Orbital Order and Superconductivity in Bilayer Nickelate Compounds

TL;DR

This paper develops a theoretical model for bilayer nickelates, explaining how orbital effects, local spins, and triplon excitations influence superconductivity and magnetic order, with implications for specific strained and doped nickelate compounds.

Contribution

It introduces a novel theory linking orbital splitting, local spin dynamics, and triplon-mediated pairing in bilayer nickelates, explaining their superconducting and magnetic behaviors.

Findings

Orbital degeneracy is lifted by a tetragonal field, localizing certain orbitals.

Superconductivity is mediated by high-energy triplon excitations.

Spin-density-wave order emerges via triplon condensation as band filling increases.

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 $3z^2-r^2$ 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.