  We study the ground-state phase diagrams of hardcore bosons with long-range
interactions on a square lattice using the linear spin-wave theory and a
cluster mean-field method. Specifically, we consider the two types of
long-range interaction: One consists only of the nearest- and
next-nearest-neighbor interactions, and the other is the dipole-dipole
interaction that decays with the interparticle distance $r$ as $\sim r^{-3}$.
It is known from previous analyses by quantum Monte Carlo methods that a
checkerboard supersolid (CSS) is absent in the ground-state phase diagram of
the former case while it is present in the latter. In the former, we find that
quantum fluctuations around mean-field solutions are enhanced by the direct
competition between the checkerboard and striped solid orders and that they
destabilize the CSS phase. On the other hand, the emergence of the CSS phase in
the latter case can be attributed to the absence of such a competition with
other solid orders. We also show that the cluster mean-field method allows for
the determination of phase boundaries in a precise quantitative manner when
scaling with respect to the cluster size is taken into account. It is found
that the phase transition between the superfluid and the solid (or CSS) is of
the first order in the vicinity of the particle-hole symmetric line.
