Bilayer Two-Orbital Model of La3Ni2O7 under Pressure

The newly discovered Ruddlesden-Popper bilayer ${\mathrm{La}}_{3}{\text{Ni}}_{2}{\mathrm{O}}_{7}$ reaches a remarkable superconducting transition temperature ${T}_{c}\ensuremath{\approx}80\text{ }\text{ }\mathrm{K}$ under a pressure of above 14 GPa. Here we propose a minimal bilayer two-orbital model of the high-pressure phase of ${\mathrm{La}}_{3}{\text{Ni}}_{2}{\mathrm{O}}_{7}$. Our model is constructed with the Ni-$3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$, $3{d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ 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, $\ensuremath{\alpha}$, $\ensuremath{\beta}$, and one hole pocket, $\ensuremath{\gamma}$, on the Fermi surface, in which the $\ensuremath{\alpha}$, $\ensuremath{\beta}$ pockets show mixing of two orbitals, while the $\ensuremath{\gamma}$ pocket is associated with $\mathrm{Ni}\text{\ensuremath{-}}{d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ orbital. The random phase approximation spin susceptibility reveals a magnetic enhancement associated with the ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ state. A higher energy model with $\mathrm{O}\text{\ensuremath{-}}p$ orbitals is also provided for further study.