Pairing symmetry is a fundamental property that characterizes a superconductor. For the iron-based high-temperature superconductors1,2, an s±-wave pairing symmetry has received increasing experimental3,4,5,6,7,8,9,10,11 and theoretical12,13,14,15,16,17,18,19,20,21 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, AxFe2Se2 (A=K,Cs) with a superconducting transition temperature (Tc) of about 30 K. We found AxFe2Se2 (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 theories12 becomes conceptually irrelevant in describing the superconducting state here. A more conventional s-wave pairing is probably a better description.
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This work is supported in part by the National Science Foundation of China, Ministry of Education of China, Science and Technology Committee of Shanghai Municipal, and National Basic Research Program of China (973 Program) under grant Nos. 2011CB921802 and 2011CBA00102.
The authors declare no competing financial interests.
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Zhang, Y., Yang, L., Xu, M. et al. Nodeless superconducting gap in AxFe2Se2 (A=K,Cs) revealed by angle-resolved photoemission spectroscopy. Nature Mater 10, 273–277 (2011). https://doi.org/10.1038/nmat2981
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