Charge Disproportionation, Mixed Valence, and Janus Effect in Multiorbital Systems: A Tale of Two Insulators

Area of Science:

• Condensed matter physics
• Quantum mechanics
• Materials science

Background:

• Multiorbital Hubbard models describe strongly correlated electron systems.
• Hund's coupling plays a crucial role in determining material properties.
• Hund's metals exhibit metallic behavior despite strong electronic interactions.

Purpose of the Study:

• To characterize the nature of the "Hund's metal" state in multiorbital Hubbard models.
• To understand the relationship between Hund's coupling and the emergence of metallic behavior.
• To connect the Hund's metal state with charge disproportionation phenomena.

Main Methods:

• Analysis of multiorbital Hubbard models.
• Comparison of models with varying Hund's coupling (positive and negative).
• Investigation across different electron fillings.

Main Results:

• Hund's metals are identified as a mixed-valence state.
• This state acts as an intermediary between a high-spin Mott insulator and a charge-disproportionated insulator.
• The charge-disproportionated insulating state is stabilized by large Hund's coupling.

Conclusions:

• The Hund's metal state is characterized as a resilient mixed-valence phase.
• A direct link is established between the Hund's metal state and charge disproportionation.
• These findings offer insights into phenomena observed in chromates and iron-based superconductors.