  Excitons in semiconductors may form correlated phases at low temperatures. We
report the observation of an exciton liquid in GaAs/AlGaAs coupled quantum
wells. Above a critical density and below a critical temperature the
photogenerated electrons and holes separate into two phases, an electron-hole
plasma and an exciton liquid, with a clear sharp boundary between them. The two
phases are characterized by distinct photoluminescence spectra and by different
electrical conductance. The liquid phase is formed by the repulsive interaction
between the dipolar excitons, and exhibits a short range order, which is
manifested in the photoluminescence lineshape.
