The recent discovery of superconductivity in the doped nickelate Nd_0.8Sr_0.2NiO₂ has opened up a great opportunity to unravel the mystery of superconductivity in correlated materials. While the electronic structure of the nickelate is similar to that of the celebrated cuprates, there is one distinct difference: not only the Ni 3dx₂-y₂ electrons but electrons in the Nd layer also form the Fermi surface. The electronic structure around the Fermi level is well described by the Ni 3dx₂-y₂, Nd 5d3z₂-r₂, and a bonding orbital made from the interstitial s and Nd 5dxy orbitals. The hybridization between the Ni 3dx₂-y₂ and Nd-layer states is small, so that the screening effect of the Nd-layer states is less effective, which leaves the Ni 3dx₂-y₂ electrons strongly correlated. On the other hand, the electron-phonon coupling constant is not strong enough to mediate superconductivity of T_c~ 10 K. These results indicate that NdNiO₂ hosts an almost isolated correlated Ni 3dx₂-y₂ orbital system with a self-doping due to the Nd-layer-state Fermi pockets [1]. Furthermore, we provide a useful guideline to eliminate the complication due to the self-doping and realize prototypical d⁹ nickelates [2].

[1] Y. Nomura et al., arXiv:1909.03942.
[2] M. Hirayama, T. Tadano et al., arXiv:1910.03974.