Nature of hole doping in ${\mathrm{Nd}}_2$${\mathrm{NiO}}_4$ and ${\mathrm{La}}_2$${\mathrm{NiO}}_4$: Comparison with ${\mathrm{La}}_2$${\mathrm{CuO}}_4$

To elucidate the hole doping mechanism, we have systematically studied the Ni and Cu K-edge x-ray-absorption spectra in doped ${\mathrm{Nd}}_2$ ${\mathrm{NiO}}_4$, ${\mathrm{La}}_2$ ${\mathrm{NiO}}_4$, and ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$. We find a significant difference in the character of the doped hole between the nickelates and cuprate. We observe that both strontium substitution and introduction of excess oxygen lead to a shift of the Ni absorption edge to higher energies, by nearly 3 eV in ${\mathrm{NaSrNiO}}_4$. This result demonstrates that the doped holes, in addition to the O 2p character, have a substantial amount of Ni 3d character (30–40 %). The intensity of the 1s-3d transition increases upon doping, consistent with increasing number of d holes. In contrast, the Cu absorption edge in the doped cuprate is shifted by much smaller amounts, indicative of a lower amount of Cu 3d character for the doped hole. The strongly mixed Ni 3d and O 2p character of the doped hole distinguishes the electronic structure of the nickelate from that of the cuprate.