Controlling free superflow, dark matter and luminescence rings of excitons in quantum well structures

TL;DR

This paper presents a theory explaining luminescence rings and dark states in excitons within quantum wells as signatures of superfluid exciton flow, proposing experiments to verify and control this superflow at nanoscale.

Contribution

It introduces a theoretical framework linking luminescence rings to superfluid exciton flow and proposes experimental methods to verify and manipulate this phenomenon.

Findings

Luminescence rings indicate superfluid exciton flow below BKT transition temperature.

Dark states are signatures of coherent exciton superflow.

Proposed experiments can verify and control exciton superflow.

Abstract

Following the discovery of Bose-Einstein condensation (BEC) in ultra cold atoms [E. Gosta, Nobel Lectures in Physics (2001-2005), World Scientific (2008)], there has been a huge experimental and theoretical push to try and illuminate a superfluid state of Wannier-Mott excitons. Excitons in quantum wells, generated by a laser pulse, typically diffuse only a few micrometers from the spot they are created. However, Butov et al. and Snoke et al. reported luminescence from indirect and direct excitons hundreds of micrometers away from the laser excitation spot in double and single quantum well (QW) structures at low temperatures. This luminescence appears as a ring around the laser spot with the dark region between the spot and the ring. Developing the theory of a free superflow of Bose-liquids we show that the macroscopic luminesce rings and the dark state are signatures of the coherent superflow of condensed excitons at temperatures below their Berezinskii-Kosterlitz-Thouless (BKT) transition temperature. To further verify the dark excitonic superflow we propose several keystone experiments, including interference of superflows from two laser spots, vortex formation, scanning of moving dipole moments, and a giant increase of the luminescence distance by applying one-dimensional confinement potential. These experiments combined with our theory will open a new avenue for creating and controlling superflow of coherent excitons on nanoscale.