For years, various systems have been considered cuprate analogues and superconductivity has been pursued profusely in some of them. Finally, in 2020 superconductivity was found in infinite-layered nickelates[1], that share some important structural and electronic-structure features with superconducting cuprates.

Infinite-layer nickelates are nominally Ni⁺: d⁹ systems, but they are metallic and are largely self-doped (the actual filling of the d band is less than 9 electrons). Our calculations[2] show that doping with Sr modifies the self-doping effect, leading to a situation more similar to that in the cuprates. E.g. Ni²⁺ dopants are non-magnetic, just like the Zhang-Rice singlets in the cuprates.

Although LaNiO₂ or NdNiO₂ are non-magnetic metals, an antiferromagnetic insulating phase exists in the phase diagram of the superconducting cuprates when the dimensionality of the system is reduced. This can be done in various ways, as our ab initio calculations show[3].

Infinite-layer nickelates are part of a more general family of layered nickelates with formula R_n+1Ni_nO_2n+2, that span different doping regimes of the Ni d band (increasing as n increases). The confinement of the NiO₂ layers introduces some differences in terms of the peculiar occupation of the Ni d_z2 orbitals and also changing the magnetic properties[4,5].

It has been found[6] that Pr₄Ni₃O₈ presents a large orbital polarization of the d manifold, resembling the electronic structure of the cuprates. Electron-doping this system[7] would recover the electronic structure of the parent phase of the cuprates, showing a promising way to search for superconductivity within this series.

[1] Danfeng Li et al., Nature 572, 624 (2019).

[2] J. Krishna et al., Phys. Rev. B 102, 224506 (2020).

[3] V. Pardo, A.S. Botana, arxiv/2012.02711 (2020).

[4] V. Pardo, W.E. Pickett, Phys. Rev. Lett. 105, 266402 (2010).

[5] A.S. Botana, V. Pardo et al., Phys. Rev. B 94, 081105 (2016).

[6] J. Zhang, A.S. Botana et al., Nature Physics 13, 864 (2017).

[7] A.S. Botana, V. Pardo, M.R. Norman, Phys. Rev. Materials 1, 021801(R) (2017).