  We have used the Stochastic Series Expansion quantum Monte Carlo method to
study interacting hard-core bosons on the square lattice, with pair-hopping
processes supplementing the standard single-particle hopping. Such pair hopping
arises in effective models for frustrated quantum magnets. Our goal is to
investigate the effects of the pair hopping process on the commonly observed
superfluid, insulating (Mott), and super-solid ground-state phases in the
standard hard-core boson model with various interaction terms. The model is
specifically motivated by the observation of finite dispersion of 2-magnon
bound states in neutron diffraction experiments SrCu$_2$(BO$_3$)$_2$. Our
results show that the pair hopping has different effects on Mott phases at
different filling fractions, "melting" them at different critical pair-hopping
amplitudes. Thus, it appears that pair hopping may have an important role in
determining which out of a potentially large number of Mott phases (stabilized
by details of the charge-diagonal interaction terms) actually survive the
totality of quantum fluctuations present.
