Unveiling hidden charge density waves in single-layer NbSe$_2$ by impurities

TL;DR

This study uses ab-initio calculations to reveal how transition metal impurities influence charge density waves and symmetry in single-layer NbSe$_2$, uncovering hidden phases and magnetic effects.

Contribution

It demonstrates how transition metal adsorption can reverse the energy hierarchy of CDW structures and induce magnetism in single-layer NbSe$_2$, revealing hidden phases.

Findings

Adsorption of Co and Mn favors hexagonal Nb-Nb clusters.

Transition metals induce magnetism and reduce symmetry.

Energy order of CDW structures is reversed by certain adsorbates.

Abstract

We employ {\it{ab-initio}} calculations to investigate the charge density waves in single-layer NbSe$_2$, and we explore how they are affected by transition metal atoms. Our calculations reproduce the observed orthorhombic phase in single-layer NbSe$_2$ in the clean limit, establishing the energy order between three different distorted structures, two consisting of triangular Nb-Nb clusters and a third, energetically unfavoured, consisting of hexagonal Nb-Nb clusters. Such energy order, in agreement with known experimental work, is reversed by the adsorption of Co and Mn, which favour the formation of hexagonal Nb-Nb clusters; this CDW structure is indeed allowed from symmetry point of view but hidden in pure single layers because it is at a higher energy. The other adsorbates, K and Ga, still favour one of the triangular Nb-Nb cluster, while suppressing the other. We report how the energy difference between such distorted structure varies with these adsorbates. Furthermore, transition metals induce magnetism and favour the reduction of the symmetry of the charge density distribution.