"Gray" BCS condensate of excitons and internal Josephson effect

TL;DR

This paper predicts that increasing exciton density leads to a mixed dark-bright BCS-like condensate, enabling an internal Josephson effect observable through photoluminescence oscillations.

Contribution

It introduces the concept of a 'gray' exciton condensate with both dark and bright components resulting from density-induced carrier exchange.

Findings

Dark exciton condensate acquires a bright component at high density

A threshold density exists for the dark-to-bright component transition

Internal Josephson oscillations could be observed in photoluminescence

Abstract

It has been recently suggested that the Bose-Einstein condensate formed by excitons in the dilute limit must be dark, i.e., not coupled to photons. Here, we show that, under a density increase, the dark exciton condensate must acquire a bright component due to carrier exchange in which dark excitons turn bright. This however requires a density larger than a threshold which seems to fall in the forbidden region of the phase separation between a dilute exciton gas and a dense electron-hole plasma. The BCS-like condensation which is likely to take place on the dense side, must then have a dark and a bright component - which makes it "gray". It should be possible to induce an internal Josephson effect between these two coherent components, with oscillations of the photoluminescence as a strong proof of the existence for this "gray" BCS-like exciton condensate.