High-order coupled cluster method calculations for the ground- and excited-state properties of the spin-half XXZ model

In this article, we present new results of high-order coupled cluster metho d (CCM) calculations, based on a Neel model state with spins aligned in the z-direction, for both the ground-and excited-state properties of the spin- half XXZ model on the linear chain, the square lattice, and the simple cubi c lattice, In particular, the high-order CCM formalism is extended to treat the excited states of lattice quantum spin systems for the first time. Com pletely new results for the excitation energy gap of the spin-half XXZ mode l for these lattices are thus determined. These high-order calculations are based on a localized approximation scheme called the LSUBm scheme in which we retain all k-body correlations defined on all possible locales of m adj acent lattice sites (k less than or equal to m). The 'raw' CCM LSUBm result s are seen to provide very good results fur the ground-state energy, sublat tice magnetization, and the value of the lowest-lying excitation energy for each of these systems. However, in order to obtain even better results, tw o types of scheme for extrapolating the LSUBm results to the limit m --> in finity (i.e., the exact solution in the thermodynamic limit) are presented. The extrapolated results provide extremely accurate results for the ground - and excited-stare properties of these systems across a wide range of valu es of the anisotropy parameter.