Wang, X.-P.; Richard, P.; Shi, X.; Roekeghem, A.; Huang, Y.-B.; Razzoli, E.; Qian, T.; Rienks, E.; Thirupathaiah, S.; Wang, H.-D.; Dong, C.-H.; Fang, M.-H.; Shi, M.; Ding, H.: Observation of an isotropic superconducting gap at the Brillouin zone centre of Tl0.63K0.37Fe1.78Se2. EPL 99 (2012), p. 67001/1-5

We performed a high-resolution angle-resolved photoemission spectroscopy study on superconducting (SC) Tl0.63K0.37Fe1.78Se2 (Tc = 29 K) in the whole Brillouin zone (BZ). In addition to a nearly isotropic ~8.2 meV 2-dimensional (2D) SC gap (2Δ/kBTc ~ 7) on quasi-2D electron Fermi surfaces (FSs) located around M(π,0,0)-A(π,0,π), we observe a ~6.2 meV isotropic SC gap (2Δ/kBTc ~ 5) 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.