Anisotropy Effects in the Excitonic Insulator

Abstract

A new insulating phase in semiconductors and semimetals has been discussed recently in the literature. This paper extends the analysis of this excitonic phase to the more general case of an anisotropic band structure of the underlying two-band model. The thermodynamic properties of the excitonic phase are worked out. Whereas the anisotropy does not lead to qualitativly new features in the case of a positive band gap (semiconductor region), important changes occur in the case of a negative band gap (semimetallic region). The latter situation is discussed in detail. Finally, the phase diagram of the excitonic phase is calculated for a particular anisotropic band model.