Excitonic Insulator

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Abstract
This paper presents theoretical considerations of a new kind of insulating phase which has recently been theoretically predicted but has as yet not been found experimentally. This phase is expected to occur when semiconductors with very small band gap or semimetals with very small band overlap are cooled to a sufficiently low temperature. The present paper first develops a BCS-like theory of the ground state and analyzes the nature of the response to a general perturbation, from which collective modes (of a sound-like nature), response to a static magnetic field, and conductivity are calculated. Finally, some discussion of the possible experimental realization of this new phase is presented.