Quantum Monte Carlo Calculation of theJ1-J2Model

The Néel and collinear order parameters of the J 1 - J 2 model are calculated by the quantum Monte Carlo simulation with the reweighting method. Systems of sizes N =4×4, 4×8 and 8×8 are treated. The thermodynamic quantities obtained by the size extrapolation suggest that the Néel order remains finite for α≤0.4 and vanishes for α>0.6, where α is defined by J 2 / J 1 , and that the collinear order remains for α≥0.7 and vanishes for α<0.6. The present result is consistent with the result of the modified spin-wave calculation, conflicting with that of the naive spin-wave theory.