Spectrum of indirect magnetoexcitons in coupled quantum wells

The indirect Mott exciton (spatially-separated electron and hole) in coupled quantum wells in crossed electric and magnetic fields is discussed. The exciton spectrum is calculated for the case where the distance between the quantum wells of the electron and hole is larger than the exciton Bohr radius. The magnetoexciton creation probability is calculated and its dependence on the electric field is found. The absorption of electromagnetic radiation between the indirect magnetoexciton levels in coupled quantum wells is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 2220–2223 (December 1997)     

Energy relaxation and transport of indirect excitons in AlAs/GaAs coupled quantum wells in magnetic field

The evolution of indirect exciton luminescence in AlAs/GaAs coupled quantum wells after excitation by pulsed laser radiation has been studied in strong magnetic fields (B⩽12 T) at low temperatures (T⩾1.3 K), both in the normal regime and under conditions of anomalously fast exciton transport, which is an indication of the onset of exciton superfluidity. The energy relaxation rate of indirect excitons measured in the range of relaxation times between several and several hundreds of nanoseconds is found to be controlled by the properties of the exciton transport, specifically, this parameter increases with the coefficient of excitonic diffusion. This behavior is qualitatively explained in terms of migration of excitons between local minima of the random potential in the plane of the quantum well. Zh. éksp. Teor. Fiz. 114, 1115–1120 (September 1998)     

Direct and spatially indirect excitons in GaAs/AlGaAs superlattices in strong magnetic fields

Luminescence and luminescence excitation spectra are used to study the energy spectrum and binding energies of direct and spatially indirect excitons in GaAs/AlGaAs superlattices having different electron and hole miniband widths in high magnetic fields perpendicular to the heterolayers. The ground state of the indirect excitons formed by electrons and holes which are spatially distributed among neighboring quantum wells is found to lie between the ground 1s state of the direct excitons and the threshold of the continuum of dissociated exciton states in the minibands. The indirect excitons have a substantial oscillator strength when the binding energy of the exciton exceeds the scale of the width of the resulting miniband. It is shown that a high magnetic field shifts a system of symmetrically bound quantum wells toward weaker bonding. At high exciton concentrations, spatially indirect excitons are converted into direct excitons through exciton-exciton collisions. Fiz. Tverd. Tela (St. Petersburg) 40, 833–836 (May 1998)     

Manifestation of collective properties of spatially indirect excitons in GaAs/AlGaAs asymmetric double quantum wells

Low-temperature (T=1.8, 4.2 K) luminescence of GaAs/Al_0.3Ga_0.7As double-coupled asymmetric quantum wells (DQW) is observed under variation of an electric field V _dc applied along the normal to the DQW plane. The FWHM of the indirect exciton (IX) emission line was found to undergo, within a certain interval of V _dc, a strong (up to a factor 3.5) narrowing, accompanied by anomalously large IX intensity fluctuations in time within the V _dc region where a strong decrease (or increase) of the FWHM is observed to occur. The dependence of the FWHM on the pumping level I _p also exhibits a substantial decrease of the FWHM within a certain I _p interval. The results obtained are discussed in the frame of an assumption which relates the observed phenomena to the onset of a condensed state in the interacting ensemble of spatially indirect excitons in DQWs. Fiz. Tverd. Tela (St. Petersburg) 41, 325–329 (February 1999)     

Far-infrared s→p ± interexciton transitions in InGaAs/GaAs coupled double quantum wells

We consider s→p ^± interexciton far-infrared (FIR) magnetooptical transitions in coupled double quantum wells (DQWs). Spatially direct (intrawell) and indirect (interwell) excitons in strained In_xGa_1−x As/GaAs symmetric DQWs with a simple valence band are considered. The evolution of the transition energies and oscillator strengths as functions of the transverse magnetic field B in different regimes is studied: we consider the direct regime (zero and low transverse electric field ℰ), the indirect regime (high ℰ), and the indirect-direct crossover (induced by increasing B at intermediate ℰ). Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 198–202 (10 August 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Direct and spatially indirect excitons in GaAs/AlGaAs superlattices in strong magnetic fields

Luminescence and luminescence excitation spectra are used to study the energy spectrum and binding energies of direct and spatially indirect excitons in GaAs/AlaAs superlattices, with different widths of the electron and hole minibands, located in a high magnetic field perpendicular to the heterolayers. It is found that the ground state of the indirect excitons formed by electrons and holes and spatially separated between neighboring quantum wells lies between the ls ground state of the direct excitons and the continuum threshold for dissociated exciton states in the minibands. Indirect excitons in superlattices have a significant oscillator strength when the binding energy of the exciton exceeds the order of the width of the resulting miniband. The behavior of the binding energy of direct and indirect heavy hole excitons during changes in the tunneling coupling between the quantum wells is established. It is shown that a strong magnetic field, which intensifies the Coulomb interaction between the electron and hole in an exciton, weakens the bond in a system of symmetrically bound quantum wells. The spatially indirect excitons studied here are analogous to first order Wannier-Stark localized excitons in superlattices with inclined bands (when an electrical bias is applied), but in the present case the localization is of purely Coulomb origin. Zh. éksp. Teor. Fiz. 112, 1106–1118 (September 1997)     

