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Towards Bose–Einstein condensation of excitons in potential traps

Abstract

An exciton is an electron–hole bound pair in a semiconductor. In the low-density limit, it is a composite Bose quasi-particle, akin to the hydrogen atom1. Just as in dilute atomic gases2,3, reducing the temperature or increasing the exciton density increases the occupation numbers of the low-energy states leading to quantum degeneracy and eventually to Bose–Einstein condensation (BEC)1. Because the exciton mass is small—even smaller than the free electron mass—exciton BEC should occur at temperatures of about 1 K, many orders of magnitude higher than for atoms. However, it is in practice difficult to reach BEC conditions, as the temperature of excitons can considerably exceed that of the semiconductor lattice. The search for exciton BEC has concentrated on long-lived excitons: the exciton lifetime against electron–hole recombination therefore should exceed the characteristic timescale for the cooling of initially hot photo-generated excitons4,5,6,7,8,9,10. Until now, all experiments on atom condensation were performed on atomic gases confined in the potential traps. Inspired by these experiments, and using specially designed semiconductor nanostructures, we have collected quasi-two-dimensional excitons in an in-plane potential trap. Our photoluminescence measurements show that the quasi-two-dimensional excitons indeed condense at the bottom of the traps, giving rise to a statistically degenerate Bose gas.

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Figure 1: Potential profiles in coupled quantum well structures in the growth direction, z, and in the x–y plane.
Figure 2: Photoluminescence intensity versus energy and one of the in-plane coordinates x at T = 1.6 K (the photoluminescence (PL) variation along the orthogonal in-plane coordinate y is the same).
Figure 3: The spatial shrinkage of the exciton cloud near the bottom of the potential trap as the temperature is reduced.
Figure 4: Anomalous properties of the traps revealed by the spatial dependence of the indirect exciton photoluminescence.

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Acknowledgements

We thank K. L. Campman for growing the high-quality coupled QW samples. This work was supported by the Office of Energy Research, Office of Basic Energy Sciences and the Division of Material Sciences of the US DOE and by the EPSRC and RFBR.

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Butov, L., Lai, C., Ivanov, A. et al. Towards Bose–Einstein condensation of excitons in potential traps. Nature 417, 47–52 (2002). https://doi.org/10.1038/417047a

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