Diagrammatic approach in the variational coupled-cluster method

We present an alternative approach based on a diagrammatic technique to the recently proposed variational coupled-cluster method. We apply this method to quantum antiferromagnetic bipartite lattices. In our analysis, infinite resummations of diagrams can be done in a straightforward manner. One such resummation reproduces spin-wave theory (SWT). Approximations beyond SWT can also be easily made. Interestingly, one such approximation produces a convergent, precise number for the order parameter of the one-dimensional isotropic model, in contrast to the well-known divergence of SWT; it also produces improved results for the two-dimensional square-lattice system. We also discuss relations and a possible combination of our approach with other established many-body theories. © 2005 The American Physical Society.