Superconducting gap symmetry of Ba0.6K0.4Fe2As2 studied by angle-resolved photoemission spectroscopy

We have performed high-resolution angle-resolved photoemission spectroscopy on the optimally doped Ba0.6K0.4Fe2As2 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 $\Delta (k)=\Delta _{0}\,{\rm cos}\,k_{x}\,{\rm cos}\,k_{y}$ formula expected for the extended s-wave symmetry. The observed finite deviation from the simple formula suggests the importance of multi-orbital effects.