Magnetic exchange coupling in theoretically-designed cuprate-analog <i>d</i>⁹ nickelates

Motivated by the discovery of superconductivity in doped nickelate R_1-xSr_xNiO₂ (R=Nd, Pr), we study the magnetic exchange coupling J in layered d⁹ nickelates, which would serve as cuprate-analog materials in the Mott-Hubbard regime. The mother compounds of the cuprates are a charge-transfer-type Mott insulator, and the J value reaches as large as about 130 meV. The large J is a unique feature of the cuprates and might play a role in realizing the high-T_c superconductivity. It is interesting to investigate whether the Mott-Hubbard-type d⁹ nickelates can also have a large J. However, one complexity is that RNiO₂ (R=Nd, Pr) is not a Mott insulator due to carrier doping from the rare-earth layer [1]. To compare the cuprates and d⁹ nickelates on an equal footing, we study theoretically-designed RbCa₂NiO₃ and A₂NiO₂Br₂ (A: a cation with the valence of 2.5⁺). These nickelates are free from the self-doping, and the Ni x²-y² orbital becomes half-filled [2]. We show that these nickelates have a significant J as large as about 100 meV, which is not far smaller than that of the cuprates [3].

[1] Y. Nomura et al., Phys. Rev. B 100, 205138 (2019) [Editor's suggestion]
[2] M. Hirayama and T. Tadano et al., Phys. Rev. B 101, 075107 (2020)
[3] Y. Nomura et al., Phys. Rev. Research 2, 043144 (2020)