Unconventional charge-density-wave gap in monolayer NbS$_2$

TL;DR

This study reveals a unique charge density wave in monolayer NbS₂ with a 3×3 superstructure and a 20 meV gap, attributed to electron-phonon interactions rather than conventional band structure effects.

Contribution

It provides the first direct evidence of a CDW in monolayer NbS₂ and links the gap features to collective phonon modes, advancing understanding of 2D CDW mechanisms.

Findings

Presence of a 3×3 CDW superstructure in monolayer NbS₂

Observation of a 20 meV gap at the Fermi level

Gap features linked to electron-phonon quasiparticles

Abstract

Using scanning tunneling microscopy and spectroscopy, for a monolayer of transition metal dichalcogenide H-NbS$_2$ grown by molecular beam epitaxy on graphene, we provide unambiguous evidence for a charge density wave (CDW) with a 3$\times$3 superstructure, which is not present in bulk NbS$_2$. 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.