Hund's metals, explained

A possible practical definition for a Hund's metal is given, as a metallic phase - arising consistently in realistic simulations and experiments in Fe-based superconductors and other materials - with three features: large electron masses, high-spin local configurations dominating the paramagnetic fluctuations and orbital-selective correlations. These features are triggered by, and increase with the proximity to, a Hund's coupling-favored Mott insulator that is realized for half-filled conduction bands. A clear crossover line is found where these three features get enhanced, departing from the Mott transition at half filling and extending in the interaction/doping plane, between a normal (at weak interaction and large doping) and a Hund's metal (at strong interaction and small doping). This phenomenology is found identically in models with featureless bands, highlighting the generality of this physics and its robustness by respect to the details of the material band structures. Some analytical arguments are also given to gain insight into these defining features. Finally the attention is brought on the recent theoretical finding of enhanced/diverging electronic compressibility near the Hund's metal crossover, pointing to enhanced quasiparticle interactions that can cause or boost superconductivity or other instabilities.