The fate of s_±superconductivity in the bilayer nickelates

The recent discovery of high-temperature superconductivity in the bilayer nickelate system La₃Ni₂O₇ [1] have led to myriad speculation as to the nature of its superconducting pair order. Due to the presence of both electron and hole pockets in the Fermi surface, many have claimed the existence of an s_± pairing state, in which the gap function, while nodeless, takes opposite signs on the Fermi surface pockets. These studies have however failed to systematically include the effects of Coulomb repulsion, which is expected to suppress the s_± state in favour of the cuprate-like d-wave state. We attempt to understand this by modelling the system as a simplified bilayer t-J model and introducing a symplectic-N treatment [2]. With this treatment, the large-N limit both preserves the time-reversal symmetry necessary for superconductivity and also introduces a new constraint that acts as a pair chemical potential, eliminating any on-site s-wave pairing. We investigate how this pair chemical potential affects the competition of s_± and d-wave, and we observe that while the d-wave state is agnostic to this new constraint, s_± is significantly suppressed for realistic parameter ranges and doping.

References:

[1] Sun H. et al., "Signatures of superconductivity near 80 K in a nickelate under high pressure", Nature (2023)

[2] Flint R. and Coleman P., "Symplectic-N t-J model and s_± superconductors", Phys. Rev. B (2012)