Orbital order and superconductivity in bilayer nickelate compounds

Abstract

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 3z2−r2 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 x2−y2 symmetry, and their Cooper pairing is mediated by the high-energy triplon excitations. As the x2−y2 band filling increases, i.e., moving further away from the Ni3+ 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 La3Ni2O7 films and electron-doped oxychloride Sr3Ni2O5Cl2 are discussed.

Source-reported materials — not catalogue approval

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