EXACT GROUND AND EXCITED-STATES OF FRUSTRATED ANTIFERROMAGNETS ON THECAV4O9 LATTICE

The experimental observation of a spin gap in the antiferromagnetic sp in-1/2 compound CaV4O9 has been attributed to the formation of a plaqu ette-resonating-valence-bond (PRVB) state in the underlying 1/5-deplet ed square lattice. We construct a spin-1/2 model on the 1/5-depleted l attice for which the PRVB state can be shown to be the exact ground st ate in a particular parameter regime. In a subspace of this parameter regime, the first excited state can be calculated exactly. In a differ ent parameter regime, the dimer state is shown to be the exact ground state.