  We investigate the phase diagram of hard-core bosons on a square lattice with
competing interactions. The hard-core bosons can be represented also by
spin-1/2 operators and the model can therefore be mapped onto an anisotropic
$J_1$-$J_2$-Heisenberg model. We find the N\'eel state and a collinear
antiferromagnetic state as classical ordered phases to be suppressed for small
ferromagnetic exchange terms $J_{1,2}^{x,y}$ and a ferromagnetic phase which
orders in the x-y-plane for large $J_{1,2}^{x,y}$. For an intermediate regime
the emergence of new quantum states like valence bond crystals or super-solids
is predicted for similar models. We do not observe any signal for long-range
order in terms of conventional order or dimer correlations in our model and
find an exponential decay in the spin correlations. Hence, all evidence is
pointing towards a quantum disordered ground state for a small region in the
phase diagram.
