determined by the holes. The ground state of the system is found to be the 2D electron–hole liquid with the energy smaller than the exciton phase. It is shown that the system decays into the spatially separated neutral electron–hole drops if the initially created charge density in the layers is smaller than the certain critical value neq.

Photoinduced charge redistribution and its influence on excitonic states in Zn(Cd)Se/ZnMgSSe/GaAs quantum-well heterostructures

The photoinduced charge redistribution in Zn(Cd)Se/ZnMgSSe/GaAs quantum-well heterostructures under different conditions of optical excitation has been investigated using scanning probe microscopy and optical spectroscopy in the temperature range from 5 to 300 K. Excitation of the samples by radiation with a photon energy greater than the band gap of Zn(Cd)Se leads to the accumulation of electrons in quantum wells, which is detected using scanning spreading resistance microscopy. For moderate excitation densities (up to 25 W/cm2) and at temperatures ranging from 80 to 100 K, the density of a quasi-two-dimensional electron gas formed in quantum wells is several orders of magnitude higher than the density of electron-hole pairs generated by the excitation radiation. The excess electron concentration in the quantum well leads to a broadening of the exciton resonances and to an increase in the relative intensity of the donor-bound exciton emission line and also determines the increase in the luminescence quantum yield with increasing excitation intensity. An additional illumination with a photon energy less than the band gap of Zn(Cd)Se decreases the concentration of excess electrons in quantum wells. The influence of the additional illumination is observed at a temperature of approximately 100 K and almost completely suppressed at 5 K. The obtained results are explained in terms of the formation of a potential barrier for electrons at the ZnMgSSe/GaAs interface and by the specific features of recombination processes in the electron-hole system containing impurity centers with different charge states.

Excitonic Phase Transition in the Extended Three-Dimensional Falicov–Kimball Model

We study the excitonic phase transition in a system of the conduction band electrons and valence band holes described by the three-dimensional (3D) extended Falicov-Kimball (EFKM) model with the tunable Coulomb interaction $U$ between both species. By lowering the temperature, the electron-hole system may become unstable with respect to the formation of the excitons, i.e, electron-hole pairs at temperature $T=T_{\Delta}$, exhibiting a gap $\Delta$ in the particle excitation spectrum. To this end we implement the functional integral formulation of the EFKM, where the Coulomb interaction term is expressed in terms of U(1) phase variables conjugate to the local particle number, providing a useful representation of strongly correlated system. The effective action formalism allows us to formulate a problem in the phase-only action in the form of the quantum rotor model and to obtain analytical formula for the critical lines and other quantities of physical interest like charge gap, chemical potential and the correlation length.

Excitonic superfluidity and screening in electron-hole bilayer systems

Superfluidity in electron-hole bilayers in graphene and GaAs has been predicted theoretically many times but not yet definitively observed. A key controversy is the correct approximation for the screening of the Coulomb interaction for the pairing. Mean-field theories using different approximations for the screening lead to diametrically contradictory predictions for superfluidity. We test these different approximations against diffusion quantum Monte Carlo results and find good agreement with the mean-field theory that uses screening in the superfluid state, but large discrepancies with other approximations for screening. This mean-field theory predicts no superfluidity in existing devices, provides pointers for new devices to generate the superfluidity, and, very importantly, it permits calculations for complicated lattices at finite temperatures, impractical in Monte Carlo.

Nonlinear optical rectification in a two-dimensional system

The optical rectification (OR) in the two-dimensional electron–hole system is investigated theoretically. An analytical expression formula of the OR coefficient is obtained via the framework of compact density matrix approach and the iterative method. The results show that the OR coefficient depends strongly on the system׳s size and the relaxation time. A more obvious OR coefficient should be found by adjusting an appropriate choice of the system parameters.