Enhanced superconductivity of Ba_0.5K_0.5Fe₂As₂ under surface potassium dosing

Surface potassium dosing has been proven to be an effective method in tuning the electron doping and enhancing the superconducting transition temperatures in both iron chalcogenides and electron doped iron pnictides. However, it is not clear how surface potassium dosing affects the hole doping and superconductivity in hole doped Fe-based superconductors. Here we performed K-dosing on Ba_0.5K_0.5Fe₂As₂, a prototypical hole-doped iron pnictide compound, and explored the electronic structure by in situ angle-resolved photoemission spectroscopy measurements. Starting from the slightly over-doped Ba_0.5K_0.5Fe₂As₂, surface K-dosing effectively reduces the hole concentration towards optimal doping and enhances the superconductivity. Intriguingly, the enhancement of superconductivity does not slow down at optimal doping, and the gap further increases with K dosing even when the carrier doping effect is saturated. Meanwhile, the quasiparticle coherence of the inner hole pockets is enhanced by higher K dosing. Our results uncover a novel scattering-reduction effect of K-dosing in Ba_1-x K _x Fe₂As₂, which collaborates with the carrier doping effect and enhances superconductivity.