Searching for key parameter for determining Tc in Fe-based superconductors: Study of P/As substitution in RFe(P,As)(O,F) [R=La and Nd]

We have investigated the relation between the temperature-dependence of resistivity and superconducting transition temperature Tc in RFeP1−xAsxO0.90F0.10 (x=0-1.0) (R=La and Nd). In contrast to the linear change of the crystal structure with increasing x, the temperature dependence of resistivity and Tc show non-monotonous x-dependence. When the As concentration x is increased, the temperature-dependence of resistivity changes from T2 to T-linear, and Tc distinctly increases in all the La compounds and the Nd ones with x<0.60. The results indicate that the substitution of As for P induces the spin fluctuation and resultantly enhances Tc. On the other hand, we could not find any relation between the temperature-dependence of resistivity and Tc in the Nd samples with x>0.60. This may suggest the existence of other parameters for determining Tc besides the antiferromagnetic correlation in this system.