Signature in sound-mode of the exciton bilayer two-dimensional superfluid transition

Abstract

Obtaining definitive evidence of exciton superfluidity in electron-hole bilayers in zero magnetic field remains a major longstanding challenge since the condensate is electrically neutral, making its phase coherence difficult to detect directly. We show that the Anderson-Bogoliubov sound velocity provides a dynamical signature of exciton superfluidity. Across the BCS-BEC crossover, the velocity is known to discontinuously drop to zero at the Berezinskii-Kosterlitz-Thouless (BKT) transition. The magnitude of the drop has a strong density dependence. We compute this behavior, with the inclusion of finite-temperature screening, and determine the BKT transition using a renormalization-group approach. We further identify a temperature window which is experimentally accessible, where vortex-antivortex excitations strongly renormalize both the sound velocity and the transition temperature.