Angle-resolved photoemission observation of isotropic superconducting gaps in isovalent Ru-substituted Ba(Fe_0.75Ru_0.25) ₂As₂

We used high-energy resolution angle-resolved photoemission spectroscopy to extract the momentum dependence of the superconducting gap of Ru-substituted Ba(Fe_0.75Ru_0.25)₂As₂ (T _c=15 K). Despite a strong out-of-plane warping of the Fermi surface, the magnitude of the superconducting gap observed experimentally is nearly isotropic and independent of the out-of-plane momentum. More precisely, we respectively observed 5.7 and 4.5 meV superconducting gaps on the inner and outer Γ-centered hole Fermi surface pockets, whereas a 4.8 meV gap is recorded on the M-centered electron Fermi surface pockets. Our results are consistent with the J₁-J₂ model with a dominant antiferromagnetic exchange interaction between the next-nearest Fe neighbors.