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Spontaneous Coherence, Pattern Formation and Kinetics of Macroscopically Ordered Exciton State

Speaker:Mr. Sen YANG
Affiliation:University of California, San Diego, USA
Date:March 31, 2009 (Tue)
Time:10:30-11:30 a.m.
Venue:Room 522, 5/F, Chong Yuet Ming Physics Building, HKU



A spatial photoluminescence pattern including external exciton rings, localized bright spots, and the macroscopically ordered exciton state (MOES) was observed in GaAS/AlGaAs coupled quantum wells. 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. I will present the experimental study of this cold exciton gas including kinetics, pattern formation and coherence. Kinetics measurements show the expansion and shrinkage of the external ring has time scale of microseconds, in good agreement with in-plane charge separation model. Commensurability is observed 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, much larger than that for a classical exciton gas. 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.

Coffee and tea will be served 20 minutes prior to the seminar.