Dark excitons and Bose-Einstein condensation in strain-trapped excitons in coupled quantum wells

It is possible to make excitons in coupled quantum wells with very long lifetime compared to their thermalization, allowing us to study equilibrium behavior. Using a localized stress to create traps for excitons in coupled quantum wells, we have demonstrated that at low temperature, high density, and large stress, the spatial pattern of photoluminescence (PL) from interwell excitons transitions to one with a dimmed center. This pattern emerges despite the center remaining the region of highest exciton density. This darkening is related to a strain-induced interaction between the light hole and heavy hole states. However, while this explanation provides a mechanism to explain many of the features, a few important predictions of this theory are not borne out by experiments. An alternate explanation is possible, utilizing an increasing population of dark (J=2) excitons and a separation of the dark and bright species. It has been proposed that a Bose-Einstein Condensate in this system would occur in a dark state, and this transition is consistent with the onset criteria of the pattern formation and explains how a slight bright/dark energy difference could lead to spatial separation of the species. Experiments employing a magnetic field to turn `dark' excitons slightly `bright' should allow the disambiguation of the role of dark excitons in this system. I will review this pattern formation and discuss data from experiments employing a magnetic field.