Crossing the Gap from $p$- to $n$-Type Doping: Nature of the States near the Chemical Potential in ${\mathrm{L}\mathrm{a}}_{2-x}{\mathrm{S}\mathrm{r}}_x{\mathrm{C}\mathrm{u}\mathrm{O}}_4$ and ${\mathrm{N}\mathrm{d}}_{2-x}{\mathrm{C}\mathrm{e}}_x{\mathrm{C}\mathrm{u}\mathrm{O}}_{4-\delta }$

We report on an x-ray absorption and resonant photoemission study on single crystals of the high-$T_c$ cuprates ${\mathrm{L}\mathrm{a}}_{2-x}{\mathrm{S}\mathrm{r}}_x{\mathrm{C}\mathrm{u}\mathrm{O}}_4$ and ${\mathrm{N}\mathrm{d}}_{2-x}{\mathrm{C}\mathrm{e}}_x{\mathrm{C}\mathrm{u}\mathrm{O}}_{4-\delta }$. Using an intrinsic energy reference, we find that the chemical potential of ${\mathrm{L}\mathrm{a}}_{2-x}{\mathrm{S}\mathrm{r}}_x{\mathrm{C}\mathrm{u}\mathrm{O}}_4$ lies near the top of the ${\mathrm{L}\mathrm{a}}_2{\mathrm{C}\mathrm{u}\mathrm{O}}_4$ valence band whereas in ${\mathrm{N}\mathrm{d}}_{2-x}{\mathrm{C}\mathrm{e}}_x{\mathrm{C}\mathrm{u}\mathrm{O}}_{4-\delta }$ it is situated near the bottom of the ${\mathrm{N}\mathrm{d}}_2{\mathrm{C}\mathrm{u}\mathrm{O}}_4$ conduction band. The data clearly establish that the introduction of Ce in ${\mathrm{N}\mathrm{d}}_2{\mathrm{C}\mathrm{u}\mathrm{O}}_4$ results in electrons being doped into the ${\mathrm{C}\mathrm{u}\mathrm{O}}_2$ planes. We infer that the states closest to the chemical potential have a Cu $3d^{10}$ singlet origin in ${\mathrm{N}\mathrm{d}}_{2-x}{\mathrm{C}\mathrm{e}}_x{\mathrm{C}\mathrm{u}\mathrm{O}}_{4-\delta }$ and a $3d^9\underline{L}$ singlet origin in ${\mathrm{L}\mathrm{a}}_{2-x}{\mathrm{S}\mathrm{r}}_x{\mathrm{C}\mathrm{u}\mathrm{O}}_4$.