Daily Overview: Today’s highlights focus on an in-depth understanding of the intrinsic electronic structure and mechanical constraint mechanisms of superconductivity in hybrid Ruddlesden-Popper nickelates. In [1], the research team, using atomic-resolution scanning tunneling microscopy, observed for the first time a nodeless U-shaped superconducting gap (approximately 14 and 20 meV) in (La,Pr)₃Ni₂O₇ ultrathin films, revealing that oxygen content control is key to obtaining the intrinsic superconducting gap. This provides direct atomic-scale evidence for nodeless pairing symmetry in bilayer nickelates. Meanwhile, [2] proposes from a mechanical perspective a shear-stress-constrained superconductivity (SSCS) framework, stating that the metastable RP lattice can enter the superconducting state only within a local shear strain window of the Ni-O framework. This view consistently explains the effects of pressure, substrate, oxygen stoichiometry, and other factors on superconductivity, and unifies phenomena in bulk high-pressure and epitaxial thin films within the same physical picture. These two works advance the understanding of core issues in the field of nickel-based superconductivity—intrinsic pairing symmetry and conditions for superconducting phase stability—from the perspectives of experimental observation and theoretical unification, respectively. arXiv submission processing window: 2026-05-15 00:00 to 2026-05-15 00:00 UTC.

1. Atomically resolved intrinsic superconducting gap in (La,Pr)3Ni2O7 films

Summary: This study employs atomic-resolution scanning tunneling microscopy and spectroscopy to characterize 1.5 unit-cell-thick (La,Pr)₃Ni₂O₇ ultrathin films grown on SrLaAlO₄. Through low-temperature ultrahigh vacuum sample transfer, an ordered √2×√2 surface reconstruction is preserved, and a U-shaped spectrum with two gap scales (approximately 14 and 20 meV) and a flat zero-bias conductance is observed in the tunneling spectra, indicating a nodeless superconducting gap. In contrast, if the sample is exposed to ultrahigh vacuum for a longer time during transfer without cooling, although the surface reconstruction and a transport superconducting onset temperature above 40 K are maintained, the tunneling spectrum becomes V-shaped, and the wide-energy spectrum shows that oxygen deficiency mixes spectral weight related to density waves. By comparing samples with different transfer times, it is determined that controlling the oxygen content is necessary to obtain an intrinsic superconducting gap, providing atomic-scale observational evidence for the intrinsic nodeless superconducting gap in bilayer nickelate ultrathin films.

2. Shear-stress-constrained superconductivity in Ruddlesden-Popper nickelates

Summary: 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.