  Quantum Monte Carlo simulations are used to investigate the two-dimensional
superfluid properties of the hard-core boson model, which show a strong
dependence on particle density and disorder. We obtain further evidence that a
half-filled clean system becomes superfluid via a finite temperature
Kosterlitz-Thouless transition. The relationship between low temperature
superfluid density and particle density is symmetric and appears parabolic
about the half filling point. Disorder appears to break the superfluid phase up
into two distinct localized states, depending on the particle density. We find
that these results strongly correlate with the results of several experiments
on high-$T_c$ superconductors.
