Fate of the excitonic insulator in the presence of phonons

TL;DR

This paper investigates how phonons influence the stability of the excitonic insulator state, revealing that lattice distortions caused by electron-phonon coupling can suppress superfluid-like excitonic condensation.

Contribution

It provides a theoretical analysis showing that phonons can induce lattice distortions that destroy the acoustic phase mode in excitonic insulators, challenging the superfluid condensate picture.

Findings

Lattice distortion can occur at the excitonic instability due to electron-phonon coupling.

Such distortions eliminate the acoustic phase mode in the excitonic insulator.

The presence of lattice degrees of freedom prevents off-diagonal long-range order.

Abstract

The influence of phonons on the formation of the excitonic insulator has hardly been analyzed so far. Recent experiments on $\rm Ta_2NiSe_5$, 1$T$-$\rm TiSe_2$, and $\rm TmSe_{0.45}Te_{0.55}$, being candidates for realizing the excitonic-insulator state, suggest, however, that the underlying lattice plays a significant role. Employing the Kadanoff-Baym approach we address this issue theoretically. We show that owing to the electron-phonon coupling a static lattice distortion may arise at the excitonic instability. Most importantly such a distortion will destroy the acoustic phase mode being present if the electron-hole pairing and condensation is exclusively driven by the Coulomb interaction. The absence of off-diagonal long-range order, when lattice degrees of freedom are involved, challenges that excitons in these materials form a superfluid condensate of Bose particles or Cooper pairs composed of electrons and holes.