Observation of Fermi-surface-dependent nodeless superconducting gaps in Ba_0.6K_0.4Fe₂As₂

We have performed a high-resolution angle-resolved photoelectron spectroscopy study on the newly discovered superconductor Ba_0.6K _0.4Fe₂As₂ (T_c=37 K). We have observed two superconducting gaps with different values: a large gap (Δ∼12 meV) on the two small hole-like and electron-like Fermi surface (FS) sheets, and a small gap (∼6 meV) on the large hole-like FS. Both gaps, closing simultaneously at the bulk transition temperature (T_c), are nodeless and nearly isotropic around their respective FS sheets. The isotropic pairing interactions are strongly orbital dependent, as the ratio 2Δ/k_BT_c switches from weak to strong coupling on different bands. The same and surprisingly large superconducting gap due to strong pairing on the two small FSs, which are connected by the (π, 0) spin-density-wave vector in the parent compound, strongly suggests that the pairing mechanism originates from the inter-band interactions between these two nested FS sheets.