Development of $ab ~initio$ method for exciton condensation and its application to $\bf TiSe_2$

TL;DR

This paper introduces an ab initio computational method combining density functional theory and many-body perturbation theory to quantitatively analyze exciton condensation, successfully applied to TiSe2 to explain its charge-density-wave state.

Contribution

The work develops a novel ab initio approach for predicting exciton condensation critical behavior, enabling quantitative analysis of excitonic insulators from first principles.

Findings

Identified exciton condensation as the origin of charge-density-wave in TiSe2.

Demonstrated lattice distortion and excitonic gap formation without phonon softening.

Provided a general methodology for searching excitonic insulators in materials.

Abstract

Exciton condensation indicating the spontaneous formation of electron-hole pair can cause the phase transition from a semimetal to an excitonic insulator by gap opening at the Fermi surface. While the idea of this excitonic insulator has been proposed for decades, current theoretical approaches can only provide qualitative descriptions, and a quantitative predicting tool is still missing. To shed insight on this problem, we developed an $ab~initio$ method based on the finite-temperature density functional theory and many-body perturbation theory to compute the exciton condensation critical behavior. Applying our approach to the monolayer $\rm TiSe_2$, we find a lattice distortion accompanied by the formation of the excitonic gap via electron-phonon coupling without phonon softening, proving that the exciton condensation is the origin of the charge-density-wave state observed in this compound. Overall, the methodology introduced in this work is general and paves the way to searching for candidate excitonic insulators in natural material systems.