Anomalous transport and luminescence of indirect excitons in AlAs/GaAs coupled quantum wells as evidence for exciton condensation

Abstract

Due to the long lifetime of indirect (interwell) excitons, exciton condensation (analogous to the Bose-Einstein condensation of bosons) is expected to occur in coupled quantum wells (CQW's). The critical conditions for the exciton condensation have been predicted to be strongly improved by high magnetic field perpendicular to the well plane. We present results of experimental study of transport and photoluminescence of indirect excitons in AlAs/GaAs CQW's at low temperatures $(T>~350$ mK) and high magnetic fields $(B<~14$ T). Strong anomalies in the transport and luminescence of indirect excitons have been observed at low temperatures and high magnetic fields: a large increase of the exciton diffusivity, a large increase of the exciton radiative decay rate and a huge noise in the integrated exciton PL intensity. An interpretation of the observed anomalies as evidence of the exciton condensation (i.e., in terms of the onset of exciton superfluidity, superradiance of the exciton condensate, and fluctuations near the phase transition) is analyzed. The parameter (temperature, exciton density, and magnetic field) dependences of the observed anomalous transport and photoluminescence of indirect excitons show that these effects are consistent with the exciton condensation in the presence of a random potential.