Effect of Disorder Outside the CuO₂ Planes on T_c of Copper Oxide Superconductors

The effect of disorder on the superconducting transition temperature T_c of cuprate superconductors is examined. Disorder is introduced into the cation sites in the plane adjacent to the CuO₂ planes of two single-layer systems, Bi_2.0Sr_1.6Ln_0.4CuO_6+δ and La_1.85-yNd_ySr_0.15CuO₄. Disorder is controlled by changing rare earth (Ln) ions with a different ionic radius in the former, and by varying the Nd content in the latter with the doped carrier density kept constant. We show that this type of disorder works as weak scatterers in contrast to the in-plane disorder produced by Zn, but remarkably reduces T_c, suggesting novel effects of disorder on high-T_c superconductivity.