Inspired by a recently proposed superconducting mechanism [1] for a new cuprate superconductor Ba₂CuO_3+δ [2], we theoretically design unconventional nickelate superconductors with near d⁸ electron configuration [3]. Although the materials that we design apparently have nothing to do with bilayer systems at first glance, electronic structure of these materials resemble that of the bilayer Hubbard model, thereby strongly enhancing superconductivity. It is known that the bilayer Hubbard model with appropriately large interlayer electron hopping exhibits s±wave superconductivity with extremely high T_c, owing to its contribution to the pairing interaction from the incipient band, but it is generally difficult to realize such a situation in actual bilayer-type materials due to various restrictions regarding the atomic orbitals. In the present study, we adopt a completely different strategy to realize the desired situation, by which we design a mixed-anion nickelates as candidates for new high-T_c superconductors. The key ingredient is that we can show, by orbital basis transformation,  that the large crystal field splitting of the orbital energy levels, induced by replacing the apical oxygens by halogens or hydrogens, is approximately equivalent to large interlayer hopping in a bilayer system.

We further find that our theory may also be related to another newly discovered superconductor (Nd,Sr)NiO₂ [4]. In this superconductor, the nickel electron configuration is expected to be close to d⁹, but this might be affected, e.g., by the reduction process during the synthesis of the material. Since the band structure of (Nd,Sr)NiO₂ itself , due to the absence of apical oxygens, resembles that of the proposed mixed anion-nickelates,  (unintentional) reduction of the electron number in (Nd,Sr)NiO₂ may also result in a similar s±wave superconductivity.

[1] K. Yamazaki et al., Phys. Rev. Res. 2, 033356 (2020).
[2] W.M. Li et al., PNAS 116, 12156 (2019).
[3] N. Kitamine et al., arXiv:2007.01153.
[4] D. Li et al., Nature 572, 624 (2019).

Keywords: nickelate superconductor, mixed-anion strategy, bilayer Hubbard model, incipient band