A full gap above the Fermi level: the charge density wave of monolayer VS2

In the weak-coupling Peierls' view, charge density wave (CDW) transitions are metal-insulator transitions, creating a gap at the Fermi level. However, with strong electron-phonon coupling, theoretically the effects of the periodic lattice distortion could be spread throughout the electronic structure and give rise to CDW gaps away from the Fermi level. Here, using scanning tunneling microscopy and spectroscopy, we present experimental evidence of a full CDW gap residing in the unoccupied states of monolayer VS2. Our ab initio calculations show anharmonic coupling of transverse and longitudinal phonons to be essential for the formation of the CDW and the full gap above the Fermi level. The CDW induces a Lifshitz transition, i.e., a topological metal-metal instead of a Peierls metal-insulator transition. Additionally, x-ray magnetic circular dichroism reveals the absence of net magnetization in this phase, pointing to a coupled CDW-antiferromagnetic ground state.