The spin gap of CaV4O9 revisited

The large-plaquette scenario of the spin gap in CaV4O9 is investigated on t he basis of extensive exact diagonalizations. We confirm the existence of a large-plaquette phase in a wide range of parameters, and we show that the most recent neutron scattering data actually require an intra-plaquette sec ond-neighbor exchange integral much larger than the inter-plaquette one, th us justifying the perturbative calculation used in the interpretation of th e neutron scattering experiments. (C) 1999 Elsevier Science B.V. All rights reserved.