Strongly enhanced charge-density-wave order in monolayer NbSe$_2$

TL;DR

This study reveals that in monolayer NbSe₂, the charge density wave transition temperature is drastically increased due to enhanced electron-phonon interactions, contrasting with the decrease in superconducting transition temperature as the layer thickness reduces.

Contribution

It provides the first detailed investigation of many-body collective phenomena in monolayer NbSe₂, highlighting the significant enhancement of charge density waves in 2D.

Findings

Charge density wave transition temperature increases from 33 K to 145 K in monolayers.

Superconducting transition temperature decreases with fewer layers.

Enhanced electron-phonon interactions drive the strong coupling regime in 2D NbSe₂.

Abstract

Two-dimensional (2D) atomic materials possess very different properties from their bulk counterparts. While changes in the single-particle electronic properties have been extensively investigated, modifications in the many-body collective phenomena in the exact 2D limit, where interaction effects are strongly enhanced, remain mysterious. Here we report a combined optical and electrical transport study on the many-body collective-order phase diagram of 2D NbSe$_2$. Both the charge density wave (CDW) and the superconducting phase have been observed down to the monolayer limit. While the superconducting transition temperature ($T_C$) decreases with lowering the layer thickness, the newly observed CDW transition temperature ($T_{\mathrm{CDW}}$) increases drastically from 33 K in the bulk to 145 K in the monolayers. Such highly unusual enhancement of CDWs in atomically thin samples can be understood as a result of significantly enhanced electron-phonon interactions in 2D NbSe$_2$, which cause a crossover from the weak coupling to the strong coupling limit. This is supported by the large blueshift of the collective amplitude vibrations observed in our experiment.