Going with the flow

Abstract

Observations of superfluid behaviour — flow without friction — of unusual character in a condensed-matter system pave the way to investigations of superfluidity in systems that are out of thermal equilibrium. It has been predicted that excitons, bound pairs of electrons and holes, should form Bose–Einstein-like condensates at relatively high temperatures. Recently, evidence has been reported of a BEC in a semiconductor microcavity formed by exciton-polaritons, which are a mixture of excitons and photons. Superfluid-like behaviour is expected in such polariton condensates, though it would be of an unusual character as microcavity polaritons are short-lived, so the condensates are not in thermal equilibrium. Amo et al. have developed a pulsed optical excitation technique to set a microcavity polariton condensate in motion and they observe clear manifestations of collective dynamics with superfluid characteristics, such as flow without resistance when crossing a defect. This work points into a new direction to study the dynamics of out-of-equilibrium condensates.