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arXiv 1304.4101cond-mat.quant-gas
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激子的玻色-爱因斯坦凝聚体的证据

Evidence for a Bose-Einstein condensate of excitons

Mathieu Alloing, Mussie Beian, Maciej Lewenstein, David Fuster, Yolanda Gonzalez, Luisa Gonzalez, Roland Combescot, Monique Combescot, Francois Dubin

摘要:

原子气体在微开尔文温度下的玻色-爱因斯坦凝聚的证明是一个引人注目的里程碑,而它对半导体激子的证据仍然是一个期待已久的里程碑。这种情况是没有预见到的,因为激子是轻质量的玻色子状粒子,预计会在几个开尔文附近发生凝聚。一个解释可以在激子的基本费米子性质中找到,它支配着激子的凝聚。准确地说,最近预测,在可获得的实验条件下,激子凝聚体应该是“灰色”的,主要的黑暗部分通过费米子交换相干耦合到一个微弱的明亮分量。由于激子大多以其光学活性而闻名,因此这种违反直觉的量子凝聚直接来自具有光学非活性的激子内部结构,即,暗的基态。在这里,我们报告令人信服的证据,这样一个“灰色”冷凝。我们使用全光学的方法,以产生微观陷阱,限制了密集的激子气体,但表现出一个令人不安的弱光发射在亚开尔文温度。这第一个指纹的“灰色”冷凝物,然后确认宏观空间相干性和线性偏振的弱激子光致发光从陷阱发出的,从理论上预测。

英文摘要:

The demonstration of Bose-Einstein condensation in atomic gases at micro-Kelvin temperatures is a striking landmark while its evidence for semiconductor excitons still is a long-awaited milestone. This situation was not foreseen because excitons are light-mass boson-like particles with a condensation expected to occur around a few Kelvins. An explanation can be found in the underlying fermionic nature of excitons which rules their condensation. Precisely, it was recently predicted that, at accessible experimental conditions, the exciton condensate shall be "gray" with a dominant dark part coherently coupled to a weak bright component through fermion exchanges. This counter-intuitive quantum condensation, since excitons are mostly known for their optical activity, directly follows from the excitons internal structure which has an optically inactive, i.e., dark, ground state. Here, we report compelling evidence for such a "gray" condensate. We use an all-optical approach in order to produce microscopic traps which confine a dense exciton gas that yet exhibits an anomalously weak photo-emission at sub-Kelvin temperatures. This first fingerprint for a "gray" condensate is then confirmed by the macroscopic spatial coherence and the linear polarization of the weak excitonic photoluminescence emitted from the trap, as theoretically predicted.

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