Two-band model for magnetism and superconductivity in nickelates

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.

Accepted by Phys. Rev. Research