Unified Origin of Magnetism and Superconductivity in La₃Ni₂O₇ Bulk and Thin Films

The discovery of high-temperature superconductivity in bilayer nickelate La₃Ni₂O₇ under pressure has sparked tremendous research interest. This excitement has been further intensified by the recent observation of superconductivity in compressed thin films at ambient pressure, although its origin and underlying mechanism remain elusive. In this work, we investigate the electronic structures and instabilities of strained thin films grown on substrates and bulk systems to identify the key factors governing superconductivity, using ab initio and functional renormalization group calculations. We find that spin fluctuations with interlayer antiferromagnetic (AFM) correlations—unique to the bilayer structure—emerge in the intermediate-coupling regime. When only one bonding and antibonding band crosses the Fermi level, a spin-density-wave (SDW) instability develops. However, when an additional bonding band approaches or crosses the Fermi level, two competing spin fluctuations with interlayer AFM correlations frustrate the SDW order but cooperate to promote strong interlayer spin-singlet pairing, corresponding to a bonding–antibonding s+- state in band space. Furthermore, this s+- pairing is enhanced by the cooperation between spin fluctuations and out-of-plane electron–phonon coupling, and remains robust against interlayer Coulomb repulsion under realistic conditions.Our study highlights the crucial role of fermiology in determining electronic instabilities and establishes a unified scenario for superconductivity in both pressurized bulk and strained thin films of bilayer nickelates.