The quantum Heisenberg antiferromagnet on the square lattice

The thermodynamic behavior of the quantum Heisenberg antiferromagnet on the square lattice (QHAF) is tackled by the pure-quantum self-consistent harmonic approximation, a semiclassical method based on the path-integral formulation of quantum statistical mechanics. Results for various properties are obtained for different values of the spin and the effects of the dipolar anisotropy are considered. Furthermore, the validity of the quantum nonlinear sigma model approach is discussed and shown to be inadequate to describe the behavior of the QHAF with spin value S greater than or equal to 1 in the experimentally accessible temperature region.