A coupled-cluster treatment of spin- 1/2 systems with nearest- and next-nearest-neighbour interactions

Abstract

The coupled-cluster method of quantum many-body theory is applied to 1D and 2D spin- 1/2 systems with nearest-neighbour and next-nearest-neighbour isotropic exchange. Several simple approximation schemes based on a Neel model state are investigated and are found to give satisfactory results in the antiferromagnetic regime. Results are given for the ground-state energy, the order parameter and the sublattice magnetization. We obtain evidence of a phase transition.