Bose-Einstein condensation in semiconductors: the key role of dark excitons

The non elementary-boson nature of excitons controls Bose-Einstein condensation in semiconductors. Composite excitons interact predominantly through Pauli exclusion; this produces dramatic couplings between bright and dark states. In microcavities, where bright excitons and photons form polaritons, they force the condensate to be linearly polarized--as observed. In bulk, they also force linear polarization, but of dark states, due to interband Coulomb scatterings. To evidence this dark condensate, we thus need indirect processes, like the shift it induces on the (bright) exciton line.