Two-band model for magnetism and superconductivity in nickelates Nd_1-xSr_xNiO₂

The recently discovered superconductivity in Nd_1-xSr_xNiO₂
provides a new opportunity for studying strongly correlated
unconventional superconductivity.
The single-hole Ni⁺ (3d⁹) configuration in the parent compound
NdNiO₂ is similar to that of Cu²⁺ in cuprates.
We suggest that after doping, the intra-orbital spin-singlet and
inter-orbital spin-triplet double-hole
(doublon) configurations of Ni²⁺ 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-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.