Hund's rule coupling promotes bad metals

There is a nice review Strong correlations from Hund's coupling by
Antoine Georges, Luca de' Medici, and Jernej Mravlje.
Section 6 describes

the main physical point of this review: Generally, the Hund’s rule coupling has a conflicting effect on the physics of the solid-state. On the one hand, it increases the critical U above which a Mott insulator is formed (Section 3); on the other hand, it reduces the temperature and the energy scale below which a Fermi liquid is formed, leading to a (bad) metallic regime in which quasiparticle coherence is suppressed (Section 4).

In particular, this means a new organising principle for strong correlated electron systems: one can have a bad metal and be a long way from a Mott insulator. "Mottness" is not the only ingredient for strong correlations.

A previous post describing some of this work includes Hund's rule coupling in multi-band metals.