Theory of “Emergence of excitonic superfluid at topological-insulator surfaces”*

We study the BCS condensation of excitons on topological-insulator surfaces. With presence of a screened Coulomb interaction between excited electrons and holes, the BCS excitons can be formed with predicted transition temperature up to 40K. The thermal fluctuation is found to disturb the long-range order by creating vortex excitations. It lowers the mean-field estimation of the transition temperature and drives the transition into the Kosterlitz-Thouless type. The BCS condensation of excitons forms ground state with quantum coherence that leads to the experimentally observed long-range transport of photocurrent. We also discuss possible generation of the topological p+ip excitonic insulators at topological-insulator surfaces.