Autolocalization of a dipolar exciton gas

We present a theory of autolocalization as a mechanism of formation of the macroscopically ordered exciton state (MOES) in semiconductor quantum wells. We show that the onset of a crystalline order having a macroscopic spatial period is possible in the systems where, in addition to long-range dipolar repulsion, the excitons exhibit resonant pairing at short distances. Our theory suggests that the central part of each condensate in the MOES may represent the long-sought bosonic analog of the Bardeen-Cooper-Schrieffer (BCS) superconductor.

Figure 1

Two-body interaction potential as a function of relative distance between the excitons for two types of scattering channels depicted schematically in the inset. The type II interaction is featureless, characterized by strong repulsion at short distances. The type I scattering channel, in which two electrons and two holes in the excitons simultaneously have antiparallel spins, can admit a quasibound state (resonance) with positive energy and finite lifetime.

Figure 2

Mean-field phase diagram of a resonant exciton condensate. VAC is the state vacuum of particles. Light gray area (X-XX) corresponds to the mixture of excitons and their biexcitonic molecules. The dark gray color is used for the pure molecular phase (XX). Solid lines depict continuous phase transition boundaries, whereas the dashed line marks the boundary of the first order [21]. In the inset we show the LDA energy profile of a harmonically trapped condensate, where one can observe the X-XX/XX transition in real space.