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

TL;DR

This study reveals a unique charge density wave in monolayer VS2 characterized by a full gap above the Fermi level and a topological transition at the Fermi level, driven by complex phonon interactions.

Contribution

It demonstrates a non-standard CDW in monolayer VS2 with a full gap above the Fermi level and highlights the role of coupled phonons and coexisting charge and spin density waves.

Findings

Full CDW gap in unoccupied states

Topological metal-metal transition at Fermi level

Absence of net magnetization in the CDW phase

Abstract

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 VS$_2$ realizes a CDW which stands out of this standard model. There is a full CDW gap residing in the unoccupied states of monolayer VS$_2$. 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.