Saturating $T^2$ resistivity in iron-based superconductors
Many correlated metals exhibit a crossover from low-temperature $ρ\propto T^{2}$ behavior to a slower increase of the resistivity at elevated temperatures. Distinguishing phonon scattering from electron--electron correlations as the origin of this crossover is experimentally challenging. We introduce a simple phenomenological framework that isolates the contribution responsible for the high-temperature saturating tendency. This approach reveals a clear contrast between systems well known for the predominance of electron--phonon scattering and several iron-based superconductors. For the latter, the conductivity can be expressed as the sum of a Fermi-liquid-like term ($\propto 1/T^{2}$) and a temperature-independent component, which together describe $ρ(T)$ up to $\sim 250$ K in LiFeAs, KFe$_2$As$_2$, and KCaFe$_4$As$_4$. In FeSe, the same approach identifies a similar regime above the nematic transition, with a pronounced change toward nearly linear-in-$T$ resistivity below it. We discuss these findings in the broader context of coherence and electronic correlations in Hund's metals.