Topological quantum phase transition between Fermi liquid phases in an Anderson impurity model

We study a generalized Anderson model that mixes two localized configurations --one formed by two degenerate doublets and the other by a triplet with single-ion anisotropy $DS_z^2$-- by means of two degenerate conduction channels. The model has been derived for a single Ni impurity embedded into an O-doped Au chain. Using the numerical renormalization group, we find a topological quantum phase transition, at a finite value $D_c,$ between two regular Fermi liquid phases of high (low) conductance and topological number $2 I_L/\pi = 0$ (-1) for $D < D_c$ ($D > D_c$), where $I_L$ is the well-known Luttinger integral. At finite temperature the two phases are separated by a non-Fermi liquid phase with fractional impurity entropy $\frac{1}{2}{\rm ln}2$ and other properties which remind those of the two-channel Kondo model.