Electronic structure of superconducting nickelates probed by resonant photoemission spectroscopy

The discovery of infinite-layer nickelate superconductors has spurred enormous interest. While the Ni¹⁺ cations possess nominally the same 3d⁹ configuration as Cu²⁺ in high-T_C cuprates, the electronic structure consistencies and variances remain elusive, due to the lack of direct experimental probes. Here, we present a soft x-ray photoemission spectroscopy study on both parent and doped infinite-layer Pr-nickelate thin films with a doped perovskite reference. By identifying the Ni character with resonant photoemission and comparison to density function theory + U calculations, we estimate U ~ 5 eV, smaller than the charge transfer energy △ ~ 8 eV, in contrast to the cuprates being charge transfer insulators. Near the Fermi level, we observe a signature of rare-earth spectral intensity in the parent compound, which is depleted upon doping. Our results demonstrate a complex interplay between the strongly correlated Ni 3d and the weakly-interacting rare-earth 5d states for the superconductivity in the nickelates.