"Exciton Liquid in Coupled Quantum Wells":
Indirect excitons in coupled quantum wells are a promising test bed for studying collective behavior in condensed matter. Exotic phases were predicted for this system and the experimental quest for observing them has been spanning a few decades. In this work, we report the observation of an exciton liquid in GaAs coupled quantum wells. Specifically, we find that above a critical density and below a critical temperature the system separates into two phases, electron-hole plasma and 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 clearly manifested in the photoluminescence lineshape.