QUANTUM MONTE-CARLO CALCULATION OF THE J(1)-J(2) MODEL

The Neel and collinear order parameters of the J1-J2 model are calculated by the quantum Monte Carlo simulation with the reweighting method. Systems of sizes N=4 x 4, 4 x 8 and 8 x 8 are treated. The thermodynamic quantities obtained by the size extrapolation suggest that the Neel order remains finite for alpha less-than-or-equal-to 0.4 and vanishes for alpha>0.6, where alpha is defined by J2/J1, and that the collinear order remains for alpha greater-than-or-equal-to 0.7 and vanishes for alpha<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.