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)     

Condensation of interwell excitons in GaAs/AlGaAs double quantum wells

Experimental observations of the collective behavior of interwell excitons in the binary quantum wells with inclined bands under bias are discussed.     

Collective behavior of interwell excitons in GaAs/AlGaAs double quantum wells

Photoluminescence spectra of interwell excitons in double GaAs/AlGaAs quantum wells (n-i-n structures) have been investigated (an interwell exciton in these systems is an electron-hole pair spatially separated by a narrow AlAs barrier). Under resonance excitation by circularly polarized light, the luminescence line of interwell excitons exhibits a significant narrowing and a drastic increase in the degree of circular polarization of photoluminescence with increasing exciton concentration. It is found that the radiative recombination rate significantly increases under these conditions. This phenomenon is observed at temperatures lower than the critical point and can be interpreted in terms of the collective behavior of interwell excitons.     

Interwell excitons in GaAs multiple quantum wells and superlattices

Il Nuovo Cimento D - The optical properties of multiple quantum wells in the transition from isolated wells to a superlattice are investigated theoretically and experimentally. For superlattices...     

Localization of negatively charged excitons in GaAs/AlGaAs quantum wells

The localization of negatively charged excitons on isolated charged donors, located in a barrier at various fixed distances L from the heteroboundary, is investigated in isolated GaAs/AlGaAs quantum wells. The energy shift of the cyclotron replica in the emission spectra of a localized excitonic complex is studied as a function of L. It is shown that in undoped samples charged excitons localize on residual donors at distances L>350 Å on account of the formation of D ^? complexes at shorter distances. It is established that a cyclotron replica arises with the recombination of an excited state and not the ground state, as previously thought, of an excitonic complex.     

Collective state of interwell excitons in GaAs/AlGaAs double quantum wells under pulse resonance excitation

The time evolution and kinetics of photoluminescence (PL) spectra of interwell excitons in double GaAs/AlGaAs quantum wells (n-i-n structures) have been investigated under the pulse resonance excitation of intrawell 1sHH excitons using a pulsed tunable laser. It is found that the collective exciton phase arises with a time delay relative to the exciting pulse (several nanoseconds), which is due to density and temperature relaxation to the equilibrium values. The origination of the collective phase of interwell excitons is accompanied by a strong narrowing of the corresponding photoluminescence line (the line width is about 1.1 meV), a superlinear rise in its intensity, a long time in the change of the degree of circular polarization, a displacement of the PL spectrum toward lower energies (about 1.5 meV) in accordance with the filling of the lowest state with the exciton Bose condensate, and a significant increase in the radiative decay rate of the condensed phase. The collective exciton phase arises at temperatures T<6 K and interwell exciton densities n=3×10¹⁰ cm^?2. Coherent properties of the collective phase of interwell excitons and experimental manifestations of this coherence are discussed.     

Phase diagram of the Bose condensation of interwell excitons in GaAs/AlGaAs double quantum wells

The luminescence of interwell excitons in GaAs/AlGaAs double quantum wells (n-i-n heterostructures) with large-scale fluctuations of random potential in the heteroboundary planes was studied at low temperatures down to 0.5 K. The properties of excitons whose photoexcited electron and hole are spatially separated in the neighboring quantum wells by a tunneling barrier were studied as functions of density and temperature. The studies were performed within domains about one micron in size, which played the role of macroscopic traps for interwell excitons. For this purpose, the sample surface was coated with a metal mask containing special openings (windows) of a micron size or smaller. Both photoexcitation and observation of luminescence were performed through these windows by the fiber optic technique. At low pumping powers, the interwell excitons were strongly localized because of the residual charged impurities, and the corresponding photoluminescence line was nonuniformly broadened. As the laser excitation power increased, a narrow line due to delocalized excitons arose in a threshold-like manner, after which its intensity rapidly increased with growing pumping and the line itself narrowed (to a linewidth less than 1 meV) and shifted toward lower energies (by about 0.5 meV) in accordance with the filling of the lowest exciton state in the domain. An increase in temperature was accompanied by the disappearance of the line from the spectrum in a nonactivation manner. The phenomenon observed in the experiment was attributed to Bose-Einstein condensation in a quasi-two-dimensional system of interwell excitons. In the temperature interval studied (0.5–3.6) K, the critical exciton density and temperature were determined and a phase diagram outlining the exciton condensate region was constructed.     

