Electronic structures and multi-orbital models of La$_3$Ni$_2$O$_7$ thin films at ambient pressure

TL;DR

This work develops a comprehensive theoretical framework for La$_3$Ni$_2$O$_7$ thin films at ambient pressure by combining DFT with tight-binding models. It introduces a double-stacked two-orbital (Ni-$d_{x^2-y^2}$ and Ni-$d_{z^2}$) model for One-UC/Three-UC slabs and extends to high-energy $dp$ models that include O-$p$ orbitals, capturing the key Fermi-surface pockets $(\alpha,\alpha',\beta,\gamma,\gamma')$ and their orbital character. The study finds that inter-stack coupling splits Ni-$d_{z^2}$ bands, yields three electron pockets and two hole pockets (with gamma pockets dominated by $d_{z^2}$), and that gamma-pocket nesting enhances magnetic correlations as shown by RPA spin-susceptibility calculations. These results link dimensionality and strain to electronic structure and magnetism, offering a solid foundation for understanding potential superconductivity in ambient-pressure nickelate thin films and guiding future experimental validations and strain-engineering strategies.

Abstract

The recent discovery of superconductivity with a transition temperature $T_c$ exceeding 40 K in La$_3$Ni$_2$O$_7$ and (La,Pr)$_{3}$Ni$_2$O$_7$ thin films at ambient pressure marks a significant breakthrough in the field of nickelate superconductors. Using density functional theory (DFT), we propose a double-stacked two-orbital effective model for La$_3$Ni$_2$O$_7$ thin film based on the Ni$-e_g$ orbitals. Our analysis of the Fermi surface reveals three electron pockets ($α,α^{\prime},β$) and two hole pockets ($γ,γ^{\prime}$), where the additional $α^{\prime}$ and $γ^{\prime}$ pockets arise from inter-stack interactions. Furthermore, we introduce a high-energy model that incorporates O$-p$ orbitals to facilitate future studies. Calculations of spin susceptibility within the random phase approximation (RPA) indicate that magnetic correlations are enhanced by nesting of the $γ$ pocket, which is predominantly derived from the Ni$-d_{z^2}$ orbital. Our results provide a theoretical foundation for understanding the electronic and magnetic properties of La$_3$Ni$_2$O$_7$ thin films.

Figures (7)

• Figure 1: Slab structure and schematic illustration of the hopping parameters used in the La$_3$Ni$_2$O$_7$ thin-film model. (a) The free-standing One-UC slab structure of La$_3$Ni$_2$O$_7$, where the two bilayers are labeled as Stack 1 and Stack 2. Green, gray, and red spheres denote La, Ni, and O atoms, respectively. The outer-apical ($d_1$ and $d_4$), inner-apical ($d_2$ and $d_3$), and in-plane ($d_5$) Ni-O bond lengths are indicated by black, red, and blue arrows, respectively. (b) Schematic illustration of the hopping parameters in the La$_3$Ni$_2$O$_7$ thin films, highlighting the Ni$-d_{x^2-y^2}$ (orange) and $d_{z^2}$ (blue) orbitals. Only nearest-neighbor hopping terms are shown.
• Figure 2: DFT-calculated band structures and partial density of states (PDOS) of La$_3$Ni$_2$O$_7$ thin films. Panels (a) and (b) correspond to calculations with $U_{eff}=0$ eV, and panels (c) and (d) correspond to $U_{eff}=2$ eV. The contributions from the Ni$-d_{x^{2}-y^{2}}$ and $d_{z^2}$ orbitals are highlighted in blue and red, respectively, while Ni$-t_{2g}$, O-$p$, and La states are represented in orange, green, and cyan, respectively. The Fermi level ($E_{F}$) is set to 0 eV. Here, $U_{eff}$ denotes the effective hubbard parameter, and UC represents unit cell.
• Figure 3: DFT-calculated two-dimensional Fermi surfaces of La$_3$Ni$_2$O$_7$ thin films for different slab models. Panels (a) and (c) correspond to the One-UC structure, while panels (b) and (d) corresponds to the Half-UC structure. The upper panels show results obtained with $U_{eff}=0$ eV, and the lower panels correspond to $U_{eff} = 2$ eV. Here, $U_{eff}$ denotes the effective hubbard parameter, and UC represents unit cell.
• Figure 4: Band structures and Fermi surfaces of the two-obital models for La$_3$Ni$_2$O$_7$ thin films. Panels (a) and (c) correspond to the double-stacked two-orbital model for the One-UC slab, while panels (b) and (d) correspond to the single-stacked two-orbital model for the Half-UC slab. The color bar indicates the orbital weights, with red and blue represent $d_{x^{2}-y^{2}}$ and $d_{z^2}$, respectively. In panels (a) and (b), the gray lines represent the DFT-calculated band structures with $U_{eff}=0$ eV. The $E_{F}$ is set to 0 eV. Here, $U_{eff}$ denotes the effective hubbard parameter, and UC represents unit cell.
• Figure 5: Band structures and Fermi surfaces of High-energy $dp$ models including oxeygen orbitals for La$_3$Ni$_2$O$_7$ thin films. Panels (a) and (c) correspond to the eleven-orbital model for the Half-UC slab, while panels (b) and (d) correspond to the twenty-two-orbital model for the One-UC slab. The color bar indicates the orbital weights, with red and blue represent $d_{x^{2}-y^{2}}$ and $d_{z^2}$, respectively. In panels (a) and (b), the gray lines represent the DFT-calculated band structures with $U_{eff}=0$ eV. The $E_{F}$ is set to 0 eV. Here, $U_{eff}$ denotes the effective hubbard parameter, and UC represents unit cell.