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 hi gh-T-c superconductors and their parent compounds. The thermodynamics of the model is approached by the pure-quantum self-consistent harmoni c approximation, that reduces the quantum problem to the study of an e ffective classical antiferromagnetic system. The effective exchange, w eakened by quantum fluctuations, enters as a temperature scale the cla ssical-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 spi n and without need for adjustable parameters, with high temperature ex pansions, quantum Monte Carlo simulations, and recent neutron and nucl ear quadrupole relaxation (NQR) experiments. (C) 1997 American Institu te of Physics.