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This study employs epitaxial La2.82Sr0.18Ni2O7 thin films (with a superconducting transition temperature of approximately 31.6 K) to systematically characterize the upper critical field and its anisotropy via high-field transport measurements up to 58 T. Near the transition temperature, superconductivity exhibits thickness-limited two-dimensional characteristics; upon cooling, the out-of-plane coherence length decreases to below the film thickness (6 nm), indicating a transition to intrinsic three-dimensional bulk superconductivity. Based on the Ginzburg-Landau model, the zero-temperature in-plane and out-of-plane upper critical fields are determined to be 82 T and 45 T, respectively, yielding an anisotropy ratio γ≈1.34, comparable to that of bulk Ruddlesden-Popper nickelates. At low temperatures, the in-plane upper critical field is strongly suppressed by the spin paramagnetic pair-breaking effect, approaching the Pauli limit (58 T), while the out-of-plane direction remains largely unaffected. This anisotropic Pauli limiting explains the reduced anisotropy of the upper critical field and supports the conclusion that superconductivity in the films is inherently three-dimensional bulk superconductivity. The results highlight the critical role of spin paramagnetic effects in determining the high-field superconducting phase diagram of these nickelates.

Inspired by the vertically electric-field-tunable superconducting properties of Ruddlesden–Popper bilayer nickelate La₃Ni₂O₇, this study employs the dynamic cluster quantum Monte Carlo method to solve the imbalanced two-orbital bilayer Hubbard model. By analyzing the electric-field-induced pairing symmetry and its evolution under undoped, hole-doped, and electron-doped conditions, we find that the s±-wave pairing originating from the d_{z²} orbital is suppressed, while the interlayer mismatch of the d_{z²} orbital and the transfer of electrons to the d_{x²-y²} orbital drive a pairing symmetry transition from s±-wave to d-wave. Interestingly, the d-wave pairing arising from the d_{x²-y²} orbital exhibits a dome-shaped behavior as a function of electric field strength. The large-scale many-body calculations are consistent with the predictions of weak-coupling methods, providing new insights into the superconducting mechanism of RP nickelates.

This study systematically investigates the strain effect in trilayer nickelate La₄Ni₃O₁₀ thin films through atomically precise synthesis, electrical transport measurements, picometer-resolution electron microscopy, and synchrotron X-ray diffraction. While compressive epitaxial strain effectively suppresses the parent density-wave order and enhances crystal symmetry (e.g., eliminating out-of-plane octahedral rotations), no superconductivity is observed even under the maximum compressive strain of -2.8%. Critical structural characterization reveals that compressive strain fails to completely eliminate the characteristic in-plane octahedral rotations in the thin films, which exhibit interlayer inequivalence between the inner and outer layers of each trilayer unit and persist robustly. Synchrotron X-ray diffraction shows that the amplitude of in-plane rotations decreases monotonically with compressive strain but does not vanish entirely. In contrast, in the bilayer system La₃Ni₂O₇, compressive strain fully suppresses all octahedral rotations, thereby inducing superconductivity. These results uncover a key difference between trilayer and bilayer systems, indicating that ambient-pressure superconductivity in trilayer nickelates cannot be achieved solely through epitaxial strain engineering, and alternative tuning methods need to be explored.

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This study employs high-brightness synchrotron X-ray diffraction for precise structural analysis of high-quality single crystals of the bilayer nickelate La₃Ni₂O₇. Using a large dynamic range detector, we successfully resolved previously overlooked weak diffraction signals, whose intensities are nearly four orders of magnitude weaker than the main Bragg reflections. These observations indicate the presence of glide mirror symmetry breaking in the crystal, leading to a polar structure (space group Ima2) instead of the previously assumed centrosymmetric model (Fmmm). Further structural refinement reveals two inequivalent nickel sites with significantly different Ni–O bond lengths. Combined with bond valence sum calculations, this suggests a checkerboard-like charge ordering of nickel sites, which, together with oxygen octahedral tilting, endows the crystal with polarity. The charge-ordered phase is structurally analogous to the polar state observed in bilayer manganites. This study establishes the polar charge-ordered state of La₃Ni₂O₇ at ambient pressure, indicating its competition with pressure-induced superconductivity, and provides critical structural insights for understanding phase competition mechanisms and the origin of pressure-induced superconductivity in bilayer nickelates.

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