Orbital selectivity in Hund's metals: The iron chalcogenides

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 [Asymptotically = to] 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.