QUANTUM MONTE-CARLO SIMULATION OF DILUTED HEISENBERG ANTIFERROMAGNETS (S=1/2) ON THE SQUARE LATTICE

The effects of inhomogeneity in a system with strong quantum fluctuations are investigated by a quantum Monte Carlo simulation (QMC). The properties of an antiferromagnetic Heisenberg model with S = 1/2 on the square lattice diluted by non-magnetic impurities are investigated. The spatial distribution of the spin correlations near impurities are found to be largely enhanced. On the other hand, from the size dependence of the square of the staggered magnetization per spin, the correlations between largely separated spins are found to be smaller than in the non-diluted case. It has been found that the existence of non-magnetic sites causes the ergodicity problem in Qmc. In order to avoid this difficulty we introduce a new type of global flip. With this type of flip QMC gives good agreement with results obtained by exact diagonalization methods even for systems with highly concentrated impurities. The methodological points are also discussed in detail.