On the Superconducting Symmetry of Fe-based Systems - Impurity Effect Studies and Neutron Scattering Measurements-

To identify the superconducting symmetry of Fe-based superconductors, we studied effects of nonmagnetic-impurities on T c , magnetic excitation spectra x″ and NMR 1/T 1 − T curve, which are sensitive to the relative signs between the order parameters on the disconnected Fermi surfaces in reciprocal space, because the symmetry is closely connected with the pairing mechanism: If the signs are opposite (symmetry S ±), the mechanism is considered to be a magnetic one, but when the signs are the same (symmetry S ++), a novel mechanism is plausible because the ordinary phonon mechanism cannot realize the T c value as high as ∼55 K found in Ln1111 (Ln=lanthanide). Results are as follows: (a) The very small rates of T c -suppression by impurity atoms M of LnFe1−y M y As0.89−x F0.11+x (M=Ni, Co, Ru) can be explained only by S ++. (b) The x″-data for Ba(Fe, Co)2As2 (T c ∼ 23 K) and Ca-Fe-Pt-As crystals seem to be well explained by S ++ rather than by S ±. (c) The nuclear magnetic resonance data can be consistently understood by S ++, too. These results suggest the S ++ symmetry and a novel pairing mechanism, which can be considered to be related to the elastic softening of C66 induced by the orbital fluctuation of the system.