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

We report a Cu K- and L3-edge resonant inelastic x-ray scattering study of charge and spin excitations of bulk Nd2-xCexCuO4, with a focus on post-growth annealing effects. For the parent compound Nd2CuO4 (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 RE2CuO4 (RE=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.