Spontaneous coherence in a cold exciton gas

• Published in: Nature (London) Vol. 483; no. 7391; pp. 584 - 588
• Main Authors: High, A. A., Leonard, J. R., Hammack, A. T., Fogler, M. M., Butov, L. V., Kavokin, A. V., Campman, K. L., Gossard, A. C.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 29-03-2012; Nature Publishing Group
• Subjects: 639/766; Bosons; Coherent states; Cold; Experiments; Gases; Humanities and Social Sciences; letter; multidisciplinary; Physics; Properties; Science; Science (multidisciplinary); Temperature; United States
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature10903

In theory, excitons can form a coherent state like a Bose–Einstein condensate, but this is difficult to produce; it is now shown that spontaneous coherence, characteristic of a condensate, can occur in a cold exciton gas.
Exciton condensate demonstrated
Like atoms, excitons — bound pairs of electrons and electron holes — can form a Bose–Einstein-condensate-like state with superfluidic properties below a critical temperature. In practice, this is difficult to achieve in a normal semiconductor owing to the short lifetime of excitons; they cannot be cooled down fast enough. This problem can be circumvented by making use of indirect excitons, which have much longer lifetimes because the electrons and holes are confined in separate quantum-well layers to prevent them from recombining too quickly. Previously, it has been shown that a cold exciton gas can be formed in this system; now, High
et al
. observe spontaneous coherence in such a gas, characteristic of a condensate.
If bosonic particles are cooled down below the temperature of quantum degeneracy, they can spontaneously form a coherent state in which individual matter waves synchronize and combine. Spontaneous coherence of matter waves forms the basis of a number of fundamental phenomena in physics, including superconductivity, superfluidity and Bose–Einstein condensation
1
,
2
. Spontaneous coherence is the key characteristic of condensation in momentum space
3
. Excitons—bound pairs of electrons and holes—form a model system to explore the quantum physics of cold bosons in solids
4
,
5
. Cold exciton gases can be realized in a system of indirect excitons, which can cool down below the temperature of quantum degeneracy owing to their long lifetimes
6
. Here we report measurements of spontaneous coherence in a gas of indirect excitons. We found that spontaneous coherence of excitons emerges in the region of the macroscopically ordered exciton state
7
and in the region of vortices of linear polarization. The coherence length in these regions is much larger than in a classical gas, indicating a coherent state with a much narrower than classical exciton distribution in momentum space, characteristic of a condensate. A pattern of extended spontaneous coherence is correlated with a pattern of spontaneous polarization, revealing the properties of a multicomponent coherent state. We also observed phase singularities in the coherent exciton gas. All these phenomena emerge when the exciton gas is cooled below a few kelvin.