Phase diagram of depleted Heisenberg model for CaV4O9

Using the quantum Monte Carlo loop algorithm we have determined the phase diagram of the 1/5-depleted Heisenberg square lattice model representing CaV4O9 as a function of the ratio of the two different couplings: bonds within a plaquette and dimer bonds between plaquettes. At isotropic coupling long range order survives the depletion, but the system is close to the quantum critical point. Already a small frustration can thus drive the system into the quantum disordered phase and explain the spin gap behavior of CaV4O9.