We present a continuous-time Monte Carlo impurity solver for multiorbital impurity models which combines a strong-coupling hybridization expansion and a weak-coupling expansion in the Hund's coupling parameter $J$. This double-expansion approach allows to treat the dominant density-density interactions $U$ within the efficient segment representation. We test the approach for a two-orbital model with static interactions, and then explain how the double expansion allows to simulate models with frequency dependent $U\left(\omega \right)$ and $J\left(\omega \right)$. The method is used to investigate spin-state transitions in a toy model for fullerides, with repulsive bare $J$ but attractive screened $J$.

• Received 6 June 2015

DOI:https://doi.org/10.1103/PhysRevB.92.115123