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 -- 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-VSe2 as a host of coexisting charge orders with distinct origins, and enable the tailoring of electronic phenomena via emergent interactions in 2D materials.
@article{arxiv.2104.12420,
title = {Coexisting charge-ordered states with distinct driving mechanisms in monolayer VSe$_2$},
author = {Rebekah Chua and Jans Henke and Surabhi Saha and Yuli Huang and Jian Gou and Xiaoyue He and Tanmoy Das and Jasper van Wezel and Anjan Soumyanarayanan and Andrew T. S. Wee},
journal= {arXiv preprint arXiv:2104.12420},
year = {2021}
}