Strong Coupling Nature of the Excitonic Insulator State in Ta$_2$NiSe$_5$

TL;DR

This paper demonstrates that Ta$_2$NiSe$_5$ exhibits an excitonic insulator state driven by strong electron-hole interactions, characterized by Bose-Einstein condensation of preformed excitons, challenging conventional band-structure descriptions.

Contribution

It provides a combined theoretical and computational analysis showing the excitonic insulator nature of Ta$_2$NiSe$_5$, highlighting the role of strong coupling and preformed excitons.

Findings

Excitonic insulator state confirmed in Ta$_2$NiSe$_5$

Preformed excitons exist above the transition temperature

Strong electron-hole attraction causes a novel insulating phase

Abstract

We analyze the measured optical conductivity spectra using the density-functional-theory-based electronic structure calculation and density-matrix renormalization group calculation of an effective model. We show that, in contrast to a conventional description, the Bose-Einstein condensation of preformed excitons occurs in Ta$_2$NiSe$_5$, despite the fact that a noninteracting band structure is a band-overlap semimetal rather than a small band-gap semiconductor. The system above the transition temperature is therefore not a semimetal, but rather a state of preformed excitons with a finite band gap. A novel insulator state caused by the strong electron-hole attraction is thus established in a real material.