Strong electronic correlations from Hund's coupling

TL;DR

This paper reviews how Hund's rule coupling, beyond Mott physics, induces strong electronic correlations in multi-orbital metals, affecting their coherence and electronic properties, especially in transition-metal oxides and iron-based superconductors.

Contribution

It provides a comprehensive overview of the role of Hund's coupling in creating strongly correlated metallic states, highlighting the concept of Hund's metals and their physical implications.

Findings

Hund's coupling influences Mott gap energetics.

It suppresses the Fermi-liquid coherence scale.

Hund's metals are relevant in transition-metal oxides and superconductors.

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 multi-orbital metallic materials which 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. The theoretical understanding and physical properties of these `Hund's metals' are reviewed, together with the relevance of this concept to transition-metal oxides of the 3d, and especially 4d series (such as ruthenates), as well as to the iron-based superconductors (iron pnictides and chalcogenides).