  We consider a two-species hard-core boson Hubbard model for a supersolid,
where the two types of bosons represent vacancies and interstitials doped into
a commensurate crystal. The on-site inter-species interaction may create bound
states of vacancies and interstitials facilitating vacancy condensation at
lower energies than in a single-species model, as suggested in an earlier mean
field study. Here we carry out quantum Monte Carlo simulation to study possible
supersolid phases of the model, corresponding to superfluid phases of the
vacancies or interstitials. At low temperatures, we find three distinct
superfluid phases. The extent of the phases and the nature of the phase
transitions are discussed in comparison to mean-field theory.
