Doping in isoelectronic cuprate superconductors

Thermoelectric power in the charge-compensated 1:2:3 family $\left({\mathrm{Ca}}_x{\mathrm{La}}_{1-x}\right)\left({\mathrm{Ba}}_{1.75-x}{\mathrm{La}}_{0.25+x}\right){\mathrm{Cu}}_3{\mathrm{O}}_y$ shows that doping occurs also under isoelectronic (constant electron concentration) conditions obtained by keeping the oxygen content $y$ constant. For all values of $x$ and $y$ there exists a single doping parameter $y-y_{\mathrm{M}-\mathrm{I}}\mathrm{}\left(x\right)\mathrm{}$, where $y_{\mathrm{M}-\mathrm{I}}$ is the value of $y$ at the metal-insulator transition, suggesting that a single mobile band controls transport and superconductor properties. Isoelectronic doping occurs via capture of mobile electrons by lower-lying low-mobility states, requiring either a shift of the band relative to these states or transfer of spectral density near the Fermi energy.