Fermi surface reconstruction and enhanced spin fluctuations in strained La$_3$Ni$_2$O$_{7}$ on LaAlO$_3$(001) and SrTiO$_3$(001)

Abstract: We explore the structural and electronic properties of the bilayer nickelate La3Ni2O7 on LaAlO3(001) and SrTiO3(001) by using density functional theory including a Coulomb repulsion term. For La$3$Ni$_2$O${7}$/LaAlO$3$(001), we find that compressive strain and electron doping across the interface result in the unconventional occupation of the antibonding Ni $3d{z^2}$ states. In sharp contrast, no charge transfer is observed for La$3$Ni$_2$O${7}$/SrTiO$3$(001). Surprisingly, tensile strain drives a metallization of the bonding Ni $3d{z^2}$ states, rendering a Fermi surface topology akin to superconducting bulk La$3$Ni$_2$O${7}$ under high pressure, yet with spin fluctuations enhanced considerably beyond pressure effects. Concomitantly, significant octahedral rotations are retained. We discuss the fundamental differences between hydrostatic pressure versus epitaxial strain and establish that strain provides a much stronger control over the Ni $e_g$ orbital polarization. The results suggest epitaxial La$3$Ni$_2$O${7}$, particularly under tensile strain, as interesting system to provide novel insights into the physics of bilayer nickelates and possibly induce superconductivity without external pressure.