Mechanism of superfluid-insulator transition in two-dimensional Bose Hubbard model

To understand the mechanism of Mott transitions in case of no magnetic influence, superfluid-insulator (Mott) transitions are studied for the S = 0 Bose Hubbard model on the square lattice, using a variational Monte Carlo approach. In trial many-body wave functions, we introduce various types of attractive correlation factors between a doubly-occupied site (doublon, D) and an empty site (holon, H), which play a central role for the transition. We propose an improved picture of D-H binding; a Mott transition occurs when the D-H pair length becomes equivalent to the minimum D-D distance, which lengths are appropriately estimated. We confirm this picture is valid for all the wave functions with attractive D-H factors we consider, and point out it can be universal for nonmagnetic Mott transitions.