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 classical-like expression for thermal averages as a temperature scale, so that one can obtain in a simple way the quantum spin correlation length from its classical counterpart. For any spin value S the results compare very well with those from experiments, quantum Monte Carlo simulations, and high-T expansion. The adequacy of our theory supports arguments previously raised against the quantitative validity of the mapping of the quantum HAF onto the quantum nonlinear sigma model.