Superconductivity with a remarkably high $T_c$ has recently been found in Sr-doped ${\mathrm{NdNiO}}_2$ thin films. While this system bears strong similarities to the cuprates, some differences, such as a weaker antiferromagnetic exchange coupling and possible high-spin moments on the doped Ni sites have been pointed out. Here, we investigate the effect of Hund coupling and crystal field splitting in a simple model system and argue that a multiorbital description of nickelate superconductors is warranted, especially in the strongly hole-doped regime. We then look at this system from the viewpoint of the spin-freezing theory of unconventional superconductivity, which provides a unified understanding of unconventional superconductivity in a broad range of compounds. ${\mathrm{Nd}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_2$ falls into a parameter regime influenced by two spin-freezing crossovers; one related to the emergent multiorbital nature in the strongly doped regime and the other related to the single-band character and square lattice geometry in the weakly doped regime.

• Received 1 October 2019
• Revised 22 November 2019

DOI:https://doi.org/10.1103/PhysRevB.101.041104