Universality of superconducting gaps in overdoped Ba_0.3K _0.7Fe₂As₂ observed by angle-resolved photoemission spectroscopy

We have performed angle-resolved photoemission spectroscopy on the overdoped Ba_0.3K_0.7Fe₂As₂ superconductor (T_c=22 K). We demonstrate that the superconducting (SC) gap on each Fermi surface (FS) is nearly isotropic whereas the gap value varies from 4.4 to 7.9 meV on different FSs. By comparing with under- and optimally doped Ba_1-xK_xFe₂As₂, we find that the gap value on each FS nearly scales with T_c over a wide doping range (0.25≤x≤0.7). Although the FS volume and the SC gap magnitude are strongly doping dependent, the multiple nodeless gaps can be commonly fitted by a single gap function assuming pairing up to the second nearest neighbor, suggesting the universality of the short-range pairing states with the s_±-wave symmetry.