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.

► Improved mechanism of nonmagnetic Mott transitions for spinless Bose Hubbard model is proposed.
► Essential concept of doublon-holon binding is discussed quantitatively by introducing two characteristic length scales.
► Mott transition point is determined when these two length scales are equivalent.