Resonant Rayleigh Scattering from Dipolar Excitonic Phases in Coupled Quantum Wells

Dipolar excitons in coupled quantum wells (CQW) offer a unique test bed for studying collective effects of a quantum degenerate system. We have recently studied the behavior of indirect excitons in this system (GaAs/AlGaAs CQW) and found an abrupt gas-liquid phase transition at a critical temperature and excitation power.
In this work, we study this transition by Resonant Rayleigh scattering measurements, known to be an insightful probe of critical phenomena. The RRS signal is expected to be strongly enhanced at the exciton resonance; with its linewidth being a measure of disorder in the sample. Here we perform the measurements in a pump–probe configuration: we create the carriers by the laser diode pump and measure the scattered intensity of the weak Ti:Sapphire probe, tuned to the NW exciton resonance. We find that in the liquid phase, at temperatures lower than 1K, the signal becomes symmetric with a significantly narrower width as compared to the signal obtained for the gas. The homogeneous nature and the narrow linewidth suggests that the disorder potential in the sample is effectively screened at these temperatures. This screening is predicted theoretically and shown to be a precursor of superfluidity.