Nickelate superconductors $--$ Multiorbital nature and spin freezing

TL;DR

This paper explores the multiorbital nature and spin-freezing phenomena in nickelate superconductors, highlighting how Hund coupling and crystal field effects influence their unconventional superconductivity.

Contribution

It demonstrates the importance of multiorbital models and spin-freezing theory to understand the superconducting behavior in doped nickelates.

Findings

Nickelate superconductors require a multiorbital description, especially in heavily doped regimes.

Spin-freezing crossovers are key to understanding the electronic phases in nickelates.

The system exhibits two distinct spin-freezing regimes related to doping levels.

Abstract

Superconductivity with a remarkably high $T_c$ has recently been found in Sr-doped 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. Sr$_{0.2}$Nd$_{0.8}$NiO$_2$ falls into a parameter regime influenced by two spin-freezing crossovers, one related to the emergent multi-orbital nature in the strongly doped regime and the other related to the single-band character and square lattice geometry in the weakly doped regime.