Spin Gap in Two-Dimensional Heisenberg Model for CaV_4O_9

Abstract

We investigate the mechanism of spin gap formation in atwo-dimensional model relevant to Mott insulators such asCaV₄O₉. From the perturbation expansion and quantum Monte Carlocalculations, the origin of the spin gap is ascribed to the four-siteplaquette singlet in contrast to the dimer gap established in thegeneralized dimerized Heisenberg model.