AN EFFICIENT IMPLEMENTATION OF HIGH-ORDER COUPLED-CLUSTER TECHNIQUES APPLIED TO QUANTUM MAGNETS

We illustrate how the systematic inclusion of multi-spin correlations of the quantum spin-lattice systems can be efficiently implemented wit hin the frame-work of the coupled-cluster method by examining the grou nd-state properties of both the square-lattice and the frustrated tria ngular-lattice quantum antiferromagnets. The ground-state energy and t he sublattice magnetization are calculated for the square-lattice and triangular-lattice Heisenberg antiferromagnets, and our best estimates give values for the sublattice magnetization which are 62% and 51% of the classical results for the square and triangular lattices, respect ively. We furthermore make a conjecture as to why previous series expa nsion calculations have not indicated Neel-like long-range order for t he triangular-lattice Heisenberg antiferromagnet. We investigate the c ritical behavior of the anisotropic systems by obtaining approximate v alues for the positions of phase transition points.