The Role of Spin-Flip Collisions in a Dark Exciton Condensate

TL;DR

This study demonstrates the formation of a dark exciton Bose-Einstein condensate in GaAs quantum wells, highlighting the role of spin-flip collisions and magnetic fields in its dynamics and density regulation.

Contribution

It reveals the impact of spin-flip collisions on dark exciton condensate density and introduces a rate equations model to explain the observed phenomena.

Findings

Condensate extends over hundreds of micrometers.

Spin-flip collisions limit dark exciton density.

Magnetic fields modify bright exciton density and system dynamics.

Abstract

We show that a Bose-Einstein condensate consisting of dark excitons forms in GaAs coupled quantum wells at low temperatures. We find that the condensate extends over hundreds of micrometers, well beyond the optical excitation region, and is limited only by the boundaries of the mesa. We show that the condensate density is determined by spin flipping collisions among the excitons, which convert dark excitons into bright ones. The suppression of this process at low temperature yields a density buildup, manifested as a temperature-dependent blueshift of the exciton emission line. Measurements under in-plane magnetic field allows us to preferentially modify the bright excitons density, and determine their role in the system dynamics. We find that their interaction with the condensate leads to its depletion. We present a simple rate equations model, which well-reproduces the observed temperature, power, and magnetic field dependence of the exciton density.