Spontaneous coherence in a cold exciton gas

摘要

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. Spontaneous coherence is the key characteristic of condensation in momentum space. Excitons-bound pairs of electrons and holes-form a model system to explore the quantum physics of cold bosons in solids. 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. 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 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.%同原子一样，“激子”（结合在一起的电子和空穴对）可形成一种象“玻色一爱因斯坦凝聚态”一样的状态，具有低于某一临界温度的超流体性质。在实践中，由于“激子”寿命短，这种状态在一种正常半导体中是难以实现的--无法以足够快的速度将它们冷却。这个问题可以通过利用“间接激子”来绕过，后者的寿命要长得多，因为电子和空穴被束缚在单独的“量子阱”层中，使空位无法太快地重新组合。以前，人们曾证明一种冷“激子”气可在这一体系中形成；现在，High等人在这样一种气体中观察到了“自发相干”现象，这是某种凝聚态的特点。