A non-crossing approximation for the study of intersite correlations

We develop a Non-Crossing: Approximation (NCA) for the effective cluster pr oblem of the recently developed Dynamical Cluster Approximation (DCA). The DCA technique includes short-ranged correlations by mapping the lattice pro blem onto a self-consistently embedded periodic cluster of size N-c. It is a fully causal and systematic approximation to the full lattice problem, wi th corrections O(1/N-c) in two dimensions. The NCA we develop is a systemat ic approximation with corrections O(1/N-c(3)). The method will be discussed in detail and results for the one-particle properties of the Hubbard model are shown. Near half filling, the spectra display pronounced features incl uding a pseudogap and non-Fermi-liquid behavior due to short-ranged antifer romagnetic correlations.