Observation of an isotropic superconducting gap at the Brillouin zone centre of Tl _0.63K _0.37Fe _1.78Se ₂

We performed a high-resolution angle-resolved photoemission spectroscopy study on superconducting (SC) Tl _0.63K _0.37Fe _1.78Se ₂ (T _c=29K) in the whole Brillouin zone (BZ). In addition to a nearly isotropic 8.2meV 2-dimensional (2D) SC gap (2Δ/k _BT _c7) on quasi-2D electron Fermi surfaces (FSs) located around M(π,0,0)-A(π,0,π), we observe a 6.2meV isotropic SC gap (2Δ/k _BT _c5) on the Z-centred electron FS that rules out any d-wave pairing symmetry and rather favors an s-wave symmetry. All isotropic SC gap amplitudes can be fit by a single-gap function derived from a local strong-coupling approach suggesting an enhancement of the next-nearest neighbor exchange interaction in the ferrochalcogenide superconductors.