Effect of dimensionality on the charge-density-wave in few-layers 2H-NbSe$_2$

TL;DR

This study explores how the dimensionality of 2H-NbSe$_2$ influences its charge density wave properties, revealing different CDW wave vectors and electronic states in bulk versus monolayer forms, supported by density functional theory.

Contribution

It provides a theoretical prediction of CDW instabilities in monolayer and bilayer NbSe$_2$, highlighting the impact of reduced dimensionality on CDW wave vectors and electronic states.

Findings

Bulk 2H-NbSe$_2$ has a CDW wave vector at 2/3 $oldsymbol{ extGamma M}$.

Monolayer NbSe$_2$ exhibits a CDW at 1/2 $oldsymbol{ extGamma M}$.

Monolayer CDW state is semimetallic, aligning with experimental observations.

Abstract

We investigate the charge density wave (CDW) instability in single and double layers, as well as in the bulk 2H-NbSe$_{2}$. We demonstrate that the density functional theory correctly describes the metallic CDW state in the bulk 2H-NbSe$_{2}$. We predict that both mono- and bilayer NbSe$_{2}$ undergo a CDW instability. However, while in the bulk the instability occurs at a momentum $\mathbf{q}_{CDW}\approx{2/3}\mathbf{\Gamma M}$, in free-standing layers it occurs at $\mathbf{q}_{CDW}\approx{1/2}\mathbf{\Gamma M}$. Furthermore, while in the bulk the CDW leads to a metallic state, in a monolayer the ground state becomes semimetallic, in agreement with recent experimental data. We elucidate the key role that an enhancement of the electron-phonon matrix element at $\mathbf{q}\approx\mathbf{q}_{CDW}$ plays in forming the CDW ground state.