Theoretical study on the pairing mechanism of a bilayer nickelate high T_c superconductor La₃Ni₂O₇ and related materials

Theoretical studies have shown that the nearly half-filled bilayer Hubbard model with large interlayer hopping exhibits high T_c s_+--wave pairing [1,2]. The gap function is momentum dependent, inidicating inter-layer, intra-unit cell pairing. We have previous proposed that the d_3z2-r2 orbitals in a bilayer nickelate La₃Ni₂O₇ can be a good candidate for realizing such a bilayer Hubbard model [3]. In fact, high T_c superconductivity was discovered in La₃Ni₂O₇ under pressure [4], and also at ambient pressure in thin films [5], which has motivated us to theoretically revisit the material. We constructed a two orbital (d_3z2-r2 and d_x2-y2) bilayer Hubbard model, and applyed the fluctuation exchange (FLEX) approximation + linearized Elisahberg equation analysis [6.7]. The gap function in the orbital representation shows that its d_3z2-r2 orbital, inter-orbital pairing component is momentum independent, indicating that the system can be essentially described by the bilayer Hubbard model. This s_+--wave pairing is found to be robust regardless of the Fermi surface topology [7]. We have also performed studies on related materials. For a trilayer nickelate La₄Ni₃O₁₀, we performed structural optimization and phonon calculations under pressure, and showed that the material exhibits tetragonal I4/mmm symmetry beyond the pressure of 15GPa [8]. In the tetragonal phase regime, we performed a FLEX analysis and found that the material can exhibit s_+--wave superconductivity with a T_c of about 20-30K, which was realized experimentally. We also predicted that a bilayer oxychloride Sr₃Ni₂O₅Cl₂ exhibits tetragonal I4/mmm symmetry at ambient pressure, and may become an ambient pressure superconductor [9]. The details of these theoretical studies will be presented in the talk.

[1] K. Kuroki et al., Phys. Rev. B 66, 184508 (2002). [2] T.A.Maier & D.J.Scalapino Phys.Rev. B 84, 180513 (2011). [3]M.Nakata et al., Phys. Rev. B 95, 214509 (2017). [4] H. Sun, et al. Nature 621, 493 (2023). [5] E. K. Ko et al., Nature 638, 935 (2024). [6] H. Sakakibara et al., Phys. Rev. Lett. 132, 106002 (2024). [7] K. Ushio et al., arXiv: 2506.20497. [8] H. Sakakibara et al., Phys. Rev. B 109, 144511 (2024). [9] M. Ochi et al., Phys. Rev. B 111, 064511 (2025).