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To understand the mechanism of Mott transitions in case of no magneticinfluence, superfluid-insulator (Mott) transitions in the S=0 Bose Hubbardmodel at unit filling are studied on the square and triangular lattices, usinga variational Monte Carlo method. In trial many-body wave functions, weintroduce various types of attractive correlation factors between adoubly-occupied site (doublon, D) and an empty site (holon, H), which play acentral role for Mott transitions, in addition to the onsite repulsive(Gutzwiller) factor. By optimizing distance-dependent parameters, we studyvarious properties of this type of wave functions. With a hint from the Motttransition arising in a completely D-H bound state, we propose an improvedpicture of Mott transitions, by introducing two characteristic length scales,the D-H binding length $\xi_{\rm dh}$ and the minimum D-D exclusion length$\xi_{\rm dd}$. Generally, a Mott transition occurs when $\xi_{\rm dh}$ becomescomparable to $\xi_{\rm dd}$. In the conductive (superfluid) state, domains ofD-H pairs overlap with each other ($\xi_{\rm dh}>\xi_{\rm dd}$); thereby D andH can propagate independently as density carriers by successively exchangingthe partners. In contrast, intersite repulsive Jastrow (D-D and H-H) factorshave little importance for the Mott transition.

Effect of Doublon-Holon Binding on Mott transition---Variational Monte Carlo Study of Two-Dimensional Bose Hubbard Models