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

TL;DR

This paper explores the nature of Hund's metals in multiorbital Hubbard models, revealing their characterization as mixed-valence states bridging two types of insulators, and connects these findings to experimental observations in chromates and iron-based superconductors.

Contribution

It introduces a new perspective on Hund's metals as mixed-valence states connecting Mott and charge-disproportionated insulators, confirmed through model comparisons.

Findings

Hund's metals are characterized as mixed-valence states.

Charge disproportionation is linked to Hund's metal behavior.

Results relate theoretical models to experimental observations in materials.

Abstract

Multiorbital Hubbard models host strongly correlated 'Hund's metals' even for interactions much stronger than the bandwidth. We characterize 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 and a charge-disproportionated insulator which is stabilized by a very large Hund's coupling. This picture is confirmed comparing models with negative and positive Hund's coupling for different fillings. Our results provide a characterization 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.