Theoretical proposal of superconductivity in hole-doped reduced bilayer nickelate La3Ni2O6: a manifestation of orbital-space bilayer model with incipient bands

TL;DR

This paper proposes that hole-doped La3Ni2O6 can exhibit superconductivity through an orbital-space bilayer model, with calculations indicating $s ext{±}$-wave pairing driven by interorbital interactions in an incipient-band regime.

Contribution

It introduces the orbital-space bilayer model (OSBM) for La3Ni2O6 and predicts superconductivity driven by interorbital interactions in an incipient-band scenario.

Findings

Large orbital level offset due to absence of outer apical oxygens.

Prediction of $s ext{±}$-wave superconductivity driven by interorbital interactions.

Different pairing mechanisms in La3Ni2O6 compared to La3Ni2O7.

Abstract

A correspondence exists between the multi-orbital Hubbard model and the bilayer Hubbard model, in which superconductivity is optimized in an incipient-band regime in both cases. In the multi-orbital system, the orbital level offset $\Delta E$ plays a role analogous to the interlayer hopping in bilayer systems, and superconductivity is enhanced for large $\Delta E$. We refer to such a multi-orbital model as an orbital-space bilayer model (OSBM). In this study, we theoretically propose that a reduced bilayer nickelate La$_3$Ni$_2$O$_6$ can be a candidate for a superconductor described by OSBM when an appropriate amount of holes is doped. By constructing a tight-binding model based on first-principles calculations, a large $\Delta E$ between the Ni $d_{x^2-y^2}$ and the other $d$ orbitals is obtained due to the absence of outer apical oxygens. Furthermore, our fluctuation exchange approximation calculations indicate the emergence of $s\pm$-wave superconductivity driven by interorbital interactions in an incipient-band situation, where the superconducting gap function changes its sign between the $d_{x^2-y^2}$ and other $d$ orbital bands. We also investigate the energetic and dynamical stability of the crystal structure under atomic substitution and pressure. Although La$_3$Ni$_2$O$_7$ and La$_3$Ni$_2$O$_6$ share a similar chemical formula, our study shows that an entirely different pairing mechanism can take place in the latter.