Double-expansion impurity solver for multiorbital models with dynamically screened U and J

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(\omega) and J(\omega). The method is used to investigate spin state transitions in a toy model for fullerides, with repulsive bare J but attractive screened J.