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. (topical review)