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 Nd0.8Sr0.2NiO2 [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 similar to 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. By an extensive materials search, we find that the Fermi pockets almost disappear in NaNd2NiO4 and NaCa2NiO3.