ARPES observation of isotropic superconducting gaps in isovalent   Ru-substituted Ba(Fe$_{0.75}$Ru$_{0.25}$)$_2$As$_2$
  We used high-energy resolution angle-resolved photoemission spectroscopy to extract the momentum dependence of the superconducting gap of Ru-substituted Ba(Fe$_{0.75}$Ru$_{0.25}$)$_2$As$_2$ ($T_c = 15$ 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 meV and 4.5 meV superconducting gaps on the inner and outer $\Gamma$-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 $J_1-J_2$ model with a dominant antiferromagnetic exchange interaction between the next-nearest Fe neighbors. 
N. Xu, P. Richard, X.-P. Wang, X. Shi, A. van Roekeghem, T. Qian, E.   Ieki, K. Nakayama, T. Sato, E. Rienks, S. Thirupathai...
cond-mat.supr-con cond-mat.str-el