Orbital localization and delocalization effects in the U 5f² configuration: Impurity problem

Anderson models, based upon quantum chemical studies of the molecule uranocene U(C₈H₈)₂, are applied to investigate the problem of an U impurity in a metal. The special point here is that the U 5f orbitals are divided into two subsets: an almost completely localized set and a considerably delocalized one. Due to the crystal field, both localized and delocalized U 5f orbitals affect the low-energy physics. A numerical renormalization-group study shows that every fixed point is characterized by a residual local spin and a phase shift. The latter changes between 0 and π/2, which indicates the competition between two different fixed points. Such a competition between the different local spins at the fixed points reflects itself in the impurity magnetic susceptibility. These different features cannot be obtained if the mixed localized/delocalized character of U 5f orbitals is neglected.