Bilayer two-orbital model of La$_3$Ni$_2$O$_7$ under pressure

TL;DR

This paper develops a minimal bilayer two-orbital model for La$_3$Ni$_2$O$_7$ under pressure, capturing its electronic structure and magnetic properties, to understand its high-temperature superconductivity at 80 K.

Contribution

The authors propose a simplified bilayer two-orbital model based on DFT calculations that accurately describes the electronic and magnetic features of La$_3$Ni$_2$O$_7$ under pressure.

Findings

Model captures key band structure and Fermi surface topology.

Reveals magnetic enhancement related to Ni-d$_{3z^2-r^2}$ orbital.

Provides a higher energy model including O-p orbitals.

Abstract

The newly discovered Ruddlesden-Popper bilayer La$_3$Ni$_2$O$_7$ reaches an remarkable superconducting transition temperature $T_c$ = 80 K under a pressure of above 14 GPa. Here we propose a minimal bilayer two-orbital model of the high-pressure phase of La$_3$Ni$_2$O$_7$. Our model is constructed with the Ni-3d$_{x^2-y^2}$, 3d$_{3z^2-r^2}$ orbitals by using Wannier downfolding of the density functional theory calculations, which captures the key ingredients of the material, such as band structure and Fermi surface topology. There are two electron pockets $\alpha$, $\beta$ and one hole pocket $\gamma$ on the Fermi surface, in which the $\alpha$, $\beta$ pockets show mixing of two orbitals, while the $\gamma$ pocket is associated with Ni-d$_{3z^2-r^2}$ orbital. The RPA spin susceptibility reveals a magnetic enhancement associating to the d$_{3z^2-r^2}$ state. A higher energy model with O-p orbitals is also provided for further study.