The recently investigated 1T-polymorph of monolayer NbSe2 revealed an insulating behaviour suggesting a star-of-David phase with 13×13 periodicity associated with a Mott insulator, reminiscent of 1T-TaS2. 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.
@article{arxiv.1803.10727,
title = {Charge-density-wave phase, mottness and ferromagnetism in monolayer $1T$-NbSe$_2$},
author = {Diego Pasquier and Oleg V. Yazyev},
journal= {arXiv preprint arXiv:1803.10727},
year = {2018}
}