Spin gap of two-dimensional antiferromagnet representing CaV4O9

We examined a two-dimensional Heisenberg model with two exchange energies, J(e) and J(c). This model describes localized spins at vanadium ions in a layer of CaV4O9, for which a spin gap was found by a recent experiment. Comparing the high-temperature expansion of the magnetic susceptibility with experimental data, we determined the exchange energies as J(e) similar or equal to 610K and J(c) similar or equal to 150 K. By numerical diagonalization we estimated the spin gap as Delta similar to 0.2J(e) similar or equal to 120 K, which is consistent with the experimental value of 107 K. Frustration by finite J(c) enhances the spin gap.