Paper

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.

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.

We report the discovery of a novel form of Ruddlesden–Popper (RP) nickelate that stands as the first example of long-range, coherent polymorphism in this class of inorganic solids. Rather than the well-known, uniform stacking of perovskite blocks ubiquitously found in RP phases, this newly discovered polymorph of the bilayer RP phase La3Ni2O7 adopts a novel stacking sequence in which single-layer and trilayer blocks of NiO6 octahedra alternate in a “1313” sequence. Crystals of this new polymorph are described in space group Cmmm, although we note evidence for a competing Imam variant. Transport measurements at ambient pressure reveal metallic character with evidence of a charge density wave transition with an onset at T ≈ 134 K. The discovery of such polymorphism could reverberate to the expansive range of science and applications that rely on RP materials, particularly the recently reported signatures of superconductivity in bilayer La3Ni2O7 with Tc as high as 80 K above 14 GPa.

This study systematically analyzes a two-site, two-orbital model of the La₃Ni₂O₇ thin-film superconductor under ambient pressure within a weakly correlated system, employing the fluctuation exchange (FLEX) approximation, with a focus on the influence of hole doping on superconducting properties. Through a detailed examination of the Fermi surface topology, it is found that when the δ pocket, formed by the d_{z²} antibonding orbital, emerges near the Γ point, the nesting between the δ and γ pockets, together with the nesting between the α and β pockets, collectively enhances s±-wave pairing at the corresponding wave vectors. The study further proposes that this enhancement mechanism of spin-fluctuation-induced pairing, driven by Fermi surface nesting, may provide a feasible route to raising the superconducting transition temperature. This work offers theoretical guidance for understanding the pairing mechanism of nickel-based thin-film superconductors under ambient pressure and for exploring higher-performance superconducting materials.

Given the urgent need to enhance the superconducting transition temperature (Tc) of La₃Ni₂O₇ single- and double-layer thin films under ambient pressure, this study proposes applying a vertical electric field to drive charge transfer for superconductivity enhancement. The vertical field drives electrons from higher-potential layers to lower-potential layers; since the Ni 3d_{z²} orbital is nearly half-filled and cannot accommodate additional electrons, the inflowing electrons primarily fill the 3d_{x²-y²} orbitals of the lower-potential layer, thereby increasing its filling rate. Using a simplified single-orbital model and a comprehensive two-orbital model, combined with slave-boson mean-field theory and density matrix renormalization group methods, numerical calculations reveal that the increased filling suppresses interlayer s-wave pairing but strongly enhances intralayer d-wave pairing, causing the bottom-layer-dominated d-wave superconductivity to rise rapidly. When the interlayer voltage reaches approximately 0.1–0.2 V, Tc can surpass the liquid nitrogen temperature (around 77 K), achieving high-temperature superconductivity in the liquid nitrogen temperature range under ambient pressure. This approach requires no high pressure and avoids chemical doping disorder, providing a feasible route to realize high-Tc superconductivity in La₃Ni₂O₇ ultrathin films, which warrants further experimental verification.

By electronic doping of the high-pressure bilayer cobaltate La₃Co₂O₇, cobalt-based La₃Ni₂O₇ analogs such as LaTh₂Co₂O₇, La₃Ni₂O₅Cl₂, and La₃Ni₂O₅Br₂ are predicted, which possess similar crystal structures and strongly correlated electronic states. Calculations based on density functional theory plus dynamical mean-field theory (DFT+DMFT) and random phase approximation (RPA) indicate that the cobalt 3d orbitals exhibit half-filled or near-half-filled occupancy, analogous to the nickel orbitals in La₃Ni₂O₇, and display strong Hund coupling and mass enhancement. The local magnetic moments (approximately 0.64 μ_B) of these cobalt-based compounds fall precisely within the optimal window for nickelate superconductivity (0.63–0.68 μ_B), strongly suggesting the possibility of high-temperature superconductivity via a similar spin fluctuation mechanism. RPA calculations further reveal that the leading pairing symmetry in electron-doped cobaltates is s-wave, belonging to the A₁ᵍ irreducible representation. This work provides a theoretical basis for achieving high-temperature superconductivity in cobalt-based systems and encourages further experimental synthesis and characterization.

This study successfully fabricated phase-pure, high-quality Ruddlesden-Popper nickelate Ln₃Ni₂O₇ thin films on LaAlO₃ and SrLaAlO₄ substrates using the giant oxidation atomic layer epitaxy (GAE) method. Films grown under strongly oxidizing ozone atmosphere exhibited superconductivity without requiring post-annealing, with optimized Ln₃Ni₂O₇/SrLaAlO₄ films achieving an onset transition temperature (Tc,onset) as high as 50 K. Systematic investigation identified four key factors governing film crystallinity and superconducting performance: precise control of cation stoichiometry suppresses secondary phase formation; complete atomic layer-by-layer coverage combined with optimized interface reconstruction reduces stacking faults; accurate regulation of oxygen content is essential for achieving a single superconducting transition and high Tc,onset. The study also revealed that deviation in cation stoichiometry leads to the formation of Ni-rich or Ni-deficient secondary phases, inducing metal-insulator transitions or highly insulating behavior, respectively, while deviations in atomic layer coverage (e.g., 101.5%) still allow superconductivity but introduce residual resistance. Interface reconstruction, such as predisposing half-unit-cell Ln₂NiO₄ or annealing the SrLaAlO₄ substrate, significantly improves film crystallinity. These findings provide important guidance for the layer-by-layer epitaxial growth of high-quality oxide high-temperature superconducting thin films.

Through first-principles calculations, this work investigates the crystal and electronic structures of the mixed bilayer-trilayer Ruddlesden-Popper nickelate La₇Ni₅O₁₇ under hydrostatic pressure and biaxial compressive strain. By analyzing the irreducible representations of dynamically unstable phonon modes in the high-symmetry P4/mmm structure, the authors identify a dynamically stable low-symmetry C2/c structure characterized by octahedral tilting. Both applied pressure and compressive strain suppress the octahedral tilting, leading to structural tetragonalization, a behavior akin to conventional Ruddlesden-Popper phases. In terms of electronic structure, the overall features under hydrostatic pressure and strain are similar, but a key difference lies in the position of the d_z² bonding band within the trilayer block: at 30 GPa pressure, this band crosses the Fermi level, whereas any magnitude of compressive strain keeps it below the Fermi level. This strain-induced electronic structure variation aligns with observations in conventional bilayer nickelates, offering critical insights into the distinct effects of pressure and strain on superconductivity in this class of materials.