Two-band model for magnetism and superconductivity in nickelates

TL;DR

This paper introduces a two-band Hubbard model for nickelates to explore the interplay of magnetism and superconductivity, revealing potential pairing symmetries and effective interactions relevant to unconventional superconductivity.

Contribution

It develops a two-band Hubbard model incorporating key orbitals and analyzes the effective exchange interactions and pairing symmetries in doped nickelates.

Findings

Identification of competing intra- and inter-orbital configurations.

Classification of possible superconducting pairing symmetries.

Derivation of effective exchange interactions among various doped states.

Abstract

The recently discovered superconductivity in Nd$_{1-x}$Sr$_x$NiO$_2$ provides a new opportunity for studying strongly correlated unconventional superconductivity. The single-hole Ni$^+$ ($3d^9$) configuration in the parent compound NdNiO$_2$ is similar to that of Cu$^{2+}$ in cuprates. We suggest that after doping, the intra-orbital spin-singlet and inter-orbital spin-triplet double-hole (doublon) configurations of Ni$^{2+}$ are competing, and we construct a two-band Hubbard model by including both the $3d_{x^2-y^2}$ and $3d_{xy}$-orbitals. The effective spin-orbital super-exchange model in the undoped case is a variant of the $SU(4)$ Kugel-Khomskii model augmented by symmetry breaking terms. Upon doping, the effective exchange interactions between spin-$\frac{1}{2}$ single-holes, spin-1 (triplet) doublons, and singlet doublons are derived. Possible superconducting pairing symmetries are classified in accordance to the $D_{4h}$ crystalline symmetry, and their connections to the superexchange interactions are analyzed.