TEMPERATURE AND SPIN-DEPENDENT CORRELATION LENGTH OF THE QUANTUM HEISENBERG-ANTIFERROMAGNET ON THE SQUARE LATTICE

The quantum Heisenberg antiferromagnet (HAF) is approached by the pure -quantum self-consistent harmonic approximation that reduces it to an effective classical HAF model. The effective exchange enters the class ical-like expression for thermal averages as a temperature scale, so t hat one can obtain in a simple way the quantum spin correlation length from its classical counterpart. For any spin value S the results comp are very well with those from experiments, quantum Monte Carlo simulat ions, and high-T expansion. The adequacy of our theory supports argume nts previously raised against the quantitative validity of the mapping of the quantum HAF onto the quantum nonlinear sigma model.