Quantum Monte Carlo study of site-diluted Heisenberg antiferromagnet on a square lattice

Ground-state phase transition of site-diluted Heisenberg antiferromagnets o n a square lattice is studied. By using the continuous-time loop algorithm, we perform large-scale quantum Monte Carlo simulation on large systems at quite low temperatures. It is found that the critical concentration of magn etic sites is independent of the spin size S, and equal to the classical pe rcolation threshold. However, the existence of quantum fluctuations makes t he critical exponents deviate from those of the classical percolation trans ition. It is found that the transition is not universal, i.e., the critical exponents depend on the spin size S.