Superconducting state of an excitonic insulator

摘要

A new state of an excitonic insulator, namely, the state with the non-dissipative electric current, is considered. It is assumed that at higher temperatures, in the metallic state, the current is induced externally, e.g. by placing the sample in a closed circuit, and electrons and holes are moving in the opposite directions carrying finite current. As temperature is decreased, transition to an ordered state due to formation of electron-hole pairs can occur, similar to the ordinary insulating state of excitonic insulator. We study the properties of such an ordered state at zero temperature and find that the spectrum of its elementary excitations has a gap, so that the state with the current is a superconductor. It is argued that this conclusion is valid in the limit of strong overlap of electron hole pairs.