  We study the single-orbital Hubbard model on the 1/5-depleted square lattice
geometry, which arises in such diverse systems as the spin-gap magnetic
insulator CaV4O9 and ordered-vacancy iron selenides, presenting new issues
regarding the origin of both magnetic ordering and superconductivity in these
materials. We find a rich phase diagram that includes a plaquette singlet
phase, a dimer singlet phase, a Neel and a block-spin antiferromagnetic phase,
and stripe phases. Quantum Monte Carlo simulations show that the dominant
pairing correlations at half filling change character from d-wave in the
plaquette phase to extended s-wave upon transition to the Neel phase. These
findings have intriguing connections to iron-based superconductors, and suggest
that some physics of multiorbital systems can be captured by a single-orbital
model at different dopings.
