We performed a resonant inelastic x-ray scattering (RIXS) study of ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_{4+\delta }$ (LSNO) at the oxygen $K$ edge to investigate the nature of the doped holes with regard to charge excitations. Charge excitations of the hole-doped nickelates are found to be almost independent of momentum transfer, indicating that the doped holes are strongly localized in character. Additionally, conspicuous changes in energy position are in temperature dependence. These characters are observed in stark contrast to those of the high-$T_c$ cuprate ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (LSCO), where delocalized doped holes form charge excitations with sizable momentum dependence in the ${\mathrm{CuO}}_2$ plane. This distinct nature of charge excitations of doped holes is consistent with the metallicity of the materials and could be caused by strong electron-phonon coupling and weak quantum spin fluctuation in the nickelates.

• Received 3 April 2020
• Revised 4 September 2020
• Accepted 30 September 2020

DOI:https://doi.org/10.1103/PhysRevB.102.165145