Condensation and pattern formation in cold exciton gases in coupled quantum wells

Bound electron?hole pairs?excitons?are light Bose particles with a mass comparable to or smaller than that of the free electron. Since the quantum degeneracy temperature scales inversely with the mass, it is anticipated that Bose?Einstein condensation of an exciton gas can be achieved at temperatures of about 1?K, orders of magnitude larger than the micro-Kelvin temperatures employed in atomic condensation. High quantum degeneracy temperatures and the possibility to control exciton density by laser photoexcitation make cold excitons a model system for studies of collective states and many-body phenomena in a system of cold bosons. Experimentally, an exciton temperature well below 1?K is achieved in a gas of indirect excitons in coupled quantum-well semiconductor heterostructures. Here, we overview phenomena in the cold exciton gases: condensation, pattern formation, and macroscopically ordered exciton states.