Effects of Long-Range Correlations on Nonmagnetic Mott Transitions in Hubbard Model on Square Lattice
The mechanism of Mott transition in the Hubbard model on a square lattice is studied without the explicit introduction of magnetic and superconducting correlations, using a variational Monte Carlo method. In trial wave functions, we consider various types of binding factors between a doubly occupied site (doublon, D) and an empty site (holon, H), like a long-range type as well as a conventional nearest-neighbor type, and add independent long-range D--D (H--H) factors. It is found that a wide choice of D--H binding factors leads to Mott transitions near the bandwidth. We modify the previous the D--H binding picture of Mott transitions by introducing two characteristic length scales, the D--H binding length \ellDH and the minimum D--D distance \ellDD, which we appropriately estimate. A Mott transition takes place at \ellDH = \ellDD. In the metallic regime (\ellDH > \ellDD), the domains of D--H pairs overlap with one another, thereby doublons and holons can move independently by changing their partners one after another. In contrast, the D--D factors give only a minor contribution to a Mott transition.