Particle correlations and evidence for dark state condensation in a cold dipolar exciton fluid

Dipolar excitons are long-lived quasi-particle excitations in semiconductor heterostructure that carry an electric dipole. Cold dipolar excitons are expected to have new quantum and classical multi-particle correlation regimes, as well as several collective phases, resulting from the intricate interplay between the many-body interactions and their quantum nature. Here we show experimental evidence of a few correlation regimes of a cold dipolar exciton fluid, created optically in a semiconductor bilayer heterostructure. In the higher temperature regime, the average interaction energy between the particles shows a surprising temperature dependence, which is evidence for correlations beyond the mean field model. At a lower temperature, there is a sharp increase in the interaction energy of optically active excitons, accompanied by a strong reduction in their apparent population. This is evidence for a sharp macroscopic transition to a dark state, as has been suggested theoretically.