Systematic inclusion of high-order multispin correlations for the spin-½ XXZ models

We apply the microscopic coupled-cluster method (CCM) to the spin-1/2 XXZ models on both the one-dimensional chain and the two-dimensional square lattice. Based on a systematic approximation scheme of the CCM developed by us previously, we carry out high-order ab initio calculations using computer-algebraic techniques. The ground-state properties of the models are obtained with high accuracy as functions of the anisotropy parameter. Furthermore, our CCM analysis enables us to study their quantum critical behavior in a systematic and unbiased manner.