Effect of Disorder Outside the CuO$_{2}$ 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 CuO$_{2}$ planes of single-layer systems, Bi$_{2}$Sr$_{1.6}$Ln$_{0.4}$CuO$_{6+y}$. Disorder is controlled by changing rare earth (Ln) ions with a different ionic radius 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.