Antiferromagnetism is relevant to high temperature (high-T c ) superconductivity because copper oxide and iron arsenide high-T c superconductors arise from electron-or hole-doping of their antiferromagnetic (AF) ordered parent compounds 1-6 . There are two broad classes of explanation for the phenomenon of antiferromagnetism: in the "local moment" picture, appropriate for the insulating copper oxides 1 , AF interactions are well described by a Heisenberg Hamiltonian 7,8 ; while in the "itinerant model", suitable for metallic chromium, AF order arises from quasiparticle excitations of a nested Fermi surface 9,10 . There has been contradictory evidence regarding the microscopic origin of the AF order in iron arsenide materials 5,6 , with some favoring a localized picture 11-15 while others supporting an
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 (Tc) 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 becomes conceptually irrelevant in describing the superconducting state here. A more conventional s-wave pairing is probably a better description.
The relation between the spin-density wave (SDW) and superconducting order is a central topic in the current research on the FeAs-based high-TC superconductors. Conflicting results exist in the LaFeAs(O, F)-class of materials, for which whether the SDW and superconductivity are mutually exclusive or they can coexist has not been settled. Here we show that for the (Ba, K)Fe2As2 system, the SDW and superconductivity can coexist in an extended range of compositions. The availability of single crystalline samples and high value of the energy gaps would make the materials a model system to investigate the high-TC ferropnictide superconductivity.
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