Phase transition of an exciton system in GaAs coupled quantum wells

We have observed a sharp reduction of the photoluminescence linewidth from a two-dimensional exciton system (coupled GaAs/AlGaAs quantum wells), at a certain critical temperature (${\mathit{T}}_{\mathit{c}}$) and under the influence of an electric field. We attribute this narrowing to a phase transition of the exciton system into an ordered state. The experiments suggest that a coherence length long enough to wash out the effect of the fluctuations, which cause the line broadening, is introduced below ${\mathit{T}}_{\mathit{c}}$.