Mussie Beian

Evidence for a Bose-Einstein condensate of excitons

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 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.

Long-lived spin coherence of indirect excitons in GaAs coupled quantum wells

We study spatially indirect excitons confined in a symmetric GaAs double quantum well. We show that the spin polarisation of very dilute gases can be optically imprinted, in both pure and superposition states. In the regime where indirect excitons can be localized, we observe at 350 mK that the excitons spin degree of freedom is frozen with a relaxation time comparable to the coherence time and to the radiative lifetime .

Publisher’s Note: Quantized Vortices and Four-Component Superfluidity of Semiconductor Excitons [Phys. Rev. Lett. 118 , 127402 (2017)]

This corrects the article DOI: 10.1103/PhysRevLett.118.127402.

Spectroscopic signatures for the dark Bose-Einstein condensation of spatially indirect excitons

We study semiconductor excitons confined in an electrostatic trap of a GaAs bilayer heterostructure. We evidence that optically bright excitonic states are strongly depleted while cooling to sub-Kelvin temperatures. In return, the other accessible and optically dark states become macroscopically occupied so that the overall exciton population in the trap is conserved. These combined behaviours constitute the spectroscopic signature for the mostly dark Bose-Einstein condensation of excitons, which in our experiments is restricted to a dilute regime within a narrow range of densities, below a critical temperature of about 1K.