Daily Overview: Today’s highlights focus on an in-depth understanding of the electronic structure of mixed Ruddlesden-Popper nickelates. [1] High-temperature and high-pressure Raman and infrared spectroscopy reveal that the structural transition from the tilted Amam phase to the untilted Fmmm phase in La₃Ni₂O₇ is accompanied by a metallization transition, but confirm that high symmetry and metallicity alone are insufficient to trigger superconductivity, providing key experimental evidence for the prerequisites of superconductivity. [2] Starting from symmetry and combining DFT+U calculations, it is pointed out that both pressurized bulk and thin films exhibit s±-wave pairing, but the dominant pairing originates from the Ni-dz² out-of-plane orbital and the Ni-dx²-y² in-plane orbital, respectively, and the Tc difference is attributed to the change in the interlayer/intralayer hopping ratio. [3] Using resonant X-ray scattering, it is found in La₂PrNi₂O₇ thin films that superconductivity exists only in regions without spin density wave order and with complete oxygen stoichiometry. It is proposed that oxygen ligand holes are mainly located on the interlayer apical oxygen, forming a stable interlayer five-spin polaron state as the superconducting ground state, emphasizing the key regulatory role of oxygen stoichiometry on interlayer coupling. These results deepen the understanding of the superconducting mechanism in nickelates from three dimensions: structure-electron coupling, pairing symmetry, and oxygen stoichiometry. arXiv submission processing window: 2026-05-05 00:00 to 2026-05-05 00:00 UTC.

1. Metallic crossover through the tilt-free transition in La$_3$Ni$_2$O$_7$ at high pressure and temperature

Summary: La3Ni2O7 undergoes a pressure-driven structural transition from a tilted Amam phase to an untilted Fmmm (or I4/mmm) phase, accompanied by the emergence of high-temperature superconductivity at 80 K. In this study, we systematically map the structural and electronic evolution of this system by combining high-pressure high-temperature Raman spectroscopy with synchrotron infrared spectroscopy. Raman measurements confirm the pressure-induced structural transition and reveal Fano line shapes, indicating enhanced electron-phonon coupling; high-temperature data show similar spectral features above 544 K, establishing for the first time the upper temperature limit of the Amam phase in the temperature–pressure phase diagram. Infrared reflectivity measurements demonstrate a concurrent metallization process, with the carrier density increasing by nearly two orders of magnitude, marking a transition from a bad metal to a good metal. These results establish a unified picture of the strong coupling between structural transformation and electronic properties, clarifying that the emergence of the untilted phase is a prerequisite for superconductivity, but that high symmetry and metallicity alone are insufficient to trigger superconductivity, providing key experimental evidence for understanding the mechanism of nickelate superconductivity.

2. Unconventional Superconductivity in $\mathrm{La_{3}Ni_{2}O_{7}}$ from the Perspective of Symmetry

Summary: This study addresses the discrepancy between bulk La₃Ni₂O₇ with high transition temperatures under high pressure and its thin-film counterpart, which superconducts at ambient pressure but with a reduced Tc, by developing a symmetry-based phenomenological approach combined with DFT+U calculations and experimentally determined Tc and structural symmetries to analyze the superconducting gap structure. The results reveal that both systems exhibit s±-wave pairing symmetry and two-band superconductivity, but the dominant microscopic pairing configurations differ: in pressurized bulk, superconductivity is primarily governed by out-of-plane pairing of Ni-dz² orbitals, whereas in thin films, in-plane pairing of Ni-dx²-y² orbitals dominates. The reduction in Tc is attributed to a decreased ratio of interlayer to intralayer hopping in the film, which shifts the dominant pairing type from out-of-plane to in-plane. These findings highlight the crucial role of symmetry in unconventional superconductivity, and the developed methodology is expected to be extendable to other unconventional superconductors.

3. Interlayer Five-Spin Polaron in Superconducting Bilayer Nickelates

  • Relevance Score: 5.6518
  • Authors: Jiarui Li, Christopher T. Parzyck, Eder G. Lomeli, Yidi Liu, Taehun Kim, Heemin Lee, Zengqing Zhuo, Eun Kyo Ko, Yaoju Tarn, Cheng-Tai Kuo, Ronny Sutarto, Chunjing Jia, Vivek Thampy, Brian Moritz, Yijun Yu, Jun-Sik Lee, Valentina Bisogni, Thomas P. Devereaux, Harold Y. Hwang, Wei-Sheng Lee
  • Affiliations: SLAC National Accelerator Laboratory, Lawrence Berkeley National Laboratory, Stanford University, Brookhaven National Laboratory, University of Florida, Canadian Light Source
  • Link: https://arxiv.org/abs/2605.02891
  • Paper page: Interlayer Five-Spin Polaron in Superconducting Bilayer Nickelates

Summary: Using resonant X-ray scattering and spectroscopy, we investigate the relationship between spin-density wave (SDW) order and superconductivity in bilayer nickelate La₂PrNi₂O₇ thin films. Superconductivity is found to emerge only in regions without SDW and with complete oxygen stoichiometry, whereas oxygen deficiency promotes SDW order, indicating phase separation between the two. Further Ni-L₃ and O-K edge spectroscopy reveal that the superconducting phase exhibits a metallic ground state dominated by Ni d⁸ and oxygen ligand-hole character; oxygen deficiency induces electron localization and the appearance of low-energy excitations. Combined with theoretical analysis, we propose that ligand holes primarily reside on the interlayer apical oxygen sites, forming stable interlayer five-spin polaron states as the ground state of superconducting bilayer nickelates. This study demonstrates that oxygen stoichiometry is a key parameter controlling interlayer coupling and electronic structure, and that SDW order is not the intrinsic parent state of superconductivity.