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This study investigates the interplay of spin density wave (SDW) and charge-anisotropy-like order with superconductivity in bilayer nickelate films by combining ozone annealing and isovalent rare-earth A-site substitution to effectively apply c-axis uniaxial pressure and tune interlayer coupling. Resonant elastic X-ray scattering shows that, upon optimizing superconductivity, the SDW transition temperature decreases from 150 K to 70 K; although weakened, the SDW persists, whereas the charge-anisotropy-like order is completely eliminated in the superconducting state. Polarization-resolved O K-edge X-ray absorption spectroscopy and electronic structure calculations indicate that both the c-axis compression induced by A-site substitution and the filling of interlayer oxygen vacancies by ozone annealing enhance interlayer Ni-O-Ni hybridization, reconstruct the Fermi surface, and suppress magnetic fluctuations near the SDW wave vector, thereby weakening the SDW instability. These results indicate that the two electronic orders are governed by different energy scales and are consistent with a spin–spinless stripe ground state, providing new clues for understanding density-wave formation and its relationship to superconductivity in bilayer nickelates.

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High-quality single crystals of the trilayer Ruddlesden–Popper nickelate La4Ni3O10 were studied using a diamond anvil cell to explicitly tune the nonhydrostatic pressure component, combined with high-pressure electrical transport and synchrotron X-ray diffraction, thereby distinguishing out-of-plane-dominated compression from enhanced in-plane biaxial compression. Enhanced in-plane biaxial compression induced a uniform, reproducible zero-resistance superconducting state at 17.2 and 18.9 GPa, with the required pressure reduced by about 60% compared with previous reports; meanwhile, the density-wave phase under low compression was completely suppressed and separated from the superconducting state, which the authors interpret as effectively ruling out soft quantum critical fluctuations of the density-wave order parameter as a pairing glue in this system. In contrast, out-of-plane-dominated compression only slowly suppressed the density wave, and zero resistance was still not achieved up to 34 GPa, with superconducting signatures being filamentary and inhomogeneous. High-pressure XRD showed that the ambient-pressure monoclinic phase transforms into a tetragonal phase under high pressure, whereas an orthorhombic phase appears as an intermediate under enhanced in-plane compression. On this basis, the authors conclude that in-plane biaxial compression is an effective means to tune nickelate superconductivity, unifies the pressure scales of bilayer and trilayer Ruddlesden–Popper structures, and is linked to results on strained thin films.

Ruddlesden-Popper nickel oxides exhibit superconductivity under both high-pressure bulk and thin-film epitaxial constraints, yet this behavior is highly dependent on sample quality, oxygen content, defects, and stress states. This paper proposes that metastable RP lattices enter the superconducting state only when the local constrained deformation of the Ni-O framework falls within a bounded shear strain window; this deformation governs octahedral rotations, interlayer Ni-O-Ni bond angles, and the coupling between Ni dz² and dx²-y² orbitals. This shear-stress-constrained superconductivity (SSCS) framework unifies previously observed phenomena such as pressure thresholds, reversibility, spatial inhomogeneity, pressure medium dependence, film-substrate sensitivity, and reproducibility challenges. The SSCS scenario does not replace the role of traditional factors such as bond angles, bond lengths, orbital occupancy, oxygen stoichiometry, or carrier density, but rather identifies the mechanical and symmetry conditions required for these factors to cooperatively stabilize the superconducting state. The brittleness and heterogeneity observed in nickel oxide superconductors are not extrinsic complexities but rather core diagnostic features of the superconducting state itself. This perspective provides specific experimental pathways for improving reproducibility and unifies the physical mechanisms underlying compressed bulk materials, epitaxial films, chemically substituted samples, and hybrid RP structures within a single conceptual framework.