Evidence for a Bose-Einstein condensate of excitons
Résumé
We report compelling evidence for a ``gray'' condensate of dipolar excitons, electrically polarised in a 25 nm wide GaAs quantum well. The condensate is composed by a macroscopic population of dark excitons coherently coupled to a lower population of bright excitons. To create the exciton condensate we use an all-optical approach in order to produce microscopic traps which confine a dense exciton gas (similar to 10(10) cm(-2)) that yet exhibits an anomalously weak photoemission at sub-kelvin temperatures. This is the first fingerprint for the ``gray'' condensate. It is then confirmed by the macroscopic spatial coherence and the linear polarization of the weak excitonic photoluminescence emitted from the trap, as theoretically predicted.