A non-crossing approximation for the study of intersite correlations

We develop a Non-Crossing Approximation (NCA) for the effective cluster problem of the recently developed Dynamical Cluster Approximation (DCA). The DCA technique includes short-ranged correlations by mapping the lattice problem onto a self-consistently embedded periodic cluster of size Nc. It is a fully causal and systematic approximation to the full lattice problem, with corrections ${\cal{O}}(1/N_{\rm c})$ in two dimensions. The NCA we develop is a systematic approximation with corrections ${\cal{O}}(1/N_{\rm c}^3)$. 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 including a pseudogap and non-Fermi-liquid behavior due to short-ranged antiferromagnetic correlations