Localized holes in superconducting lanthanum cuprate

Copper NQR spectra demonstrate the existence of a second, anomalous copper site in lanthanum cuprate whose character is independent of the method of doping. We present a systematic NMR/NQR study of ${\mathrm{La}}_2{\mathrm{CuO}}_{4+\delta }$ for a range of $\delta$, which demonstrates that the density of such sites increases with $\delta$. Analysis of our results and of published data from ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ indicate that a substantial fraction of the doped holes in these materials localize in ${\mathrm{CuO}}_6$ octahedra adjacent to the out-of-plane dopants, and that the anomalous sites are those neighboring a site occupied by such a pinned hole. Thus, superconductivity occurs in a ${\mathrm{CuO}}_2$ plane containing many localized holes.