Spontaneous Coherence and Spin Texture in a Cold Exciton Gas

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Speaker: Leonid Butov, University of California, San Diego

Date: September 14, 2011

Event: Solid-State Lighting Science Workshop in Novel Emitters and Nanostructured Materials

Abstract: An indirect exciton is a bound pair of an electron and a hole confined in spatially separated layers. Indirect excitons have long lifetimes and long spin relaxation times, can cool down to low temperatures well below the temperature of quantum degeneracy, can travel over large distances before recombination, and can be in situ controlled by voltage. Due to these properties, indirect excitons form a model system both for the studies of basic properties of cold bosons and for the development of optoelectronic devices. In this contribution, we report on the observation of a pattern of spontaneous coherence, spin texture, and phase singularities in a cold gas of indirect excitons in a GaAs/AlGaAs coupled quantum well structure. The observed features of spin texture include a vortex of linear polarization with polarization perpendicular to the radial direction around an exciton source and a periodic spin texture around the macroscopically ordered exciton state formed in the exciton ring. Extended spontaneous coherence of excitons is observed in the region of the polarization vortices and in the region of the macroscopically ordered exciton state. The coherence length in these regions is much larger than in a classical gas, indicating a coherent exciton state with a much narrower than classical exciton distribution in momentum space, characteristic of a condensate. The observed phase singularities include phase domains and fork-like dislocations in the interference pattern. Extended spontaneous coherence, spin texture, and phase singularities are spatially correlated and emerge when the exciton gas is cooled below a few Kelvin.