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arXiv:1506.08020

Spectroscopic Signatures for Bose-Einstein Condensation of Dark Excitons in a Trap

Mussie Beian, Mathieu Alloing, Romain Anankine, E. Cambril, K. Merghem, Carmen Gómez, Aristide Lemaître, François Dubin

Published Jun 26, 2015• 2 citations
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Abstract

We study spatially indirect excitons confined in a 10 microns wide electrostatic trap. At sub-Kelvin bath temperatures, we show that the optical emission from the trap is anomalously weak in a regime where the excitons photoluminescence is homogeneously broadened and narrow-band. Particularly, we observe a loss of photoluminescence below a few Kelvin which violates classical expectations. Through a careful calibration of our experiments, we show that the photoluminescence loss can only be interpreted as the signature for Bose-Einstein stimulation of excitons from bright to dark states.

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We study spatially indirect excitons confined in a 10 microns wide electrostatic trap. At sub-Kelvin bath temperatures, we show that the optical emission from the trap is anomalously weak in a regime where the excitons photoluminescence is homogeneously broadened and narrow-band. Particularly, we observe a loss of photoluminescence below a few Kelvin which violates classical expectations. Through a careful calibration of our experiments, we show that the photoluminescence loss can only be interpreted as the signature for Bose-Einstein stimulation of excitons from bright to dark states.
Summary form only given. In the dilute limit excitons behave as weakly interacting Bose particles and are expected to undergo the Bose-Einstein condensation (BEC). Because the exciton mass is very small at experimentally accessible exciton densities the critical temperature for exciton BEC should reach several Kelvins, i.e., about six orders of magnitude higher than the T/sub c/ for atom BEC. To create degenerate Bose-gas of excitons we propose quasi-2D gas of indirect excitons in coupled quantum wells (CQWs) which have two primary advantages for realizing a quantum degenerate exciton gas and, ultimately, the BEC-like phase transition: (i) the long lifetime of indirect excitons and (ii) the short exciton cooling time.
We calculate the spatially resolved optical emission spectrum of a weakly interacting Bose gas of excitons confined in a three dimensional potential trap due to interband transitions involving weak direct and phonon mediated exciton-photon interactions. Applying the local-density approximation, we show that for a noncondensed system the spatiospectral lineshape of the direct process reflects directly the shape of the potential. The existence of a Bose-Einstein condensate changes the spectrum in a characteristic way so that it directly reflects the constant chemical potential of the excitons and the renormalization of the quasiparticle excitation spectrum. Typical examples are given for parameters of the lowest yellow excitons in Cu2O{\text{Cu}}_{2}\text{O}Cu2​O.