Possible incipient-band-induced unconventional superconductivity in nickelates with large crystal field splitting

We theoretically design nickelate superconductors with d^8+δ electron configuration [1]. We exploit the mixed anion strategy to enhance the crystal field splitting, so that only the d_x²-y² band intersects the Fermi level, while all the other four 3d bands are incipient. The idea is inspired by a recent study [2] which shows that a two-orbital model with large level offset exhibits high T_c s±wave superconductivity when one of the bands is incipient. Through orbital basis transformation, the large level offset in the two-orbital model is shown to correspond to a large interlayer hopping in a bilayer system, which is known to give rise to high T_c s±wave superconductivity.
We further find that our theory may also be related to a newly discovered superconductor (Nd,Sr)NiO₂ [3]. In this superconductor, the electron configuration is expected to be close to d⁹, but this might be affected, e.g., by the reduction process during the synthesis. Since the band structure of (Nd,Sr)NiO₂ itself , due to the absence of apical oxygens, resembles that of the mixed anion-nickelates, it may also give rise to a similar s±wave superconductivity.
[1] N. Kitamine et al., arXiv:2007.01153.
[2] K. Yamazaki et al., Phys. Rev. Res. 2, 033356 (2020).
[3] D. Li et al., Nature 572, 624 (2019)