Extended coupled-cluster treatment of correlations in quantum magnets

The spin-half XXZ model on the linear chain and the square lattice are examined with the extended coupled-cluster method (ECCM) of quantum many-body theory. We are able to describe both the Ising Heisenberg phase and the XY-Heisenberg phase, starting from known wave functions in the Ising limit and at the phase transition point between the XY-Heisenberg and ferromagnetic phases, respectively, and by systematically incorporating correlations on top of them. The ECCM yields good numerical results via a diagrammatic approach, which makes the numerical implementation of higher-order truncation schemes feasible. In particular, the best nonextrapolated coupled-cluster result for the sublattice magnetization is obtained, which indicates the employment of an improved wave function. Furthermore, the ECCM finds the expected qualitatively different behaviors of the linear-chain and square-lattice cases. [S0163-1829(99)02530-8].