Successive phase transitions at finite temperatures toward the supersolid state in a three-dimensional extended Bose-Hubbard model

We study the finite temperature properties of the extended Bose-Hubbard model on a cubic lattice. This model exhibits the so-called supersolid state. To start with, we investigate the ground-state phase diagram and find the supersolid state in the region of underdoped (less than the half-filled) density in contrast to the cases of one- and two-dimensional systems where the superfluid state and the solid state show the phase separation. Next, we investigate ordering processes at finite temperatures by quantum Monte Carlo simulations and find successive superfluid and solid phase transitions. There, we find that the two order parameters compete with each other. We establish a finite temperature phase diagram, which contains the superfluid, the solid, the supersolid, and the disordered phases. We develop a mean-field theory to analyze the ordering processes and compare the result with that obtained by simulations and discuss the mechanism of the competition of these two orders. We also study how the supersolid region shrinks as the on-site repulsion becomes strong.