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Free superflow of excitons in a dark state and luminescence rings in quantum well structures

Excitons in quantum wells, generated by a laser pulse, typically diffuse only a few micrometers from the spot where they are created. Solving the Gross-Pitaevskii equation we describe a free superflow of a two-dimensional slowly decaying Bose liquid. In contrast to previous theoretical models describing the luminescence ring formation in quantum wells via the conventional diffusion of electrons and holes, we explain this phenomenon as a sequence of superfluidity of excitons in a dark state. We propose a few key experiments which can further verify the exciton superfluid.