Doublon–holon binding effects on Mott transitions in two-dimensional Bose Hubbard model Original Research Article

A mechanism of Mott transitions in a Bose Hubbard model on a square lattice is studied, using a variational Monte Carlo method. Besides an onsite correlation factor, we introduce a four-body doublon–holon factor into the trial state, which considerably improves the variational energy and can appropriately describe a superfluid–insulator transition. Its essence consists in binding (and unbinding) of a doublon to a holon in a finite short range, identical with the cases of fermions. The features of this transition are qualitatively different from those of Brinkman–Rice-type transitions.