Superconducting gap symmetry of Ba_0.6K_0.4Fe ₂As₂ studied by angle-resolved photoemission spectroscopy

We have performed high-resolution angle-resolved photoemission spectroscopy on the optimally doped Ba_0.6K_0.4Fe₂As ₂ compound and determined the accurate momentum dependence of the superconducting (SC) gap in four Fermi-surface sheets including a newly discovered outer electron pocket at the M-point. The SC gap on this pocket is nearly isotropic and its magnitude is comparable (Δ∼11 meV) to that of the inner electron and hole pockets (∼12 meV), although it is substantially larger than that of the outer hole pocket (∼6 meV). The Fermi-surface dependence of the SC gap value is basically consistent with the Δ(k)=Δ₀ cos k_x cos k_y formula expected for the extended s-wave symmetry. The observed finite deviation from the simple formula suggests the importance of multi-orbital effects.