Quantum phase transition of two-dimensional diluted Heisenberg antiferromagnet

Quantum phase transition of site-diluted and bond-diluted Heisenberg antife rromagnets on square lattices is studied. By using the novel continuous-tim e loop algorithm, we perform quantum Monte Carlo simulations on quite large r lattices at extremely lower temperatures than the previous numerical stud ies. It is found that the antiferromagnetic long-range order at T = 0 persi sts so long as a cluster of magnetic sites percolates, that is, the critica l concentration is equal to the classical percolation threshold, in both of the site-diluted and bond-diluted cases. Furthermore, we find that some cr itical exponents, such as the mag netization exponent beta, are non-classic al and strongly depend on the spin size S. On the other hand, we show that the correlation-length exponent nu is universal and is equal to the classic al value (nu = 4/3).