Designing nickelate superconductors with large crystal field splitting exploiting mixed-anion strategy

Based on a combination of first principles and many-body calculations, we design mixed-anion nickelate superconductors with d^8+δ electron configuration [1]. We adopt halogens or hydrogens as out-of-plane anions, which results in the band structure with large energy level offset between d_x²_-y² and the other 3d orbitals. We find that if small amount of electrons are doped into the system, the four 3d band other than d_x²_-y² being incipient and s±wave superconductivity is strongly enhanced. This result can be understood by the mathematical equivalence between the two-orbital model and the bilayer Hubbard model [2]. In fact, we can show, by orbital basis transformation, that the large level offset is approximately equivalent to large interlayer hopping in a bilayer system. The bilayer Hubbard model is known to exhibit high T_c s±wave superconductivity when the interlayer hopping is several times larger than the intralayer one and one of the bands located below Fermi level.
We further find that our theory may also be related to a newly discovered superconductor (Nd,Sr)NiO₂ [3]. Details on this point will also be given in the presentation.
[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)