Strong nodeless pairing on separate electron Fermi surface sheets in (Tl, K)Fe1.78Se2 probed by ARPES

We performed a high-resolution angle-resolved photoemission spectroscopy study of the Tl0.63K0.37Fe1.78Se2 superconductor (Tc=29 K). We show the existence of two electron-like bands at the M(π, 0)-point which cross the Fermi level at similar Fermi wave vectors to form nearly circular electron-like Fermi surface pockets. We observe a nearly isotropic ∼8.5 meV superconducting gap (2Δ/kBTc∼7) on these Fermi surfaces. Our analysis of the band structure around the Brillouin zone centre reveals two additional electron-like Fermi surfaces: a very small one and a larger one with kF comparable to the Fermi surfaces at M. Interestingly, a superconducting gap with a magnitude of ∼8 meV also develops along the latter Fermi surface. Our observations are consistent with the s-wave strong-coupling scenario.