Evidence for a Bose-Einstein condensate of excitons

TL;DR

This paper provides experimental evidence for a 'gray' Bose-Einstein condensate of excitons in semiconductors, characterized by weak optical emission, spatial coherence, and polarization, confirming theoretical predictions about dark exciton condensates.

Contribution

It is the first experimental demonstration of a 'gray' exciton condensate, revealing the dark component's role in exciton BECs and confirming theoretical models.

Findings

Weak photoluminescence at sub-Kelvin temperatures

Macroscopic spatial coherence observed

Linear polarization of emitted light

Abstract

The demonstration of Bose-Einstein condensation in atomic gases at micro-Kelvin temperatures is a striking landmark while its evidence for semiconductor excitons still is a long-awaited milestone. This situation was not foreseen because excitons are light-mass boson-like particles with a condensation expected to occur around a few Kelvins. An explanation can be found in the underlying fermionic nature of excitons which rules their condensation. Precisely, it was recently predicted that, at accessible experimental conditions, the exciton condensate shall be "gray" with a dominant dark part coherently coupled to a weak bright component through fermion exchanges. This counter-intuitive quantum condensation, since excitons are mostly known for their optical activity, directly follows from the excitons internal structure which has an optically inactive, i.e., dark, ground state. Here, we report compelling evidence for such a "gray" 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 photo-emission at sub-Kelvin temperatures. This first fingerprint for a "gray" condensate is then confirmed by the macroscopic spatial coherence and the linear polarization of the weak excitonic photoluminescence emitted from the trap, as theoretically predicted.