Commensurate Supersolid of Three-Dimensional Lattice Bosons

Using quantum Monte Carlo simulations, we show that a perfect {\it commensurate} checkerboard supersolid is stable in the soft-core Bose-Hubbard model with nearest-neighbor repulsions on a cubic lattice. In conventional cases, supersolids are realized by doped-defect-condensation mechanism where doped bosons or holes into a perfect crystal act as interstitials or vacancies, delocalizing in the crystal order. However, in the above model, a supersolid state is stabilized even at the commensurate filling 1/2 {\it in the absence of doping}. By our grand canonical simulations, we obtain a ground-state phase diagram that suggests the existence of supersolid at commensurate density (commensurate supersolid). In order to obtain direct evidence of the commensurate supersolid, we perform simulation at a fixed density $\rho=1/2$ and deny a possibility of phase separations. From snapshots, we confirm that the commensurate supersolid is realized by unconventional mechanism where interstitial-vacancy pairs are created in a perfect crystal and separate from each other, delocalizing in the crystal order.