Meta-Plaquette Expansion for the Triplet Excitation Spectrum in   CaV$_4$O$_9$
  We study antiferromagnetic, $S=1/2$ Heisenberg models with nearest and second neighbor interactions on the one-fifth depleted square lattice which describes the spin degrees of freedom in the spin-gap system CaV$_4$O$_9$. The meta-plaquette expansion for the triplet excitation spectrum is extended to fifth order, and the results are compared with experimental data on CaV$_4$O$_9$. We attempt to locate the phase boundary between magnetically ordered and gapped phases. 
Zheng Weihong, J. Oitmaa, and C. J. Hamer (Univ. of New South Wales,   Australia)