Capping Effects in Parent and Superconducting Nd_1−xSr_xNiO₂

The discovery of superconductivity in infinite layer nickelates (Nd_1−xSr_xNiO₂) has attracted a lot of attention. So far, superconductivity in Nd_1−xSr_xNiO₂ is a prerogative of thin films making the role of substrates and interfaces crucial. On this end, capping has been shown to affect the interface structure highlighting its active role on the electronic and magnetic properties of nickelates. However, only few spectroscopic studies specifically address this topic. Here, we use Resonant Inelastic X-ray Scattering (RIXS) to investigate the influence of the SrTiO₃ capping layer on the excitations of Nd_1−xSr_xNiO₂ (x = 0 and 0.2). Spin excitations are observed in Nd_1−xSr_xNiO₂regardless of capping. While doping softens the spin excitations, capping moderately hardens them in both parent and superconducting samples and increases their spectral weight. Additionally, the hybridization between Ni 3d and Nd 5d orbitals is reduced by capping, as evidenced by a weaker charge transfer peak at ∼ 0.6 eV. These observations can be rationalized phenomenologically in the context of the polar discontinuity at the NdNiO₂-SrTiO₃ interface leading to the atomic and electronic reconstruction, possibly extending to the whole film. Finally, our findings uncover the effects of SrTiO₃ capping on the spin and charge excitations in Nd_1−xSr_xNiO₂ and pave the way for a general understanding of infinite layer nickelates thin films.