An Improved Cluster Approximation for Antiferromagnet

Abstract

Cluster approximation for the antiferromagnets is improved by extending the effective Hamiltonian method so that there appears no anti-Néel point. It is applied to the S=1⁄2 antiferromagnet on a square lattice with anisotropic exchange interaction which includes the anisotropy parameter η. This system is reduced to the Heisenberg model or to the Ising model according as η goes to 1 or 0. The variations of the Néel point T_N and the specific heat with η are calculated. The results of the specific heat curves for η\gtrsim0.9 exhibit a broad maximum above T_N where a small spike appears.