Decisive role of electron-phonon coupling for phonon and electron instabilities in transition metal dichalcogenides

TL;DR

This paper demonstrates that electron-phonon coupling is the key factor driving charge density wave formation in transition metal dichalcogenides, through an ab-initio approach that accurately captures phonon and electron instabilities.

Contribution

It introduces a systematic ab-initio framework to evaluate the roles of Fermi surface nesting and electron-phonon coupling in CDW formation, highlighting EPC's decisive influence.

Findings

EPC softens phonon frequencies at CDW vectors

Phonon instabilities become imaginary due to EPC

Correct prediction of CDW gap distribution requires EPC inclusion

Abstract

The origin of the charge density wave (CDW) in transition metal dichalcognides has been in hot debate and no conclusive agreement has been reached. Here, we propose an ab-initio framework for an accurate description of both Fermi surface nesting and electron-phonon coupling (EPC) and systematically investigate their roles in the formation of CDW. Using monolayer 1H-NbSe$_2$ and 1T-VTe$_2$ as representative examples, we show that it is the momentum-dependent EPC softens the phonon frequencies, which become imaginary (phonon instabilities) at CDW vectors (indicating CDW formation). Besides, the distribution of the CDW gap opening (electron instabilities) can be correctly predicted only if EPC is included in the mean-field model. These results emphasize the decisive role of EPC in the CDW formation. Our analytical process is general and can be applied to other CDW systems.