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 $3z^2-r^2$ 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.

7 pages, 4 figures, small changes in the text, published version

Cited by in corpus (1)

• Origin of Spin Stripes in Bilayer Nickelate La${}_3$Ni${}_2$O${}_7$