2025/06/19 by Khaliullin, Giniyat, Chaloupka, Jiří · 2 citations
#FOS: Physical sciences #Strongly Correlated Electrons (cond-mat.str-el)
paper · doi:10.48550/arxiv.2506.16360
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.