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Here we report the first successful synthesis of single crystals of the samarium-based trilayer Ruddlesden–Popper nickelate Sm₄Ni₃O₁₀₋δ using a high-pressure, high-temperature (3.25 GPa, 1400 °C) flux method. Single-crystal X-ray diffraction and powder diffraction refinement show that the structure adopts the orthorhombic Pbca space group, with an Ni–O–Ni bond angle along the c axis of about 152.4°, significantly deviating from 180°. Magnetization and transport measurements at ambient pressure consistently reveal a density-wave transition at about 180 K; applying pressure up to 80 GPa partially suppresses the insulating behavior and density-wave order, but no superconductivity is observed. Density functional theory calculations indicate that the 3d_z² and 3d_x²−y² orbitals are separated from the other t₂g orbitals and are the main contributors to the Fermi surface. These results suggest that the severe deviation of the Ni–O–Ni bond angle may hinder the emergence of superconductivity, and the newly synthesized Sm₄Ni₃O₁₀₋δ provides a unique platform for studying the fundamental physics of Ruddlesden–Popper nickelates.

Oxygen-deficient nickelate thin films were fabricated on highly reduced and conductive SrTiO₃ substrates, and through Meissner effect and transport measurements, a superconducting onset temperature of 50–70 K was observed, with zero resistance achieved at 20–25 K, indicating the presence of superconductivity in island-like regions within the film. A giant paramagnetic Meissner effect peak appeared at approximately 48 K, further supporting the occurrence of a superconducting transition near this temperature. Additionally, a non-reciprocal, hysteresis-free superconducting diode effect was observed, with its polarity fully polarizable and reversible. The thin films comprise a mixture of various Ruddlesden–Popper phases, including the infinite-layer phase. These enhanced superconducting properties are attributed to the synergistic effect between the oxygen-deficient films and the highly reduced SrTiO₃ substrates.

Bilayer Ruddlesden-Popper nickelates exhibit superconductivity near 80 K under high pressure, and recent work has stabilized RA₃Ni₂O₇ (RA = rare earth or alkaline earth element) superconducting thin films at ambient pressure via epitaxial strain, enabling transport, spectroscopic, microscopic, and device measurements. This review summarizes experimental progress on ambient-pressure superconducting bilayer nickelate thin films, covering synthesis routes, oxygen stoichiometry, substrate-induced strain, normal-state transport, superconducting properties, doping phase diagrams, and momentum-resolved electronic structure. Key unresolved issues include the reproducibility of phase-pure ultrathin films, the microscopic origin of the two-step superconducting transition, the roles of oxygen defects and substrate doping, the position of the Ni 3dz₂-derived γ band, and the pairing symmetry. The review concludes that future work must establish more quantitative links between crystal structure, orbital reconstruction, and superconductivity to deepen the understanding of this unconventional high-temperature superconducting system.

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