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 ${\mathrm{L}\mathrm{a}}_{2-x}{\mathrm{S}\mathrm{r}}_x{\mathrm{C}\mathrm{u}\mathrm{O}}_4$.

Figure 1

Room temperature Cu $2p$ XPS spectra of (a) ${\mathrm{N}\mathrm{d}}_{1.85}{\mathrm{C}\mathrm{e}}_{0.15}{\mathrm{C}\mathrm{u}\mathrm{O}}_{4-\delta }$ and (b) ${\mathrm{N}\mathrm{d}}_2{\mathrm{C}\mathrm{u}\mathrm{O}}_4$.

Figure 2

Cu $L_3$ XAS spectra of ${\mathrm{N}\mathrm{d}}_{2-x}{\mathrm{C}\mathrm{e}}_x{\mathrm{C}\mathrm{u}\mathrm{O}}_{4-\delta }$ normalized on the Nd $M_5$ absorption intensity as shown in the inset.

Figure 3

Upper panel: resonant photoemission of ${\mathrm{N}\mathrm{d}}_{1.85}{\mathrm{C}\mathrm{e}}_{0.15}{\mathrm{C}\mathrm{u}\mathrm{O}}_{4-\delta }$, ${\mathrm{N}\mathrm{d}}_2{\mathrm{C}\mathrm{u}\mathrm{O}}_4$, ${\mathrm{L}\mathrm{a}}_2{\mathrm{C}\mathrm{u}\mathrm{O}}_4$, and ${\mathrm{L}\mathrm{a}}_{1.85}{\mathrm{S}\mathrm{r}}_{0.15}{\mathrm{C}\mathrm{u}\mathrm{O}}_4$ at the Cu $L_3$ edge. The energies of the spectra are aligned on the Cu $3d^8$ ${}^{1}G$ final states. Lower panel: off-resonance spectra at a photon energy 5 eV below the resonance. Arrows indicate the valence band onset of ${\mathrm{N}\mathrm{d}}_2{\mathrm{C}\mathrm{u}\mathrm{O}}_4$ and the Fermi level of ${\mathrm{N}\mathrm{d}}_{1.85}{\mathrm{C}\mathrm{e}}_{0.15}{\mathrm{C}\mathrm{u}\mathrm{O}}_{4-\delta }$.

Figure 4

On- and off-resonance photoemission spectra at the Nd $M_5$ absorption edge.

Figure 5

On- and off-resonance photoemission spectra at the Cu $L_3$ absorption edge.