Superconductivity of LaFe_<1-y>Co_yAsO_<1-x>F_x(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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We have prepared the superconducting system LaFe_<1-y>Co_yAsO_<1-x>F_x (x=0.11) and carried out measurements of their electrical resistivity ρ and superconducting diamagnetism. ^<75>As- and ^<135>La-NMR studies have also been carried out. The Knight shift observed for ^<75>As has been found to be suppressed by the superconductivity; for ^<135>La, the shift is almost insensitive to the superconductivity. This result presents rather strong experimental evidence for singlet pairing. The Co doping effect on the superconducting transition temperature T_c is not so significant as expected for superconductors with nodes, suggesting that the potential scattering does not seem to primarily suppress the superconductivity. Even for superconductors without nodes, it may not be so trivial to expect this small effect, if there are two different (disconnected) Fermi surfaces whose order parameters have opposite signs. As a possible explanation of the observed T_c suppression, which is found to be small, seems to be related with the loss of the itinerant nature of the electrons.
社団法人日本物理学会の論文
2008-10-15