Formation of a two-dimensional single-component correlated electron system and band engineering in the nickelate superconductor <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>NdNiO</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>

TLDR

Superconductivity was recently observed in the infinite‑layer nickelate Nd0.8Sr0.2NiO2, prompting investigation of how the Ni 3dₓ₂₋ᵧ₂ electrons in the NiO₂ layer couple to electrons in the Nd layer. The authors aim to determine whether an almost isolated single‑orbital Ni 3dₓ₂₋ᵧ₂ system can be realized and to explore the possibility of a more ideal single‑orbital Mott‑Hubbard regime. They find that the low‑energy electronic structure of NdNiO₂ requires three orbitals—Ni 3dₓ₂₋ᵧ₂, Nd 5d₃z²₋r², and a bonding orbital from interstitial s and Nd 5d_xy—but the Ni–Nd hybridization is negligible, the Nd layer only modestly screens the Hubbard U (by 10–20 %), electron‑phonon coupling is too weak to support Tc ≈ 10 K, and the material behaves as an almost.

Abstract

Motivated by the recent experimental discovery of superconductivity in the infinite-layer nickelate Nd0.8Sr0.2NiO2 [Li et al., Nature 572, 624 (2019)], we study how the correlated Ni 3dx2-y2 electrons in the NiO2 layer interact with the electrons in the Nd layer. We show that three orbitals are necessary to represent the electronic structure around the Fermi level: Ni 3dx2-y2, Nd 5d3z2-r2, and a bonding orbital made from an interstitial s orbital in the Nd layer and the Nd 5dxy orbital. By constructing a three-orbital model for these states, we find that the hybridization between the Ni 3dx2-y2 state and the states in the Nd layer is tiny. We also find that the metallic screening by the Nd layer is not so effective in that it reduces the Hubbard U between the Ni 3dx2-y2 electrons just by 10--20 %. On the other hand, the electron-phonon coupling is not strong enough to mediate superconductivity of Tc ~ 10 K. These results indicate that NdNiO2 hosts an almost isolated correlated 3dx2-y2 orbital system. We further study the possibility of realizing a more ideal single-orbital system in the Mott-Hubbard regime. We find that the Fermi pockets formed by the Nd-layer states dramatically shrink when the hybridization between the interstitial s state and Nd 5dxy state becomes small. By an extensive materials search, we find that the Fermi pockets almost disappear in NaNd2NiO4 and NaCa2NiO3.