THE COUPLED-CLUSTER METHOD APPLIED TO THE XXZ MODEL USING A PLANAR MODEL STATE

The coupled-cluster method (CCM) is applied to the spin-1/2 XXZ model for the 1D chain and the 2D square lattice in the region -1 < Delta < 1. A CCM model state is chosen in which all spins lie in the xy plane with nearest-neighbour spins anti-parallel. Very good agreement with t he exactly known ground-state energies in ID is obtained, and good agr eement with Monte Carlo results is obtained for the 2D case. The subla ttice magnetization is obtained, and evidence of phase transition poin ts in both 1D and 2D is presented. The critical behaviour of the ID gr ound-state energy near the ferromagnetic phase transition point is inv estigated.