Charge density waves and Fermi level pinning in monolayer and bilayer SnSe$_2$

TL;DR

This study reveals the emergence of charge density waves and Fermi level pinning in monolayer and bilayer SnSe$_2$, driven by Fermi surface nesting and interface effects, with implications for 2D material electronic properties.

Contribution

It demonstrates the presence of a 2×2 charge density wave in monolayer and bilayer SnSe$_2$ and links it to Fermi surface nesting and interface-induced gap states, offering new insights into 2D CDW mechanisms.

Findings

Charge density wave observed in monolayer and bilayer SnSe$_2$.

Fermi level pinned inside the semiconductor band gap.

Fermi surface nesting drives CDW formation.

Abstract

Materials with reduced dimensionality often exhibit exceptional properties that are different from their bulk counterparts. Here we report the emergence of a commensurate 2 $\times$ 2 charge density wave (CDW) in monolayer and bilayer SnSe$_2$ films by scanning tunneling microscope. The visualized spatial modulation of CDW phase becomes prominent near the Fermi level, which is pinned inside the semiconductor band gap of SnSe$_2$. We show that both CDW and Fermi level pinning are intimately correlated with band bending and virtual induced gap states at the semiconductor heterointerface. Through interface engineering, the electron-density-dependent phase diagram is established in SnSe$_2$. Fermi surface nesting between symmetry inequivalent electron pockets is revealed to drive the CDW formation and to provide an alternative CDW mechanism that might work in other compounds.