Exciton condensation driven by bound states of Green's functions zeros

TL;DR

This paper investigates how bound states of Green's function zeros can drive exciton condensation, revealing complex interactions between poles and zeros that lead to exotic insulator phases in correlated topological systems.

Contribution

It uncovers the role of Green's function zeros in exciton formation and phase transitions in quantum spin-Hall and Mott insulators, introducing a novel mechanism for exciton condensation.

Findings

Green's function zeros emerge in the quantum spin-Hall regime.

An exotic quantum spin-Hall insulator with chiral edge states is identified.

Evidence of bound states between zeros' valence and conduction bands is presented.

Abstract

The interaction driven transition between quantum spin-Hall and Mott insulators in the Bernevig, Hughes and Zhang model is studied by dynamical cluster approximation, and found to be accompanied by the emergence of Green's function zeros already in the quantum spin-Hall regime. The non-trivial interplay between Green's function poles and zeros leads to an exotic quantum spin-Hall insulator exhibiting two chiral branches of edge Green's function poles and one of zeros. When symmetry breaking is allowed, a non-topological excitonic insulator is found to intrude between quantum spin-Hall and Mott insulators. We find evidence that excitons in the Mott insulator, which become soft at the transition to the excitonic insulator, are actually bound states between valence and conduction bands of Green's function zeros, rather than between lower and upper Hubbard bands.