Daily Overview: Today’s highlights focus on deepening the understanding of the electronic structure of hybrid Ruddlesden-Popper nickelates. In [1], the research team, using ultra-low-temperature scanning tunneling microscopy/spectroscopy, observed for the first time in (La,Pr)₃Ni₂O₇ films an energy-symmetric, flat-bottom U-shaped superconducting gap with zero residual density of states and a gap exceeding 40 meV, achieving zero-resistance temperatures above 20 K at ambient pressure. The temperature- and magnetic-field-dependent characteristics of the gap indicate a nodeless gap function at very low temperatures, revealing the nature of high-temperature superconductivity in bilayer nickel oxides. In [2], by comparing the spin and orbital excitations of undoped infinite-layer PrNiO₂ and insulating CaCuO₂ using momentum-resolved and polarization-resolved RIXS, it was found that both support three-dimensional antiferromagnetic order with similar three-dimensional spin-spin correlations. However, the Ni-dxy peak energy is significantly lower and the dispersion direction is opposite, attributed to different orbital superexchange coupling mechanisms. Meanwhile, spin fluctuations in the nickelate have lower energy, and doped charges are more localized. These two works, from the perspectives of superconducting gap and magnetic orbital dynamics, respectively, deepen the understanding of the electronic structure and pairing mechanism in nickel-based superconductors. arXiv submission processing window: 2026-05-18 00:00 to 2026-05-18 00:00 UTC.

1. Observation of flat-bottom U-shaped energy gap in high-Tc nickelate (La,Pr)3Ni2O7 thin films

Summary: Using ultra-low-temperature scanning tunneling microscopy/spectroscopy and electrical transport measurements, this study reports for the first time the observation of an energetically symmetric, flat-bottomed U-shaped superconducting gap with zero residual density of states and a gap magnitude exceeding 40 meV in (La,Pr)₃Ni₂O₇ thin films. Transport measurements reveal an onset superconducting transition temperature above 40 K and a zero-resistance temperature above 20 K under ambient pressure. The tunneling spectra exhibit unconventional temperature evolution: as temperature increases, the U-shaped gap rapidly fills and transforms into a V-shaped gap; meanwhile, the gap decreases under a 14-tesla c-axis magnetic field. These temperature and magnetic field dependencies are consistent with superconducting gap behavior, indicating the existence of a nodeless gap function at ultralow temperatures. This discovery unveils the nature of high-temperature superconductivity in bilayer nickelates and provides important insights into achieving superconductivity above the boiling point of liquid nitrogen under ambient or zero pressure.

2. Spin and orbital excitations in undoped infinite layers: a comparison between superconducting PrNiO2 and insulating CaCuO2

Summary: This study systematically compares the spin and orbital excitation properties of undoped superconducting infinite-layer nickelate PrNiO₂ and insulating cuprate CaCuO₂ using momentum-resolved and polarization-resolved resonant inelastic X-ray scattering (RIXS) measurements. The results show that the in-plane magnetic exchange integral of PrNiO₂ (approximately 46 meV) is significantly smaller than that of CaCuO₂ (approximately 82 meV), while the out-of-plane exchange integrals are similar (approximately 6–7 meV), indicating that both materials support three-dimensional antiferromagnetic order with comparable three-dimensionality of spin-spin correlations. The orbital excitations (intra-3d transitions) are well described by a single-ion model, but the Ni-dxy peak energy is notably lower than that of Cu-dxy, with opposite dispersion directions—nickelate exhibits orbital excitation propagation driven by nearest-neighbor orbital superexchange coupling, whereas cuprate is dominated by next-nearest-neighbor coupling. Despite a significant difference in charge-transfer energy (larger in the nickelate), the spin and orbital excitation characteristics are generally highly similar, with key distinctions only in the energy and dispersion of the Ni-dxy peak, attributed to differing orbital superexchange coupling mechanisms. This work reveals the core commonalities in magnetism and orbital dynamics between infinite-layer nickelates and cuprates, while also indicating smaller spin fluctuation energies and stronger localization of doped charges on metal sites in the nickelates.