Interlayer pairing mechanism for bilayer nickelate superconductors

The discovery of superconductivity in Ruddlesden-Popper bilayer nickelates under both high pressure and ambient conditions has opened a new paradigm for exploring unconventional superconductivity. This review provides a brief survey of theoretical progress on bilayer nickelate superconductors. Drawing from the key experimental observations, we summarize essential physical ingredients including the hybridized Ni-3 d x 2-y squared and 3 d z squared electronic structure, orbital-dependent electronic correlation, Hund's coupling, and strong interlayer magnetic coupling. The fundamental theoretical models including the bilayer two-orbital Hubbard model and its minimal t - J variants are introduced. Starting from the atomic-limit interlayer valence bond picture of the half-filled d z squared orbital, we elaborate on strong correlation interlayer pairing mechanisms based on different limiting considerations. Specific emphasis is placed on the hybridization mechanism, where the d z squared local singlet pairs provide the pairing energy and their hybridization with itinerant d x 2-y squared promotes superconducting phase coherence. We further analyze the pairing symmetry, the dependence of T c on various internal and external parameters, the nontrivial normal state properties including the Fermi liquid, non-Fermi liquid, weakly insulating and pseudogap behaviors. Effects of pressure tuning, oxygen content, and Kondo scattering induced by oxygen vacancies are also discussed. Finally, weak correlation theories based on spin fluctuations associated with Fermi surface nesting are briefly covered.