Charge-density-wave phase, mottness and ferromagnetism in monolayer $1T$-NbSe$_2$

TL;DR

This study investigates the electronic and magnetic properties of monolayer 1T-NbSe2, revealing a charge-density-wave phase with Mott insulating behavior and weak ferromagnetism, using first-principles calculations and many-body theories.

Contribution

It provides the first comprehensive first-principles analysis of the charge-density-wave, Mott insulating, and ferromagnetic phases in monolayer 1T-NbSe2, highlighting its unique magnetic interactions.

Findings

Identified an incommensurate charge-density-wave instability.

Confirmed the star-of-David phase as the most stable CDW.

Found weak ferromagnetic coupling in the Mott phase.

Abstract

The recently investigated $1T$-polymorph of monolayer NbSe$_2$ revealed an insulating behaviour suggesting a star-of-David phase with $\sqrt{13}\,\times\sqrt{13}$ periodicity associated with a Mott insulator, reminiscent of $1T$-TaS$_2$. In this work, we examine this novel two-dimensional material from first principles. We find an instability towards the formation of an incommensurate charge-density-wave (CDW) and establish the star-of-David phase as the most stable commensurate CDW. The mottness in the star-of-David phase is confirmed and studied at various levels of theory: the spin-polarized generalized gradient approximation (GGA) and its extension involving the on-site Coulomb repulsion (GGA+$U$), as well as the dynamical mean-field theory (DMFT). Finally, we estimate Heisenberg exchange couplings in this material and find a weak nearest-neighbour ferromagnetic coupling, at odds with most Mott insulators. We point out the close resemblance between this star-of-David phase and flat-band ferromagnetism models.