UC San Diego

Indirect excitons in GaAS/AlGaAs coupled quantum wells have small mass, long lifetime and short cooling time. Those enable us to create a cold exciton gas. I will present the experimental study of this cold exciton gas including kinetics, pattern formation and coherence. A spatial photoluminescence pattern including external exciton rings, localized bright spots, and the macroscopically ordered exciton state (MOES) was observed. While the external ring is a classical object by itself it is the region where the coldest exciton gas is created: the external ring is far from the hot excitation spot and the excitons in the ring are formed from well-thermalized carriers. The MOES - an array of beads with spatial order on macroscopic length appears abruptly in the ring at temperatures below a few Kelvin, where the thermal de Broglie wavelength is comparable to the interparticle separation and the exciton gas is nonclassical. We performed the first time-resolved kinetics measurements in this system. Simulations based on in-plane charge separation model gave good agreements with experimental results. From the simulation fits, diffusion constant of electrons and holes were obtained. We also observed commensurability in MOES: the exciton density wave is stable when there is an integer number of MOES wavelength between defects. In the external ring, a strong enhancement of spatial coherence length is observed below a few Kelvin. The increase of the coherence length is correlated with the macroscopic spatial ordering of excitons. The coherence length at the lowest temperature corresponds to a very narrow spread of the exciton momentum distribution, much smaller than that for a classical exciton gas.