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

Knowledge of the symmetry of the superconducting order parameter is essential to understand the origin of superconductivity itself. Studies on the recently discovered heavily doped Fe 2 Se 2 now show that in these compounds the order parameter has a relatively simple symmetry compared with most other Fe-based superconductors, questioning again the generality of the results obtained so far. Pairing symmetry is a fundamental property that characterizes a superconductor. For the iron-based high-temperature superconductors 1 , 2 , an s ± -wave pairing symmetry has received increasing experimental 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 and theoretical 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 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 ~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 12 becomes conceptually irrelevant in describing the superconducting state here. A more conventional s -wave pairing is probably a better description.