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] Original Research Article

We have investigated the relation between the temperature-dependence of resistivity and superconducting transition temperature T c 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 T c show non-monotonous x -dependence. When the As concentration x is increased, the temperature-dependence of resistivity changes from T 2 to T -linear, and T c distinctly increases in all the La compounds and the Nd ones with x<0.60x<0.60. The results indicate that the substitution of As for P induces the spin fluctuation and resultantly enhances T c. On the other hand, we could not find any relation between the temperature-dependence of resistivity and T c in the Nd samples with x>0.60x>0.60. This may suggest the existence of other parameters for determining Tc besides the antiferromagnetic correlation in this system.