Evidence for a Bose-Einstein condensate of excitons (Vol. 45 No.5-6)

At sufficiently low temperatures, boson-like quantum particles can condense in the ground-state of the system and then form a particular realm called a Bose-Einstein condensate. Semiconductor excitons, i.e. electron-hole pairs bound by Coulomb attraction, shall undergo Bose-Einstein condensation under a priori easily accessible experimental conditions, e.g. below a few Kelvins. However, due to their composite nature, excitons exhibit a dark ground-state, i.e. optically inactive, which has certainly contributed to the lack of signature of exciton condensation obtained through conventional optical probes.

In this letter, the authors reveal the dark nature of excitons Bose-Einstein condensation. They report an excitonic quantum statistical distribution marked by a dominant (~ 90%) fraction of dark excitons at sub-Kelvin temperatures. The exciton condensate emits a weak photoluminescence with macroscopic spatial coherence and linear polarization. These signal a multi-component exciton condensation, with a weak bright component coherently coupled to a dominant dark part, as theoretically predicted.

M. Alloing et al., “Evidence for a Bose-Einstein condensate of excitons”, EPL, 107, 10012 (2014)
[Abstract]