Plaquette Resonating-Valence-Bond Ground State of CaV4O9

Abstract
A theoretical model is presented to explain the spin gap observed for CaV4O9. The underlying lattice of the 1/5-depleted square lattice favors a formation of the plaquette resonating-valence-bond state. Inclusion of the frustrating second neighbor interaction enhances this tendency, leading to a quantum disordered state of a two-dimensional spin-1/2 Heisenberg model with a sufficiently big spin gap compatible with experiments.