Strong Correlations from Hund’s Coupling

TLDR

Strong electronic correlations arise not only near Mott insulators but also from Hund’s rule coupling in multiorbital metals, with the average shell occupancy acting as a key control parameter. The paper reviews the theory and physical properties of Hund’s metals and their relevance to 3d/4d transition‑metal oxides and iron‑based superconductors. The review discusses how Hund’s coupling reduces the Fermi liquid coherence scale and alters Mott‑gap energetics, shaping the physics of Hund’s metals. The strongest Hund’s‑coupling effects appear when the shell occupancy deviates from half‑filling or single occupancy.

Abstract

Strong electronic correlations are often associated with the proximity of a Mott-insulating state. In recent years however, it has become increasingly clear that the Hund’s rule coupling (intra-atomic exchange) is responsible for strong correlations in multiorbital metallic materials that are not close to a Mott insulator. Hund’s coupling has two effects: It influences the energetics of the Mott gap and strongly suppresses the coherence scale for the formation of a Fermi liquid. A global picture has emerged recently, which emphasizes the importance of the average occupancy of the shell as a control parameter. The most dramatic effects occur away from half-filling or single occupancy. We review the theoretical understanding and physical properties of these Hund’s metals, together with the relevance of this concept to transition-metal oxides (TMOs) of the 3d, and especially 4d, series (such as ruthenates), as well as to the iron-based superconductors (iron pnictides and chalcogenides).