Using scanning tunneling microscopy and spectroscopy, for a monolayer of transition metal dichalcogenide H-NbS2 grown by molecular beam epitaxy on graphene, we provide unambiguous evidence for a charge density wave (CDW) with a 3×3 superstructure, which is not present in bulk NbS2. Local spectroscopy displays a pronounced gap of the order of 20 meV at the Fermi level. Within the gap low energy features are present. The gap structure with its low energy features is at variance with the expectation for a gap opening in the electronic band structure due to a CDW. Instead, comparison with \it{ab initio} calculations indicates that the observed gap structure must be attributed to combined electron-phonon quasiparticles. The phonons in question are the elusive amplitude and phase collective modes of the CDW transition. Our findings advance the understanding of CDW mechanisms in two dimensional materials and their spectroscopic signatures.
@article{arxiv.2307.13791,
title = {Unconventional charge-density-wave gap in monolayer NbS$_2$},
author = {Timo Knispel and Jan Berges and Arne Schobert and Erik G. C. P. van Loon and Wouter Jolie and Tim Wehling and Thomas Michely and Jeison Fischer},
journal= {arXiv preprint arXiv:2307.13791},
year = {2024}
}