Post-growth annealing effects on charge and spin excitations in Nd2−xCexCuO4

We report a Cu $K$- and ${L}_{3}$-edge resonant inelastic x-ray scattering study of charge and spin excitations of bulk ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4}$, with a focus on post-growth annealing effects. For the parent compound ${\mathrm{Nd}}_{2}{\mathrm{CuO}}_{4}$ $(x=0)$, a clear charge-transfer gap is observed in the as-grown state, whereas the charge excitation spectra indicate that electrons are doped in the annealed state. This is consistent with the observation that annealed thin-film and polycrystalline samples of $R{E}_{2}{\mathrm{CuO}}_{4}$ $(\mathrm{RE}=\text{rare earth})$ can become metallic and superconducting at sufficiently high electron concentrations without Ce doping. For $x=0.16$, a Ce concentration for which it is known that oxygen reduction destroys long-range antiferromagnetic order and induces superconductivity, we find that the high-energy spin excitations of non-superconducting as-grown and superconducting annealed crystals are nearly identical. This finding is in stark contrast to the significant changes in the low-energy spin excitations previously observed via neutron scattering.