  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.
