  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.
