Observation of Electridelike s States Coexisting with Correlated d Electrons in NdNiO_{2}

Despite exhibiting a similar d_{x^{2}-y^{2}} band character to cuprates, infinite-layer nickelates host additional electron pockets that distinguish them from single-band cuprates. The elusive orbital origin of these electron pockets has led to competing theoretical scenarios. Here, using polarization-dependent and resonant angle-resolved photoemission spectroscopy, we determine the orbital character of the Fermi surfaces in NdNiO_{2}. Our data reveal that the electronlike pocket arises predominantly from interstitial s states, with negligible contributions from rare-earth 5d and 4f orbitals near the Fermi level. The observation of well-defined quantum-well states indicates a uniform distribution of these interstitial electrons throughout the film thickness. By comparing with electronic structure of LaNiO_{2}, we find that the rare-earth element modulates the Ni-derived bands and hopping integrals through a chemical pressure effect. These findings clarify the role of rare-earth elements in shaping the low-energy electronic structure and establish the presence of electridelike interstitial s states in a correlated oxide system, where electrons occupy lattice voids rather than atomic orbitals. The electridelike character offers new insight into the self-doping and superconductivity in infinite-layer nickelates.