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

We investigate strong electron-electron correlation effects on two-dimensional van der Waals materials Nb₃X₈ (X =Cl ,Br ,andI ). We find that the monolayers Nb₃X₈ 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 ≈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_x²−y²+i d_{x y} superconducting pairing instability is found. We also construct a tunable bilayer Hubbard system for two sliding Nb₃X₈ layers. The bilayer system displays a crossover between the band insulator and Mott insulator.