Strong nodeless pairing on separate electron Fermi surface sheets in (Tl, K)Fe _1.78 Se ₂ probed by ARPES

Abstract

We performed a high-resolution angle-resolved photoemission spectroscopy\nstudy of the Tl$_{0.63}$K$_{0.37}$Fe$_{1.78}$Se$_2$ superconductor ($T_c=29$\nK). We show the existence of two electronlike bands at the M$(\\pi, 0)$ point\nwhich cross the Fermi level at similar Fermi wave vectors to form nearly\ncircular electronlike Fermi surface pockets. We observe a nearly isotropic\n$\\sim$ 8.5 meV superconducting gap ($\\Delta/k_BT_c\\sim 7$) on these Fermi\nsurfaces. Our analysis of the band structure around the Brillouin zone centre\nreveals two additional electronlike Fermi surfaces: a very small one and a\nlarger one with $k_F$ comparable to the FS pockets at M. Interestingly, a SC\ngap with a magnitude of $\\sim$ 8 meV also develops along the latter FS. Our\nobservations are consistent with the s-wave strong coupling scenario.\n