Formation of a two-dimensional single-component correlated electron system and band engineering in the nickelate superconductor NdNiO₂

Motivated by the recent experimental discovery of superconductivity in the infinite-layer nickelate Nd_0.8Sr_0.2NiO₂ [Li et al., Nature (London) 572, 624 (2019)], we study how the correlated Ni 3d_x²-_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 3d_x²-_y², Nd 5d_3z²-_r², and a bonding orbital made from an interstitial s orbital in the Nd layer and the Nd 5d_xy orbital. By constructing a three-orbital model for these states, we find that the hybridization between the Ni 3d_x²-_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 3d_x²-_y² electrons just by 10%-20%. On the other hand, the electron-phonon coupling is not strong enough to mediate superconductivity of T_c ∼ 10 K. These results indicate that NdNiO₂ hosts an almost isolated correlated 3d_x²-_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 5d_xy state becomes small.