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Exciton Liquid in Coupled Quantum Wells

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Science  03 Jan 2014:
Vol. 343, Issue 6166, pp. 55-57
DOI: 10.1126/science.1243409

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Exciton Liquid

Excitons, bound states of electrons and holes (states “vacated” by electrons), can be found in semiconductors and have long been predicted to form correlated phases at sufficiently large densities and low temperatures. Stern et al. (p. 55) studied the behavior of spatially indirect excitons, which consist of electrons and holes residing in spatially separated, but coupled, quantum wells. The excitons were created through a combination of photoexcitation and electric gating. At high enough laser power and low enough temperatures, a new phase with a distinct photoluminescence signature appeared with behavior consistent with that of a classical liquid of excitons.

Abstract

Excitons in semiconductors may form correlated phases at low temperatures. We report the observation of an exciton liquid in gallium arsenide/aluminum gallium arsenide–coupled quantum wells. Above a critical density and below a critical temperature, the photogenerated electrons and holes separate into two phases: an electron-hole plasma and an exciton liquid, with a clear sharp boundary between them. The two phases are characterized by distinct photoluminescence spectra and by different electrical conductance. The liquid phase is formed by the repulsive interaction between the dipolar excitons and exhibits a short-range order, which is manifested in the photoluminescence line shape.

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