Interwell radiative recombination in the presence of random potential fluctuations in GaAs/AlGaAs biased double quantum wells

The interwell radiative recombination from biased double quantum wells (DQW) in pin GaAs/AlGaAs heterostructures is investigated at different temperatures and external electrical fields. The luminescence line of interwell recombination of spatially separated electron-hole pairs exhibits systematic narrowing with temperature increase from 4.5 to 30 K. A theoretical model is presented which explains the observed narrowing in terms of lateral thermally activated tunneling of spatially separated e-h pairs localized by random potential fluctuations in the quantum wells. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 8, 580–585 (25 April 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Field-induced lifetime enhancement and ionization of excitons in GaAs/AlGaAs quantum wells

Picosecond and nanosecond luminescence studies of GaAs/AlGaAs quantum wells exposed to electric fields perpendicular to the layers are reported. The drastic red-shift of the photoluminescence with the electric field strength is accompanied by a strong increase of the electron-hole recombination lifetime. These features are most dominant for the wider wells due to the stronger polarization of the confined electron-hole pairs. Our observations are consistent with the model of the Quantum-Confined Stark Effect. In contrast, for high fields in the regime of ∼100 kV/cm field ionization limits the confinement of electrons and holes and leads to a strong decrease of the photoluminescence yield and lifetime with increasing field strength.     

Interwell excitons in semimagnetic semiconductor double quantum wells in an external magnetic field

Physics of the Solid State - The use of semimagnetic semiconductor double quantum wells in an external magnetic field is suggested for the separation of exciton charges. The exciton energies and...     

Giant burst of the emission line intensity of spatially indirect excitons in GaAs/AlGaAs double quantum wells

A study is reported on the low-temperature (T≤30 K) photoluminescence in the spatially indirect exciton line in GaAs/Al_0.33Ga_0.67As double quantum wells as a function of optical pump power and applied electric field. We have revealed a giant (threefold) burst of luminescence intensity in a part of the spectral profile of the indirect-exciton line occurring at certain values of the external electric field, temperature, and optical pumping. We have also observed that this part of the indirect-exciton line profile varies in intensity (fluctuates) with time with a characteristic period of tens of seconds. The results obtained are discussed within a model of the Bose-Einstein condensation of a system of two-dimensional bosons that have, besides the free, a discrete energy spectrum lying below the bottom of the free-state band.     

A diffusion anomaly in dipole-oriented two-dimensional excitons in GaAs/AlGaAs coupled quantum wells

Il Nuovo Cimento D - We have observed an anomaly in the low-temperature photoluminescence of dipole-oriented long-lived excitons in a coupled quantum well under an electrical bias. A discrepancy...     

High-frequency magneto-oscillations in GaAs/AlGaAs quantum wells

We have observed very high-frequency, highly reproducible magneto-oscillations in modulation doped GaAs/AlGaAs quantum well structures. The oscillations are periodic in an inverse magnetic field (1/B) and their amplitude increases with temperature up to T approximately 700 mK. Being initially most pronounced around the filling factor nu=1/2, they move towards lower nu with increasing T. Front and back-gating data imply that these oscillations require a coupling to a parallel conducting layer. A comparison with existing oscillation models renders no explanation.     

Free and localized positively charged excitons in the emission spectrum of GaAs/AlGaAs quantum wells

The recombination emission spectra of an excitonic complex (A ⁰ X) localized on a neutral acceptor, which have previously been attributed to a positively charged exciton (X ⁺), are investigated. Satellites arising around the main luminescence line as a result of recoil processes during recombination of the complex which leave the surviving hole in an excited state are observed and investigated. It is shown in a computational model based on the Luttinger Hamiltonian that the energy splittings between the main line and the satellites correspond to an in-barrier impurity center located a definite distance from the well. It is shown that as the magnetic field increases, a transition is observed from the singlet ground state of the complex to a multiplet state. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 3, 223–228 (10 August 1998)     

Exciton behavior in GaAs/AlGaAs coupled double quantum wells with interface disorder

In this work, we present a detailed study on the optical properties of two GaAs/Al_0.35Ga_0.65As coupled double quantum wells (CDQWs) with inter-well barriers of different thicknesses, by using photoluminescence (PL) spectroscopy. The two CDQWs were grown in a single sample, assuring very similar experimental conditions for measurements of both. The PL spectrum of each CDQW exhibits two recombination channels which can be accurately identified as the excitonic e₁-hh₁ transitions originated from CDQWs of different effective dimensions. The PL spectra characteristics and the behavior of the emissions as a function of temperature and excitation power are interpreted in the scenario of the bimodal interface roughness model, taking into account the exciton migration between the two regions considered in this model and the difference in the potential fluctuation levels between those two regions. The details of the PL spectra behavior as a function of excitation power are explained in terms of the competition between the band gap renormalization (BGR) and the potential fluctuation effects. The results obtained for the two CDQWs, which have different degrees of potential fluctuation, are also compared and discussed.     

Charged excitons in GaAs quantum wells

Il Nuovo Cimento D - We implement optical spectroscopy to study charged excitons (trions) in modulation-doped GaAs/AlGaAs quantum wells. We observe for the first time several new trions: the...