An experimental technique for the indirect manipulation and detection of electron spins entangled in two-dimensional magnetoexcitons has been developed. The kinetics of the spin relaxation has been investigated. Photoexcited spin-magnetoexcitons were
found to exhibit extremely slow relaxation in specific quantum Hall systems, fabricated in high mobility GaAs/AlGaAs structures, namely, the relaxation time reaches values over one hundred microseconds. A qualitative explanation of this spin-relaxation kinetics is presented. Its temperature and magnetic field dependencies are discussed within the available theoretical framework.
Correlations of luminescence intensity have been studied under Bose-Einstein condensation of dipolar excitons in the temperature range of 0.45-4.2 K. Photoexcited dipolar excitons were collected in a lateral trap in GaAs/AlGaAs Schottky-diode heteros
tructure with single wide (25 nm) quantum well under applied electric bias. Two-photon correlations were measured with the use of a classical Hanbury Brown - Twiss intensity interferometer (time resolution ~0.4 ns). Photon bunching has been observed near the Bose condensation threshold of dipolar excitons determined by the appearance of a narrow luminescence line of exciton condensate at optical pumping increase. The two-photon correlation function shows super-poissonian distribution at time scales of system coherence (<~1 ns). No photon bunching was observed at the excitation pumping appreciably below the condensation threshold. At excitation pumping increasing well above the threshold, when the narrow line of exciton condensate grows in the luminescence spectrum, the photon bunching is decreasing and finally vanishes - the two-photon correlator becomes poissonian reflecting the single-quantum-state origin of excitonic Bose condensate. Under the same conditions a first-order spatial correlator, measured by means of the luminescence interference from spatially separated condensate parts, remains significant. The discovered photon bunching is rather sensitive to temperature: it drops several times with temperature increase from 0.45 K up to 4.2 K. If assumed that the luminescence of dipolar excitons collected in the lateral trap reflects directly coherent properties of interacting exciton gas, the observed phenomenon of photon bunching nearby condensation threshold manifests phase transition in interacting exciton Bose gas.
