Quantum Monte Carlo investigation of the 2D Heisenberg model with S=1/2

The two-dimensional (2D) Heisenberg model with anisotropic exchange (Delta= 1-J(x)/J(z)) and S=1/2 is investigated by the quantum Monte Carlo method. T he energy, susceptibility, specific heat, spin-spin correlation functions, and correlation radius are calculated. The sublattice magnetization (sigma) and the Neel temperature of the anisotropic antiferromagnet are logarithmi c functions of the exchange anisotropy: 1/sigma + 1 + 0.13(1) ln(1/Delta). Crossover of the static magnetic structural factor as a function of tempera ture from power-law to exponential occurs for T-c/J approximate to 0.4. The correlation radius can be approximated by 1/xi =2.05T(1.0(6))/exp(1.0(4)/ T). For La2CuO4 the sublattice magnetization is calculated as sigma=0.45, t he exchange is J=(1125=1305) K; for Er(2)CuO(4)J similar to 625 K and the e xchange anisotropy Delta similar to 0.003. The temperature dependence of th e static structural magnetic factor and the correlation radius above the Ne el temperature in these compounds can be explained by the formation of topo logical excitations (spinons). (C) 1999 American Institute of Physics. [S10 63-7834(99)02401-6].