O ct 1 99 5 Spin Gap of Two - Dimensional Antiferromagnet Representing CaV 4 O 9 Kazuhiro Sano and Ken ’ ichi Takano

We examined a two-dimensional Heisenberg model with two kinds of exchange energies, Je and Jc. This model describes localized spins at vanadium ions in a layer of CaV4O9, for which a spin gap is found by a recent experiment. Comparing the high temperature expansion of the magnetic susceptibility to experimental data, we determined the exchange energies as Je ≃ 610 K and Jc ≃ 150 K. By the numerical diagonalization we estimated the spin gap as ∆ ∼ 0.2Je ≃ 120 K, which consists with the experimental value 107 K. Frustration by finite Jc enhances the spin gap.