The numerical renormalization group and multi-orbital impurity models

The numerical renormalization group (NRG) is a highly versatile and accurate method for the simulation of (effective) fermionic impurity models. Despite that the cost of NRG is exponential in the number of orbitals, by now, symmetric three-band calculations have become available on a routine level. [1] Here we present a recent detailed study on the spin-orbital separation in a three-band Hund metal with relevance for iron-pnictides via the dynamical mean field theory (DMFT). [2] In cases, finally, where the orbital symmetry is broken, we demonstrate that interleaved NRG [3] still offers an accurate alternative approach within the NRG with dramatically improved numerical efficiency at comparable accuracy relative to conventional NRG. \\[2ex] [1] Weichselbaum, Annals of Physics {\bf 327}, 2972 (2012) \\ [2] Stadler et al, PRL {\bf 115}, 136401 (2015) \\ [3] Mitchell et al, PRB {\bf 89}, 121105(R) (2014)