Mottness in two-dimensional van der Waals Nb3X8 monolayers (X=Cl,Br,andI)

Abstract

We investigate strong electron-electron correlation effects on two-dimensional van der Waals materials ${\mathrm{Nb}}_{3}{X}_{8} (X=\mathrm{Cl},\mathrm{Br},\phantom{\rule{0.28em}{0ex}}\text{and}\phantom{\rule{0.28em}{0ex}}\mathrm{I})$. We find that the monolayers ${\mathrm{Nb}}_{3}{X}_{8}$ are ideal systems close to the strong correlation limit. They can be described by a half-filled single band Hubbard model in which the ratio between the Hubbard, $U$, and the bandwidth, $W, U/W\ensuremath{\approx}5--10$. Both Mott and magnetic transitions of the material are calculated by the slave boson mean-field theory. Doping the Mott state, a ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}+i{d}_{xy}$ superconducting pairing instability is found. We also construct a tunable bilayer Hubbard system for two sliding ${\mathrm{Nb}}_{3}{X}_{8}$ layers. The bilayer system displays a crossover between the band insulator and Mott insulator.