Daily Overview: The May 16 digest keeps two papers that are directly relevant to nickelate superconductivity. The first uses atomic-resolution scanning tunneling microscopy and spectroscopy to observe a nodeless U-shaped superconducting gap in (La,Pr)₃Ni₂O₇ films, emphasizing that oxygen-content control is essential for obtaining intrinsic spectra. The second proposes a shear-stress-constrained superconductivity picture, using local constrained deformation of the Ni-O framework to connect superconductivity in high-pressure bulk and epitaxial thin-film Ruddlesden-Popper nickelates. Together, these two works provide complementary views on pairing signatures, sample sensitivity, and reproducibility in nickelate superconductors.

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

Summary: This study employs atomic-resolution scanning tunneling microscopy and spectroscopy to characterize the superconducting thin film of (La,Pr)₃Ni₂O₇ with 1.5 unit-cell thickness grown on SrLaAlO₄. By maintaining a well-ordered √2×√2 surface reconstruction through low-temperature ultrahigh vacuum sample transfer, highly reproducible U-shaped tunneling spectra were obtained in rapidly transferred samples, exhibiting two superconducting gap magnitudes (approximately 14 meV and 20 meV) and an extended zero-conductance plateau, indicating a nodeless superconducting gap in the system. In contrast, samples exposed to a relatively higher vacuum environment for an extended period without cooling during transfer, despite retaining surface reconstruction and a superconducting onset transition temperature above 40 K, displayed V-shaped spectra. Broad-energy-range spectra further revealed that oxygen loss introduces spectral weight associated with density waves. By comparing spectral variations under different transfer durations, this study demonstrates that oxygen content control is a prerequisite for obtaining the intrinsic superconducting gap. It provides direct atomic-scale observation of a nodeless superconducting gap in bilayer nickelate thin films, offering crucial experimental evidence for understanding the pairing symmetry of superconductivity in this system.

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

Summary: This paper proposes the “Shear Stress-Confined Superconductivity” (SSCS) scenario to provide a unified understanding of the superconducting behavior in Ruddlesden-Popper (RP) nickelates under high-pressure bulk and epitaxial thin-film conditions. The core idea is that the metastable RP lattice enters a superconducting state only when the local constrained deformation of the Ni-O framework falls within a finite shear strain window. This deformation controls the rotation of NiO₆ octahedra, the interlayer Ni-O-Ni bond angle, and the hybridization between Ni dz² and dx²-y² orbitals. The SSCS scenario consistently explains a range of experimental phenomena, including the pressure threshold (~14 GPa) in bulk materials, the reversibility of superconductivity, spatial inhomogeneity and filamentary superconducting features, sensitivity to the pressure medium, the dependence of superconductivity in thin films on substrate quality, and poor reproducibility across different samples. The paper emphasizes that hydrostatic pressure or epitaxial strain alone is not the direct cause of superconductivity; rather, the key lies in the local structural confinement state achieved through shear stress. In bulk materials, intrinsic shear stress generated by non-hydrostatic pressure drives the structure into a favorable configuration; in thin films, the biaxial strain provided by the substrate already supplies this confinement, so additional pressure can further enhance Tc. The SSCS scenario also explains how chemical doping, mixed RP phases, and optimized pressure media enhance superconductivity by modulating the shear stress distribution. This framework underscores that the fragility and heterogeneity of nickelate superconductors are not extrinsic disturbances but intrinsic features of the superconducting state itself. Future directions for improving reproducibility include controlling the average local confinement and its spatial distribution width in samples, and employing local probes such as nitrogen-vacancy centers to correlate stress, stoichiometry, and superconducting response. In summary, SSCS provides a unified quantitative framework for understanding RP nickelate superconductivity across bulk materials and thin films.