Phase diagram of the excitonic insulator

TL;DR

This paper develops a method to construct phase diagrams for excitonic insulators in two-band models, applying it to a Coulomb-interacting system, revealing a steeple-like phase diagram contrary to previous expectations.

Contribution

The authors introduce a quantitative scheme for phase diagram construction of excitonic insulators and demonstrate its application to a long-range Coulomb interaction model.

Findings

The phase diagram exhibits a steeple-like shape.

The generalized gap equation controls phase transitions.

Numerical results challenge traditional expectations.

Abstract

Motivated by recent experiments, which give strong evidence for an excitonic insulating phase in $\rm TmSe_{0.45}Te_{0.55}$, we developed a scheme to quantitatively construct, for generic two-band models, the phase diagram of an excitonic insulator. As a first application of our approach, we calculated the phase diagram for an effective mass two-band model with long-range Coulomb interaction. The shielded potential approximation is used to derive a generalized gap equation controlling for positive (negative) energy gaps the transition from a semi-conducting (semi-metallic) phase to an insulating phase. Numerical results,obtained within the quasi-static approximation, show a steeple-like phase diagram in contrast to long-standing expectations.