On the superconducting pairing mechanism of Fe-based systems—Is it new or well-known ?

To clarify the superconducting mechanism of Fe-based superconductors, we have studied three physical quantities sensitive to the sign change between their superconducting order parameters ΔΔ on the disconnected hole- and electron-Fermi-surfaces around ΓΓ and M points, effects of nonmagnetic impurities on the superconducting transition temperature TcTc, magnetic excitation spectra and NMR longitudinal relaxation rates 1/T1T1, because the nonexistence (existence) of the sign change seems to directly correspond to the new (already known) mechanism of the pairing. The results show that the sign change does not exist, suggesting a new pairing mechanism in the Fe-based superconductors.

► Superconducting symmetry of Fe-based superconductors has been studied.
► Small rates of TcTc suppression by nonmagnetic impurities exclude the S±S± symmetry.
► Results of NMR and neutron measurements can be understood by the S++ symmetry.