Susceptibility and triplet excitation spectrum for CaV4O9:: Third-order perturbation expansions

The uniform susceptibility and tripler excitation spectrum of the Heisenberg antiferromagnet on the one-fifth depleted square lattice are calculated using the third-order perturbation expansion method from the viewpoint of the weakly coupled meta-plaquette picture. Both the susceptibility and tripler excitation spectrum data for CaV4O9 are simultaneously explained. The origin of the relatively large spin gap in this compound is discussed.