Effect of Ln-Site Disorder on T_c of Oxypnictide Superconductor LnFeAsO_1−xF_x (Ln=Nd, Ce–Gd, and La–Dy)

Abstract

The effect of disorder on the transition temperature T_c of the oxypnictide superconductor LnFeAsO_1−xF_x is explored. Disorder is controlled by ionic-radius mismatch of the Ln-site elements with the doped carrier density kept constant. The x-ray diffraction and x-ray absorption fine structure experiments confirmed that the averaged lattice parameters and the local fluctuation of the Fe–As bond length are identical but, nevertheless, T_c decreases in the order of Ln=Nd, Ce_0.62Gd_0.38, and La_0.62Dy_0.38. This type of weak disorder reduces T_c possibly via local fluctuation of the Fe–As bond angle.