Abstract
Using pair breaking by Cu-site magnetic impurities as a probe, we demonstrate that the root of superconductivity is located in the ‘charge reservoir’ regions of La2-βSrβCuOx and Nd2-zCezCuOx (with x ≈ 4), namely in the regions occupied by interstitial dopant oxygen, and not in the cuprate planes. The fractional changes in T c induced by non-magnetic impurities in both materials are the same, indicating that Nd2-zCezCuOx, like La2-βSrβCuOx, is a hole superconductor.