Superconductivity in nickelate and cuprate superconductors with strong bilayer coupling

Abstract

The discovery of superconductivity at 80 K under high pressure in ${\mathrm{La}}_{3}{\mathrm{Ni}}_{2}{\mathrm{O}}_{7}$ presents the groundbreaking confirmation that high-${T}_{c}$ superconductivity is a property of strongly correlated materials beyond cuprates. We use density functional theory calculations of the band structure of ${\mathrm{La}}_{3}{\mathrm{Ni}}_{2}{\mathrm{O}}_{7}$ under pressure to verify that the low-energy bands are composed almost exclusively of Ni $3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ and O $2p$ orbitals. We deduce that the Ni $3{d}_{{z}^{2}}$ orbitals are essentially decoupled by the geometry of the high-pressure structure and by the effect of the Ni Hund coupling being strongly suppressed, which results from the enhanced interlayer antiferromagnetic interaction between $3{d}_{{z}^{2}}$ orbitals and the strong intralayer hybridization of the $3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbitals with O $2p$. By introducing a tight-binding model for the Fermi surfaces and low-energy dispersions, we arrive at a bilayer $t\text{\ensuremath{-}}{t}_{\ensuremath{\perp}}\text{\ensuremath{-}}J$ model with strong interlayer hopping, which we show is a framework unifying ${\mathrm{La}}_{3}{\mathrm{Ni}}_{2}{\mathrm{O}}_{7}$ with cuprate materials possessing similar band structures, particularly the compounds ${\mathrm{La}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{6}$, ${\mathrm{Pb}}_{2}{\mathrm{Sr}}_{2}{\mathrm{YCu}}_{3}{\mathrm{O}}_{8}$, and ${\mathrm{EuSr}}_{2}{\mathrm{Cu}}_{2}{\mathrm{NbO}}_{8}$. We use a renormalized mean-field theory to show that these systems should have $(d+is)$-wave superconductivity, with a dominant $d$-wave component and the high ${T}_{c}$ driven by the near-optimally doped $\ensuremath{\beta}$ band, while the $\ensuremath{\alpha}$ band adds an $s$-wave component that should lead to clear experimental signatures.

Topics & Concepts