Charge and energy transfer in double asymmetric quantum wells with quantum dots

The luminescence and luminescence excitation spectra of CdSe/ZnSe quantum dots are studied in a set of double quantum wells with the ZnSe barrier of width 14 nm, the same amount of a deposited CdSe layer forming a deep well and shallow wells with different depths. It is found that for a certain relation between the depths of shallow and deep wells in this set, conditions are realized under which the exciton channel in the luminescence excitation spectrum of a shallow well dominates in the region of kinetic exciton energies exceeding 10 longitudinal optical phonons above the bottom of the exciton band of the ZnSe barrier. A model is developed for the transfer of electrons, holes, and excitons between the electronic states of shallow and deep quantum wells separated by wide enough barriers. It is shown that the most probable process of electronic energy transfer between the states of shallow and deep quantum wells is indirect tunneling with the simultaneous excitation of a longitudinal optical phonon in the lattice. Because the probability of this process for single charge carriers considerably exceeds the exciton tunneling probability, a system of double quantum wells can be prepared in which, in the case of weak enough excitation, the states of quantum dots in shallow quantum wells will be mainly populated by excitons, which explains experimental results obtained.     

Influence of nonequilibrium phonons on exciton luminescence in CdTe/CdMnTe quantum wells

The luminescence method has been employed for the first time to detect nonequilibrium phonons in CdTe quantum wells with Cd_0.6Mn_0.4Te barriers. The method makes use of the giant Zeeman splitting of exciton states in CdTe/(Cd,Mn)Te quantum wells and is promising for application in high-sensitivity subterahertz phonon spectrometry. The method can also be useful in revealing the spin-phonon coupling mechanisms in diluted magnetic semiconductors. Fiz. Tverd. Tela (St. Petersburg) 40, 816–819 (May 1998)     

Phase diagram of the bose condensation of dipolar excitons in GaAs/AlGaAs quantum-well heterostructures

Experimental studies of the phase diagram of Bose condensation in a system of spatially indirect dipolar excitons in GaAs/AlGaAs quantum wells are reviewed. The properties of spatially periodic patterns arising in the luminescence of the exciton Bose condensate in a ring-shaped potential trap and the coherence of the condensate luminescence are discussed.     

Free and bound excitons in silver bromide under hydrostatic pressure

Abstract

Employing resonant Raman and luminescence spectroscopy, various exciton states and phonon modes are studied at low temperature in AgBr under hydrostatic pressure up to 0.7 GPa. The deformation potential for the indirect free exciton gap and mode Grüneisen parameters for various phonons are determined. Excitons bound to neutral donors and isoelectronic iodine are found to essentially derive from L-point valence band states.     

Dimensional effects and magnetic field influence on excitons in coupled quantum dots and coupled quantum wells

Direct and indirect excitons in coupled quantum wells and in coupled quantum dots are studied. We consider excitons with two-dimensional, quasi-two-dimensional and three-dimensional carriers. Problems were investigated for a wide range of characteristic parameters—confining to potential steepness, distances between quantum wells or dots, effective width of wells and magnetic fields. The mutual influence of the controlling parameters of the problem on exciton properties is analyzed. Energy and wave function spectra were calculated and dispersion law and effective masses were obtained.     

Theory of indirect exciton photoluminescence in elevated quantum trap

Inspired by an experiment of indirect excitons photoluminescence (PL) in elevated quantum trap (High et al., 2009), we theoretically investigate the energy relaxation and nonlinear interactions of indirect excitons in coupled quantum wells. It is shown that, when increasing the laser power, the intensity reversion of two PL peaks is due to the phonon necklace effect. In addition, we use a nonlinear Schrödinger equation including attractive two-body, repulsive three-body interactions and the excitation power dependence of energy distribution to understand the exciton states. This model gives a natural account for the PL blue shift with the increase of the excitation power. This study thus provides an alternative way to understand the underlying physics of the exciton dynamics in coupled potential wells.     

