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

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-VSe2. Systematic scanning tunneling microscopy experiments reveal that bilayer VSe2 largely retains the bulk electronic structure, hosting a tri-directional CDW. However, monolayer VSe2 exhibits a dimensional crossover, hosting two CDWs with distinct wavelengths. 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 arises from enhanced electron-electron interactions in the 2D limit. These findings establish monolayer-VSe2 as the first host of coexisting charge orders with distinct origins, opening the door to tailoring electronic phenomena via emergent interactions in 2D materials.