Anderson impurity model at finite Coulomb interactionU: Generalized noncrossing approximation

We present an extension of the noncrossing approximation (NCA), which is widely used to calculate properties of Anderson impurity models in the limit of infinite Coulomb repulsion →U∞, to the case of finite U. A self-consistent conserving pseudoparticle representation is derived by symmetrizing the usual NCA diagrams with respect to empty and doubly occupied local states. This requires an infinite summation of skeleton diagrams in the generating functional thus defining the ``symmetrized finite-U NCA'' (SUNCA). We show that within SUNCA the low-energy scale TK (Kondo temperature) is correctly obtained, in contrast to other simpler approximations discussed in the literature.