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 (T-c = 29 K). We show the existence of two electron-like bands at the M(pi, 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 similar to 8.5meV superconducting gap (2 Delta/k(B)T(c) similar to 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 k(F) comparable to the Fermi surfaces at M. Interestingly, a superconducting gap with a magnitude of similar to 8meV also develops along the latter Fermi surface. Our observations are consistent with the s-wave strong-coupling scenario. Copyright (c) EPLA, 2011