Charge disproportionation, mixed valence, and Janus effect in multiorbital systems: A tale of two insulators

Multiorbital Hubbard models host strongly correlated "Hund's metals" even for interactions much stronger than the bandwidth. We characterise this interaction-resilient metal as a mixed-valence state. In particular, it can be pictured as a bridge between two strongly correlated insulators: a high-spin Mott insulator, favoured by a large Coulomb interaction, and a charge-disproportionation insulator, stabilised by a large Hund's coupling. The mixed-valence metallic state emerges in the region where the two competing parent insulators are degenerate. This picture is further strengthened by comparing models with negative and positive Hund's coupling at different fillings. Our results provide a characterisation of the Hund's metal state and connect its presence with charge disproportionation, which has indeed been observed in chromates and proposed to play a role in iron-based superconductors.