Janus-faced influence of the Hund's rule coupling in strongly correlated materials

TL;DR

This paper explores how Hund's rule coupling influences electronic correlations in multi-band metals, revealing its role in promoting metallic, insulating, or bad-metallic states depending on electron filling, and providing a classification for transition-metal oxides.

Contribution

It introduces a novel classification framework for 3d and 4d transition-metal oxides based on Hund's coupling effects, explaining diverse correlation phenomena.

Findings

Hund's coupling affects quasiparticle coherence and Mott proximity differently depending on filling.

A strongly correlated bad-metal state can exist far from a Mott insulator.

The classification explains high Nél temperatures in Tc-based perovskites.

Abstract

We show that in multi-band metals the correlations are strongly affected by the Hund's rule coupling, which depending on the filling promotes metallic, insulating or bad-metallic behavior. The quasiparticle coherence and the proximity to a Mott insulator are influenced distinctly and, away from single- and half-filling, in opposite ways. A strongly correlated bad-metal far from a Mott phase is found there. We propose a concise classification of 3d and 4d transition-metal oxides within which the ubiquitous occurrence of strong correlations in Ru- and Cr-based oxides, as well as the recently measured high N\'eel temperatures in Tc-based perovskites are naturally explained.