Abstract Body

The recently discovered Ruddlesden-Popper bilayer superconductor La₃Ni₂O₇ has attracted widespread attention due to its superconducting transition temperature exceeding the boiling point of liquid nitrogen, reaching approximately 80 K under a pressure. The superconducting phase under high pressure has an orthorhombic structure of Fmmm space group with the 3d_x2−y2 and 3d_z2 orbitals of Ni cations strongly mixing with oxygen 2p orbitals. Our density functional theory calculations indicate that the superconductivity emerges coincidently with the metallization of the σ-bonding bands under the Fermi level, consisting of the 3d_z2 orbitals with the apical oxygen ions connecting the Ni–O bilayers. Here we propose a bilayer two-orbital model for La₃Ni₂O₇ under high pressure, primarily based on the 3d_x2−y2 and 3d_z2 orbitals of Ni. Through Wannier downfolding and symmetry analysis, we obtain parameters such as electron hopping and site-energy, which provide an excellent description of the electronic band structure and Fermi surface. We find that the Fermi surface of the high-pressure phase consists of three pockets, with α and β being electron pockets, and γ being a hole pocket (mainly originating from the 3d_z2 orbital). To explicitly consider the physics of O-p orbitals, we introduce a higher energy model (eleven-orbital model).  Based on these models, we study the charge transfer, Zhang-Rice singlet bands, pairing symmetry, and superconducting transition temperature in La₃Ni₂O₇. We obtain a comprehensive superconducting phase diagram in the doping plane and find that the La₃Ni₂O₇ under pressure is situated roughly in the optimal doping regime of the phase diagram. Recently, the discovery of superconductivity in Ruddlesden-Popper (RP) La₄Ni₃O₁₀ under pressure has further expanded the realm of nickelate-based superconductor family. Based on the DFT results, we further propose a trilayer two-orbital model by performing Wannier downfolding on Ni-eg orbitals. Our model reveals four Fermi surface sheets with α, β, β′, γ pockets, bearing resemblance to that of bilayer La₃Ni₂O₇. According to the model, our calculated spin susceptibility under random phase approximation predicts an analogous magnetic signal at q = (π/2, π/2), which is more associated with nesting within β, β′ pocktes. Finally, a high energy sixteen-orbital model with direct dp, pp hoppings is proposed, which implies that La₄Ni₃O₁₀ also lies in charge-transfer picture within Zaanen-Sawatzky-Allen scheme. Our exposition of electronic reconstructions and multi-orbital models shed light on theoretical electronic correlation study and experimental exploration for lower pressure in RP series.

References

[1] Nature 621, 493 (2023).

[2] Phys. Rev. Lett. 131, 126001 (2023).

[3]Nature Communications 15，4373 (2024).

[4] SCPMA 67, 117403 (2024).

[5] SCPMA 67, 117402 (2024).

[6] arXiv:2308.16564 (NPJ Quantum Materials accepted).

[7] Phys. Rev. B 110, 014503 (2024).

[8] Chinese Phys. Lett. 41 077402 (2024).

[9] arXiv: 2404.11001

[10] arXiv:2405.19161.

[11] arXiv:2407.19213.

Acknowledgment

This project is supported by NKRDPC-2022YFA1402802, NSFC-92165204, Leading Talent Program of Guangdong Special Projects (201626003), Guangdong Provincial Key Laboratory of Magnetoelectric Physics and Devices (No. 2022B1212010008), and Guangdong Research Center for Magnetoelectric Physics.

Figure 1. Hopping integrals in the bilayer two-orbital model of La₃Ni₂O₇ with the Ni-𝑑_{𝑥2−𝑦2} (red) and 𝑑_{3𝑧2−𝑟2} (blue) orbitals. (b) Fermi surfaces determined by the bilayer two-orbital model. Reprinted from Ref. [2].

Keywords: High-Tc superconductivity, Nickelate superconductor, Theoretical Models, Pairing symmetry, Pressure