Correlation length of the isotropic quantum Heisenberg antiferromagnet

The quantum Heisenberg antiferromagnet on the square lattice is known to model the magnetic interactions in the copper ion planes of many high-T-c superconductors and their parent compounds. The thermodynamics of the model is approached by the pure-quantum self-consistent harmonic approximation, that reduces the quantum problem to the study of an effective classical antiferromagnetic system. The effective exchange, weakened by quantum fluctuations, enters as a temperature scale the classical-like expressions for the thermal averages, and the quantum spin correlation length is then obtained from its classical counterpart in a simple way. The theory compares very well, for any value of the spin and without need for adjustable parameters, with high temperature expansions, quantum Monte Carlo simulations, and recent neutron and nuclear quadrupole relaxation (NQR) experiments.