Metastable energy distribution and localization of spatially indirect excitons

We report the electric-field-induced localization of extremely-long-lived excitons, with lifetimes of up to 5 ms in coupled GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum wells. The effect is manifested in a field-sensitive temperature dependence of the exciton recombination time that is well fit by a model based on a field-dependent mobility edge. The observed field dependence of the exciton through the transition from a mobile to a localized gas is consistent with an exciton gas that is in metastable equilibrium, and is not consistent with either a Bose distribution or with a spatial distribution dominated by dipole-dipole repulsion.