Experimental Study of the Exciton Gas Liquid Transition in Coupled Quantum Wells

We study the exciton gas-liquid transition in GaAs/AlGaAs coupled quantum wells. Below a critical temperature, Tc=4.8K, and above a threshold laser power density the system undergoes a phase transition into a liquid state. We determine the density temperature phase diagram over the temperature range 0.1 to 4.8K. We find that the latent heat increases linearly with temperature at T<1.1K, similarly to a Bose Einstein condensate transition, and becomes constant at 1.1<T<4.8K. Resonant Rayleigh scattering measurements reveal that the disorder in the sample is strongly suppressed and the diffusion coefficient sharply increases with decreasing temperature at T<Tc, allowing the liquid to spread over large distances away from the excitation region. We suggest that our findings are manifestations of a quantum liquid behavior.