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Low-temperature physics

Excitons condense

Nano Lett. 12, 2605–2609 (2012)

At low temperature, a collection of bosons can condense into a state described by a single, collective wavefunction. Such condensates have been created using atoms, photons and magnons, allowing unique quantum phenomena to be observed. Now Arthur Gossard and colleagues at the University of California at Santa Barbara and the University of California at San Diego have demonstrated condensation of electron–hole pairs, which are called excitons, inside an electrostatic trap.

Because electrons and holes have a tendency to recombine quickly, Gossard and co-workers trapped one of each in separate semiconductor quantum wells. The resulting indirect excitons were spatially confined by an electric field created by a diamond-shaped electrode. Emission from spatially separated excitons then created interference patterns, whose dependence on temperature and exciton density confirmed the existence of an exciton condensate over the entire area of the trap. The coherence of the condensate was also shown to be spontaneous, and not induced by the laser light used to excite the excitons.

Condensation occurred at a lattice temperature of around 2 K, which is high relative to the temperatures typically required for atomic condensates. Furthermore, excitons are very different from bosons that have been condensed before and, therefore, exciton condensates are expected to exhibit a variety of unique behaviours.

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Excitons condense. Nature Nanotech (2012).

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