Hund’s coupling assisted orbital-selective superconductivity in Ba_1-xK_xFe₂As₂

While the superconducting transition temperature of hole-doped Ba_1-xK_xFe₂As₂ decreases past optimal doping, superconductivity does not completely disappear even for the fully doped KFe₂As₂ compound. In fact, superconductivity is robust through a Lifshitz transition where electron bands become hole-like around the zone corner at x~0.7, thus challenging the conventional understanding of superconductivity in iron-based systems. High resolution angle-resolved photoemission spectroscopy is used to investigate the superconducting gap structure, as well as the normal state electronic structure, around optimal doping and across the Lifshitz transition. Our findings reveal a largely orbital-dependent superconducting gap structure, where the more strongly correlated d_xy band has a vanishing superconducting gap at higher doping, aligning with the Hund’s metal behavior observed in the normal state. Notably, the superconducting gap on the d_xy band disappears before the Lifshitz transition, suggesting that the Fermi surface topology may play a secondary role. We discuss how these results point to orbital-selective superconducting pairing and how strong correlations via Hund’s coupling may shape superconducting gap structures in iron-based and other multiorbital superconductors.