In the standard model of charge density wave (CDW) transitions, the displacement along a single phonon mode lowers the total electronic energy by creating a gap at the Fermi level, making the CDW a metal--insulator transition. Here, using scanning tunneling microscopy and spectroscopy and ab initio calculations, we show that VS2 realizes a CDW which stands out of this standard model. There is a full CDW gap residing in the unoccupied states of monolayer VS2. At the Fermi level, the CDW induces a topological metal-metal (Lifshitz) transition. Non-linear coupling of transverse and longitudinal phonons is essential for the formation of the CDW and the full gap above the Fermi level. Additionally, x-ray magnetic circular dichroism reveals the absence of net magnetization in this phase, pointing to coexisting charge and spin density waves in the ground state.
@article{arxiv.2101.01140,
title = {A full gap above the Fermi level: the charge density wave of monolayer VS2},
author = {Camiel van Efferen and Jan Berges and Joshua Hall and Erik van Loon and Stefan Kraus and Arne Schobert and Tobias Wekking and Felix Huttmann and Eline Plaar and Nico Rothenbach and Katharina Ollefs and Lucas Machado Arruda and Nick Brookes and Gunnar Schoenhoff and Kurt Kummer and Heiko Wende and Tim Wehling and Thomas Michely},
journal= {arXiv preprint arXiv:2101.01140},
year = {2022}
}