Michael Stern, Vladimir Umansky, Israel Bar-Joseph
We report optical spectroscopy studies and electrical transport measurements that provide evidence for the formation of a dark exciton condensate in GaAs coupled quantum wells. We find that below a critical temperature and above a threshold power the photoluminescence from the sample separates into two spatial regions, each characterized by a distinct spectrum, with a clear dark boundary between them. The phase separation is accompanied by the onset of strong non-radiative recombination in one of the regions. We show that this darkening is due to suppression of spin-flip processes between dark to bright excitons, giving rise to a build up of a large population of dark excitons. We find that the condensate is formed through a slow nucleation process, which typically takes a few seconds. Electrical transport measurements show that the condensate region is characterized by relatively low conductivity, indicating its excitonic nature.
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http://arxiv.org/abs/1203.0837
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