Anomalous influence of magnetic field on the indirect exciton in GaAs/AlGaAs double quantum wells

A study is reported of exciton luminescence in GaAs double quantum wells produced in electric and magnetic fields. It has been found that the indirect-exciton line (IX) behaves anomalously, namely, one observes a magnetic-field-induced low-energy shift of the IX line, and the onset of periodic (≈5 s) fluctuations in the IX-line intensity. Fiz. Tverd. Tela (St. Petersburg) 40, 803–805 (May 1998)     

Sasers: resonant transitions in narrow-gap semiconductors and in exciton system in coupled quantum wells

Two schemes for steady stimulated phonon generation (saser, i.e., phonon laser) are proposed. The first scheme exploits a narrow-gap indirect semiconductor or analogous indirect gap semiconductor heterostructure where the tuning into resonance of one-phonon transition of electron–hole recombination can be carried out by external pressure, magnetic or electric fields. The second scheme uses one-phonon transition between direct and indirect exciton levels in coupled quantum wells. The tuning into the resonance of this transition can be accomplished by engineering of dispersion of indirect exciton by external in-plane magnetic and normal electric fields. In the second scheme the magnitude of phonon wave vector is determined by magnitude of in-plane magnetic field and, therefore, such a saser is tunable. Both schemes are analyzed and estimated numerically.     

Midinfrared photoluminescence from SnTe/PbTe/CdTe double quantum wells grown by molecular beam epitaxy

Molecular beam epitaxial growth and photoluminescence (PL) properties of SnTe/PbTe/CdTe double quantum wells (DQWs) on (1 0 0)-oriented GaAs substrates are reported. These DQWs were consisted of a very thin SnTe/PbTe QW nested in a 10-nm-thick PbTe/CdTe QW. Efficient midinfrared PL was observed from the DQWs at 300 K in agreement with the coherent SnTe/PbTe growth on the thick CdTe barrier layer. The PL peak wavelength of the DQWs was found to increase with the SnTe thickness d by covering a wide range of the 3–5 μm atmospheric window with d≤2.5 monolayer.     

Bose condensation of two-dimensional dipolar excitons: Simulation by the quantum Monte Carlo method

The Bose condensation of two-dimensional dipolar excitons in quantum wells is numerically studied by the diffusion Monte Carlo simulation method. The correlation, microscopic, thermodynamic, and spectral characteristics are calculated. It is shown that, in structures of coupled quantum wells, in which low-temperature features of exciton luminescence have presently been observed, dipolar excitons form a strongly correlated system.     

Temperature properties of exciton luminescence from CdTe quantum wells with different thicknesses in the CdTe/CdMnTe structure

The optical spectra of the CdTe/Cd_0.7Mn_0.3Te structure containing three CdTe quantum wells with nominal thicknesses of 16, 8, and 4 monolayers have been investigated. The temperature dependences of parameters of the exciton luminescence spectra (integrated intensity, full-width at half-maximum, position of the maximum, Stokes shift) for quantum wells with different thicknesses differ substantially. These differences are explained by a strong thickness dependence of the energy of Coulomb coupling in the exciton, the energy of localization of the exciton on bulges of the quantum well, and the degree of penetration of the exciton wave function into the barrier. At high excitation power densities, the emission contours of the quantum wells with thicknesses of 8 and 16 monolayers contain short-wavelength tails that correspond to optical transitions between excited quantum-well levels.     

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)     

Influence of Polarization-Induced Electric Field on Subband Structure in AlxGa1-xN/GaN Double Quantum Wells

The influence of the polarization-induced electric field and other parameters on the subband structure in Al_xGa_1-xN /GaN coupled double quantum wells (DQWs) has been studied by solving the Schrödinger and Poisson equations self-consistently. It is found that the polarization effect leads to an asymmetric potential profile of Al_xGa_1-xN/GaN DQWs although the two wells have the same width and depth. The polarization effect also leads to a very large Stark shift between the odd and the even order subband levels that can reach 0.54eV. Due to the polarization-induced Stark shift, the wavelength of the intersubband transition between the first odd order and the second even order subband levels becomes smaller, which is useful for realization of optoelectronic devices operating within the telecommunication window region.     

Stimulated scattering of indirect excitons in coupled quantum wells: signature of a degenerate Bose-gas of excitons.

We observe and analyze strongly nonlinear photoluminescence kinetics of indirect excitons in GaAs/AlGaAs coupled quantum wells at low bath temperatures, > or = 50 mK. The long recombination lifetime of indirect excitons promotes accumulation of these Bose particles in the lowest energy states and allows the photoexcited excitons to cool down to temperatures where the dilute 2D gas of indirect excitons becomes statistically degenerate. Our main result--a strong enhancement of the exciton scattering rate to the low-energy states with increasing concentration of the indirect excitons--reveals bosonic stimulation of exciton scattering, which is a signature of a degenerate Bose-gas of excitons.