On the dark nature of exciton Bose-Einstein condensate

Details On the dark nature of exciton Bose-Einstein condensate On the dark nature of exciton Bose-Einstein condensate

2008-07-29

arxiv.org/abs/0807.4724v1

Physics

Condensed Matter

Strongly Correlated Electrons

6 pages, 6 figures

Scientific paper

We show that for the very same reason that excitons are bright, i.e. coupled to photons, they have a higher energy than dark excitons, even for electrons spatially separated from holes, such as in a double quantum well. Indeed, the same channel which produces the finite electron-hole effective overlap responsible for the absorption and emission of photon allows for Coulomb interband exchange processes, which are nothing but a sequence of virtual recombination and creation of one electron-hole pair. Consequently, this additional repulsive electron-hole Coulomb exchange interaction exists for bright excitons, but not for dark excitons. If we now remember that dark excitons with spins $\pm 2$ are formed in a natural way through carrier exchange between two opposite spin bright excitons, we are led to predict that in a double quantum well sample with one trap -- a configuration appropriate to get high density -- exciton Bose-Einstein condensation should appear when cooling down the sample as a dark spot made of $(\pm 2)$ excitons at the center of the trap.

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