In-Plane Ni-O-Ni Bond Angles as Structural Fingerprints of Superconductivity in Layered Nickelates: Effects of Pressure, Strain, Layering, and Correlations

TL;DR

This study uses DFT+$U$ calculations to identify in-plane Ni-O-Ni bond angles as key structural indicators of superconductivity in layered nickelates, showing how pressure, strain, and correlations influence their electronic and structural properties.

Contribution

It demonstrates that in-plane Ni-O-Ni bond angles serve as structural fingerprints for superconductivity, linking external conditions to electronic structure and phase behavior in layered nickelates.

Findings

Bond angles correlate with $T_c$ and can serve as proxies.

Pressure and strain straighten bond angles, affecting superconductivity.

Electronic correlations influence structural transitions and orbital hybridization.

Abstract

We investigate the structural and electronic conditions conducive to superconductivity in layered nickelates using density functional theory with Hubbard corrections (DFT+$U$). For both the bilayer and 1-3 polymorphs of La$_3$Ni$_2$O$_7$, we find that the in-plane Ni-O-Ni bond angles under pressure strongly correlate with the experimentally observed superconducting transition temperature ($T_c$) dome, and may serve as a reasonable proxy. Under compressive strain, the bond angles straighten, peaking near 2\% strain-consistent with experimental reports of superconductivity in strained bilayer thin films. However, the bond angles at this strain are more bent than those achieved under hydrostatic pressure, correlating with a lower $T_c$. We show that increasing the number of NiO$_2$ layers, as in La$_4$Ni$_3$O$_{10}$, or substituting heavier rare-earth elements (e.g., Pr) raises the pressure required to reach the structural configuration associated with superconductivity. Our results indicate that these systems require higher external pressure to achieve in-plane bond straightening. Varying the on-site Coulomb interaction $U$ reveals that stronger electronic correlations delay the structural transition and favor high-spin states. This suggests that moderate correlation strength may be optimal for superconductivity, with stronger correlation preventing the formation of favorable bond geometries. Electronic structure analysis shows that the Ni $e_g$ orbitals dominate near the Fermi level and shift downward with pressure, enhancing Ni-O hybridization. These results highlight how pressure and strain tune structural features that may be essential for engineering high-$T_c$ phases in nickelate superconductors.