FFLO Excitonic State in the Three-Chain Hubbard Model for Ta$_2$NiSe$_5$

TL;DR

This study explores the FFLO excitonic state in a three-chain Hubbard model for Ta$_2$NiSe$_5$, revealing a pressure-induced transition from uniform to finite-momentum excitonic phases, with a crossover from BEC to BCS regimes.

Contribution

It introduces a detailed phase diagram showing the emergence of FFLO excitonic states and the BEC-BCS crossover in Ta$_2$NiSe$_5$ under varying energy gaps and pressure.

Findings

Identification of FFLO excitonic state at finite momentum q.

Observation of a phase transition from uniform to FFLO state under pressure.

Demonstration of BEC-BCS crossover in the excitonic phase diagram.

Abstract

The three-chain Hubbard model for Ta$_2$NiSe$_5$ known as a candidate material for the excitonic insulator is investigated over the wide range of energy gap $D$ between the two-fold degenerate conduction bands and the nondegenerate valence band including both semiconducting ($D>0$) and semimetallic ($D<0$) cases. In the semimetallic case, the difference of the band degeneracy inevitably causes the imbalance of each Fermi wavenumber, resulting in a remarkable excitonic state characterized by the condensation of excitons with finite center-of-mass momentum $q$, the so-called Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) excitonic state. With decreasing $D$ corresponding to increasing pressure, the obtained excitonic phase diagram shows a crossover from BEC ($D\simg 0$) to BCS ($D\siml 0$) regime, and then shows a distinct phase transition at a certain critical value $D_c(<0)$ from the uniform ($q=0$) to the FFLO ($q\ne 0$) excitonic state, as expected to be observed in Ta$_2$NiSe$_5$ under high pressure.