Spontaneous Coherence in a Cold Exciton Gas

TL;DR

This paper reports the observation of spontaneous coherence and phase singularities in a cold gas of indirect excitons, indicating the formation of a quantum condensate at temperatures below a few Kelvin.

Contribution

It provides experimental evidence of spontaneous coherence and vortex structures in a cold exciton gas, advancing understanding of exciton condensation in solid-state systems.

Findings

Extended coherence length in exciton gas indicates a condensate.

Observation of phase singularities in the coherent exciton state.

Coherence emerges below a few Kelvin, characteristic of quantum degeneracy.

Abstract

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 due to their long lifetimes. Here, we report on the measurement of spontaneous coherence in a gas of indirect excitons. We found that extended 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. We also observed phase singularities in the coherent exciton gas. Extended spontaneous coherence and phase singularities emerge when the exciton gas is cooled below a few Kelvin.