Nodeless superconducting gap in AxFe2Se2 (A = K, Cs) revealed by angle-resolved photoemission spectroscopy

Pairing symmetry is a fundamental property that characterizes a superconductor. For the iron-based high-temperature superconductors, an s(+/-)-wave pairing symmetry has received increasing experimental and theoretical support. More specifically, the superconducting order parameter is an isotropic s-wave type around a particular Fermi surface, but it has opposite signs between the hole Fermi surfaces at the zone centre and the electron Fermi surfaces at the zone corners. Here we report the low-energy electronic structure of the newly discovered superconductors, A(x)Fe(2)Se(2) (A = K, Cs) with a superconducting transition temperature (T-c) of about 30 K. We found A(x)Fe(2)Se(2) (A = K, Cs) is the most heavily electron-doped among all iron-based superconductors. Large electron Fermi surfaces are observed around the zone corners, with an almost isotropic superconducting gap of similar to 10.3 meV, whereas there is no hole Fermi surface near the zone centre, which demonstrates that interband scattering or Fermi surface nesting is not a necessary ingredient for the unconventional superconductivity in iron-based superconductors. Thus, the sign change in the s(+/-) pairing symmetry driven by the interband scattering as suggested in many weak coupling theories becomes conceptually irrelevant in describing the superconducting state here. A more conventional s-wave pairing is probably a better description.