AN AB-INITIO COUPLED-CLUSTER THEORY OF QUANTUM SPIN LATTICES AND THEIR QUANTUM CRITICAL-BEHAVIOR

Strongly interacting quantum spin-lattice models exhibit a wide variet y of phases with diverse and subtle magnetic ordering properties. Thei r detailed description within a unified microscopic framework poses a real challenge for the many-body theorist. By specific application to the spin-1/2 anisotropic Heisenberg model on a square lattice, we show how the ab initio coupled cluster method, which has already been very successfully applied to a wide variety of quantum many-body and held- theoretic systems, may be very efficiently and systematically implemen ted for spin-lattice models. Results for such local properties as the ground-state energy and sublattice magnetization are thereby obtained which are on a par with those from the best of the available alternati ve methods. Furthermore, we demonstrate explicitly how the coupled clu ster method now also provides an effective and fully microscopic tool to yield systematic and accurate estimates of the zero-temperature qua ntum phase transition boundaries between states of different quantum o rder, as well as of the critical behaviour of the system in the vicini ty of the transition points.