Orbital selectivity in Hund's metals: The iron chalcogenides

Abstract

We show that electron correlations lead to a bad metallic state in chalcogenides FeSe and FeTe despite the intermediate value of the Hubbard repulsion $U$ and Hund's rule coupling $J$. The evolution of the quasiparticle weight $Z$ as a function of the interaction terms reveals a clear crossover at $U\ensuremath{\simeq}2.5$ eV. In the weak coupling limit $Z$ decreases for all correlated $d$ orbitals as a function of $U$ and beyond the crossover coupling they become weakly dependent on $U$ while strongly dependent on $J$. A marked orbital dependence of the $Z$'s emerges even if in general the orbital-selective Mott transition only occurs for relatively large values of $U$. This two-stage reduction of the quasiparticle coherence due to the combined effect of Hubbard $U$ and the Hund's $J$ suggests that the iron-based superconductors can be referred to as Hund's correlated metals.