Hund Physics Landscape of Two-Orbital Systems

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Chemistry

Background:

• Superconductivity in infinite-layer nickelates (RE_{1-δ}Sr_{δ}NiO_{2}) has recently been discovered.
• The role of Hund coupling (J) in these and related systems is not fully understood, particularly in two-orbital models.

Purpose of the Study:

• To investigate the influence of Hund coupling (J) in a quarter-filled two-orbital Hubbard model.
• To identify and characterize different correlated metallic regimes arising from varying Hund coupling strengths.
• To establish criteria for Hund metallicity in two-orbital systems.

Main Methods:

• Theoretical study of a quarter-filled two-orbital Hubbard model.
• Analysis of the model's phase diagram, focusing on regions of negative effective Coulomb interaction.
• Identification of distinct correlated metallic states based on Hund coupling (J) effects.

Main Results:

• A unique region of negative effective Coulomb interaction was identified, differing from three- and five-orbital models.
• Four distinct correlated metallic regimes were discovered: one near a Mott insulator and three others ('intermediate' metal, weak Hund metal, valence-skipping metal) away from Mottness.
• Criteria for characterizing these novel metallic states were proposed.

Conclusions:

• The study establishes the existence of Hund metallicity in two-orbital systems.
• The findings provide a theoretical framework for understanding the electronic properties of novel nickelate superconductors.
• The identified metallic regimes offer new avenues for exploring correlated electron phenomena.