Hund's metal physics: from SrNiO$_2$ to NdNiO$_2$.

We study the normal state electronic structure of the recently discovered infinite-layer nickelate superconductor, Nd$_{1-x}$Sr$_x$NiO$_2$. Using SrNiO$_2$ as a reference, we find that Nd$_{1-x}$Sr$_x$NiO$_2$ is a multi-orbital electronic system with characteristic Hund's metal behaviors, such as metallicity, the importance of high-spin configurations, tendency towards orbital differentiation, and the absence of magnetism in regimes which are ordered according to static mean-field theories. In addition, our DFT+DMFT calculations with exact double counting scheme show that despite large charge carrier doping from SrNiO$_2$ to LaNiO$_2$, the Ni-$3d$ total occupancy is barely changed due to the decreased hybridization with the occupied oxygen-2$p$ states, and increased hybridization with the unoccupied La-5$d$ states. Our results are in good agreement with the existing resonant inelastic x-ray scattering measurements and pave the way to understand the pairing mechanism of Nd$_{1-x}$Sr$_x$NiO$_2$.