Theory of condensation of indirect excitons in a trap

We present theoretical studies of condensation of indirect excitons in a trap. Our model quantifies the effect of screening of the trap potential by indirect excitons on exciton condensation. The theoretical studies are applied to a system of indirect excitons in a GaAs/AlGaAs coupled quantum well structure in a diamond-shaped electrostatic trap where exciton condensation was studied in earlier experiments. The estimated condensation temperature of the indirect excitons in the trap reaches hundreds of milliKelvin

Pattern Formation as a Signature of Quantum Degeneracy in a Cold Exciton System

The development of a Turing instability to a spatially modulated state in a photoexcited electron-hole system is proposed as a novel signature of exciton Bose statistics. We show that such an instability, which is driven by kinetics of exciton formation, can result from stimulated processes that build up near quantum degeneracy. In the spatially uniform 2d electron-hole system, the instability leads to a triangular lattice pattern while, at an electron-hole interface, a periodic 1d pattern develops. We analyze the mechanism of wavelength selection, and show that the transition is abrupt (type I) for the uniform 2d system, and continuous (type II) for the electron-hole interface.Comment: 5 pages, 3 figure

Kinetics of Exciton Emission Patterns and Carrier Transport

We report on the measurements of the kinetics of expanding and collapsing rings in the exciton emission pattern. The rings are found to preserve their integrity during expansion and collapse, indicating that the observed kinetics is controlled by charge carrier transport rather than by a much faster process of exciton production and decay. The relation between ring kinetics and carrier transport, revealed by our experiment and confirmed by comparison with a theoretical model, is used to determine electron and hole transport characteristics in a contactless fashion.Comment: 6 pages, 4 figure

Kinetics of indirect excitons in the optically-induced exciton trap

We report on the kinetics of a low-temperature gas of indirect excitons in the optically-induced exciton trap. The excitons in the region of laser excitation are found to rapidly -- within 4 ns -- cool to the lattice temperature T = 1.4 K, while the excitons at the trap center are found to be cold -- essentially at the lattice temperature -- even during the excitation pulse. The loading time of excitons to the trap center is found to be about 40 ns, longer than the cooling time yet shorter than the lifetime of the indirect excitons. The observed time hierarchy is favorable for creating a dense and cold exciton gas in optically-induced traps and for in situ control of the gas by varying the excitation profile in space and time before the excitons recombine.Comment: 4 pages, 3 figure

Ring-shaped spatial pattern of exciton luminescence formed due to the hot carrier transport in a locally photoexcited electron-hole bilayer

A consistent explanation of the formation of a ring-shaped pattern of exciton luminescence in GaAs/AlGaAs double quantum wells is suggested. The pattern consists of two concentric rings around the laser excitation spot. It is shown that the luminescence rings appear due to the in-layer transport of hot charge carriers at high photoexcitation intensity. Interestingly, one of two causes of this transport might involve self-organized criticality (SOC) that would be the first case of the SOC observation in semiconductor physics. We test this cause in a many-body numerical model by performing extensive molecular dynamics simulations. The results show good agreement with experiments. Moreover, the simulations have enabled us to identify the particular kinetic processes underlying the formation of each of these two luminescence rings.Comment: 14 pages, 16 figures. Final versio

Ballistic spin transport in exciton gases

Traditional spintronics relies on spin transport by charge carriers, such as electrons in semiconductor crystals. This brings several complications: the Pauli principle prevents the carriers from moving with the same speed; Coulomb repulsion leads to rapid dephasing of electron flows. Spin-optronics is a valuable alternative to traditional spintronics. In spin-optronic devices the spin currents are carried by electrically neutral bosonic quasi-particles: excitons or exciton-polaritons. They can form highly coherent quantum liquids and carry spins over macroscopic distances. The price to pay is a finite life-time of the bosonic spin carriers. We present the theory of exciton ballistic spin transport which may be applied to a range of systems where bosonic spin transport has been reported, in particular, to indirect excitons in coupled GaAs/AlGaAs quantum wells. We describe the effect of spin-orbit interaction of electrons and holes on the exciton spin, account for the Zeeman effect induced by external magnetic fields, long range and short range exchange splittings of the exciton resonances. We also consider exciton transport in the non-linear regime and discuss the definitions of exciton spin current, polarization current and spin conductivity.Comment: 16 pages, 10 figures to be published in Phys. Rev.