Superconductivity of LaFe1-yCoyAsO1-xFx

We have prepared the superconducting system LaFe1-yCoyAsO1-xFx (x=0.11) and carried out measurements of their electrical resistivity p and superconducting diamagnetism. As-75- and La-135-NMR studies have also been carried out. The Knight shift observed for As-75 has been found to be suppressed by the superconductivity; for La-135, 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.