Orbital order and superconductivity in bilayer nickelate compounds

We propose a theory for bilayer nickelate materials, where a large tetragonal field—intrinsic or induced by epitaxial strain—lifts the orbital degeneracy and localizes the 3 z 2 − r 2 orbital states. These states host local spins S = 1 / 2 bound into singlets by strong interlayer coupling, and their dynamics is described by weakly dispersive singlet-triplet excitations (“triplons”). The charge carriers occupy the wide bands of x 2 − y 2 symmetry, and their Cooper pairing is mediated by the high-energy triplon excitations. As the x 2 − y 2 band filling increases, i.e., moving further away from the Ni 3 + valence state, the indirect Ruderman-Kittel-Kasuya-Yosida interactions between local spins induce spin-density-wave order via triplon condensation. Implications of the model for compressively strained La 3 Ni 2 O 7 films and electron-doped oxychloride Sr 3 Ni 2 O 5 Cl 2 are discussed.