Spin excitations in Nd 1 − x Sr x NiO 2 and YBa 2 Cu 3 O 7 − δ : The influence of Hubbard U

We use resonant inelastic x-ray scattering (RIXS) to compare the doping dependence of magnetic excitations of an infinite-layer nickelate to those of a prototypical superconducting cuprate. The polarization analysis of RIXS spectra establishes the dominant spin-flip nature of the midinfrared peak in both cases. Hole doping leads to opposite behavior of the magnetic energy in the two materials. By fitting the data with an original Hubbard-based model for dynamic susceptibility, we find that $t$ is comparable in the two materials while $U$ is about twice larger in the nickelate. This finding accounts for the smaller magnetic bandwidth of nickelates and for its decrease upon doping.