Coexisting charge-ordered states with distinct driving mechanisms in monolayer VSe$_2$

TL;DR

This study uncovers a novel coexisting charge density wave state in monolayer VSe₂, driven by different mechanisms, highlighting the complex electronic phases possible in two-dimensional materials.

Contribution

It demonstrates the emergence of two distinct charge density waves in monolayer VSe₂, one bulk-like and one unconventional, driven by different physical mechanisms.

Findings

Monolayer VSe₂ hosts two coexisting CDWs with different origins.

The unconventional CDW may result from enhanced electron-electron interactions.

Bilayer VSe₂ retains bulk-like electronic structure with a single CDW.

Abstract

Thinning crystalline materials to two dimensions (2D) creates a rich playground for electronic phases, including charge, spin, superconducting, and topological order. Bulk materials hosting charge density waves (CDWs), when reduced to ultrathin films, have shown CDW enhancement and tunability. However, charge order confined to only 2D remains elusive. Here we report a distinct charge ordered state emerging in the monolayer limit of $1T$-VSe$_2$. Systematic scanning tunneling microscopy experiments reveal that bilayer VSe$_2$ largely retains the bulk electronic structure, hosting a tri-directional CDW. However, monolayer VSe$_2$ -- consistently across distinct substrates -- exhibits a dimensional crossover, hosting two CDWs with distinct wavelengths and transition temperatures. Electronic structure calculations reveal that while one CDW is bulk-like and arises from the well-known Peierls mechanism, the other is decidedly unconventional. The observed CDW-lattice decoupling and the emergence of a flat band suggest that the new CDW could arise from enhanced electron-electron interactions in the 2D limit. These findings establish monolayer-VSe$_2$ as a host of coexisting charge orders with distinct origins, and enable the tailoring of electronic phenomena via emergent interactions in 2D materials.