Convergent Expansions for Properties of the Heisenberg Model forCaV4O9

We have constructed high-order $T=0\mathrm{}$ expansions for the elementary excitation spectra, and high-temperature expansions for the susceptibility, for the $S=1\mathrm{}/2$ Heisenberg antiferromagnet believed to describe the spin-gap system ${\mathrm{CaV}}_4{\mathrm{O}}_9$. Existing susceptibility data are analyzed using these theoretical results. If nearest- and second-neighbor interactions are in the ratio 2:1, there should be clear indications in both neutron and Raman scattering.