Distinct Transport Behaviors of LaFe1-yCoyAsO1-xFx ($x=0.11$) between the Superconducting and Nonsuperconducting Metallic $y$ Regions Divided by $y \sim 0.05$

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Electrical resistivities, Hall coefficients, and thermoelectric powers have been measured for polycrystalline samples of LaFe1-yCoyAsO1-xFx ($x=0.11$) with various values of $y$. The results show that there exists a clear distinction of these transport behaviors between the superconducting and nonsuperconducting metallic regions of $y$ divided by the boundary value $y_{\text{c}}\sim 0.05$. We have found that the behaviors in both regions are very similar to those of high-$T_{\text{c}}$ Cu oxides in the corresponding phases. If they reflect, as in the case of Cu oxides, the effects of strong magnetic fluctuations, the energy scale of the fluctuations is considered to be smaller than that of the high-$T_{\text{c}}$ Cu oxides by a factor of ${\sim}1/2$. Arguments on the electronic nature and superconducting symmetry are presented on the basis of the observed small rate of the $T_{\text{c}}$ suppression caused by Co doping.
2009-04-15