GREEN-FUNCTION THEORY OF THE 2-DIMENSIONAL HEISENBERG-MODEL - SPIN-WAVE IN SHORT-RANGE ORDER

The two dimensional Heisenberg model is studied with a Green's function decoupling scheme in the ferromagnetic and the antiferromagnetic cases. We employ Kondo and Yamaji's formulation and improve it quantitatively. The present theory does not violate the rotational symmetry and the sum rule of the correlation function. We calculate excitation spectrum, long range order, correlation functions, correlation length, energy, and spin susceptibility. The results are compared with those of the Monte Carlo simulations and the experiments of La2CuO4. Qualitative and semi-quantitative agreements are obtained. In particular, the temperature dependence of the susceptibility indicated by Monte Carlo simulations is reproduced over all temperature region.