LAUSR

We have investigated the superconducting (SC) gap on hole Fermi surfaces (FSs) of optimally substituted $\mathrm{Ba}(\mathrm{F}{\mathrm{e}}_{0.65}\mathrm{R}{\mathrm{u}}_{0.35}{)}_{2}\mathrm{A}{\mathrm{s}}_{2}$ by angle-resolved photoemission spectroscopy (APRES) using bulk-sensitive 7 eV laser and synchrotron radiation. It was found that, whereas the gap is isotropic in the ${k}_{x}\ensuremath{-}{k}_{y}$ plane, the gap magnitudes of two resolved hole FSs show similar ${k}_{z}$ dependences and decrease as ${k}_{z}$ approaches $\ensuremath{\sim}2\phantom{\rule{0.16em}{0ex}}\ensuremath{\pi}/c$ (i.e., around the $Z$ point), unlike the other Fe-based superconductors reported so far, where the SC gap of only one hole FS shows a strong ${k}_{z}$ dependence. This unique gap structure can be understood in the scenario that the ${d}_{{z}^{2}}$ orbital character is mixed into both hole FSs due to finite spin-orbit coupling (SOC) and is reproduced by calculation within the random phase approximation including the SOC.