Magnetic versus nonmagnetic ion substitution effects onTcin the La-Sr-Cu-O and Nd-Ce-Cu-O systems

The effects of a substitution for Cu by other 3d metals (Ni, Co, and Zn) on ${\mathit{T}}_{\mathit{c}}$ in the Nd-Ce-Cu-O system was studied and compared with effects of the same ions on ${\mathit{T}}_{\mathit{c}}$ in the La-Sr-Cu-O system. We found (1) Zn suppresses ${\mathit{T}}_{\mathit{c}}$ more slowly in the Nd than in the La systems, so the disorder produced by the nonmagnetic ions is less important in the Nd system, (2) Ni and Co depress ${\mathit{T}}_{\mathit{c}}$ more quickly in the Nd than in the La system, showing that the magnetic pair breaking is stronger in the Nd system, and (3) in the La system the magnetism of the dopant has no effect on ${\mathit{T}}_{\mathit{c}}$. Thus, within the same chemical system (cuprates) we find that as the correlation length is increased one obtains more familiar BCS-type behavior. We suggest that much of the behavior of the various high-${\mathit{T}}_{\mathit{c}}$ oxides may be simply a function of the correlation length.