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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.

The discovery of high-temperature superconductivity near 80 K in bilayer nickelate La3Ni2O7 under high pressures has renewed the exploration of superconducting nickelate in bulk materials. The extension of superconductivity in other nickelates in a broader family is also essential. Here, we report the experimental observation of superconducting signature in trilayer nickelate La4Ni3O10 under high pressures. By using a modified sol-gel method and post-annealing treatment under high oxygen pressure, we successfully obtained polycrystalline La4Ni3O10 samples with different transport behaviors at ambient pressure. Then we performed high-pressure electrical resistance measurements on these samples in a diamond-anvil-cell apparatus. Surprisingly, the signature of possible superconducting transition with a maximum transition temperature (T c) of about 20 K under high pressures is observed, as evidenced by a clear drop of resistance and the suppression of resistance drops under magnetic fields. Although the resistance drop is sample-dependent and relatively small, it appears in all of our measured samples. We argue that the observed superconducting signal is most likely to originate from the main phase of La4Ni3O10. Our findings will motivate the exploration of superconductivity in a broader family of nickelates and shed light on the understanding of the underlying mechanisms of high-T c superconductivity in nickelates.

Recently, the bilayer nickelate La3Ni2O7 has been discovered as a new superconductor with transition temperature Tc near 80 K under high pressure1–3. Despite extensive theoretical and experimental work to understand the nature of its superconductivity4–29, the requirement of extreme pressure restricts the use of many experimental probes and limits its application potential. Here we present signatures of superconductivity in La3Ni2O7 thin films at ambient pressure, facilitated by the application of epitaxial compressive strain. The onset Tc varies roughly from 26 to 42 K, with higher Tc values correlating with smaller in-plane lattice constants. We observed the co-existence of other Ruddlesden–Popper phases within the films and dependence of transport behaviour with ozone annealing, suggesting that the observed low zero resistance Tc of around 2 K can be attributed to stacking defects, grain boundaries and oxygen stoichiometry. This finding initiates numerous opportunities to stabilize and study superconductivity in bilayer nickelates at ambient pressure, and to facilitate the broad understanding of the ever-growing number of high temperature and unconventional superconductors in the transition metal oxides.

Signatures of superconductivity near 80 K in a nickelate under high pressure

Researchers integrated soft point-contact Andreev reflection spectroscopy into a palm-type cubic anvil high-pressure cell, employing substrate anchoring and an external wire branching strategy to stably form multiple point-contact junctions under hydrostatic pressures up to 15 GPa. Benchmark measurements on the elemental superconductor Nb verified the method’s reliability and yielded a zero-temperature superconducting energy gap ratio of 2Δ(0)/k_B T_c ≈ 3.3. Further application to the kagome metal superconductor CsCr3Sb5 and the bilayer nickelate superconductor La2PrNi2O7 revealed sharp zero-bias conductance peaks strikingly different from those of conventional BCS superconductors, and their evolution with temperature, magnetic field, and pressure was systematically investigated. Analysis indicates that these spectroscopic features are consistent with unconventional superconductivity and possible d-wave pairing symmetry, providing direct spectroscopic evidence for understanding their pairing mechanism. This work successfully establishes a high-pressure experimental platform that bridges macroscopic electrical transport and microscopic spectroscopic probes, opening a new avenue for broadly exploring the pairing symmetries of pressure-induced unconventional superconductors.