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This study theoretically analyzes the superconductivity of La₃Ni₂O₇ thin films under ambient pressure. A model Hamiltonian is constructed based on first-principles structure optimization, with the in-plane lattice constants fixed to those of the experimental substrates (LSAT, LAO, and SLAO), and an additional model employing experimentally determined crystal structures is used. The linearized Eliashberg equation is solved using the fluctuation exchange approximation (FLEX) with full momentum- and frequency-dependent Green’s functions and pairing interactions. The results indicate that the electronic structure, including the presence or absence of the γ Fermi pocket, depends on the adopted crystal structure and whether +U corrections are included in the band calculations, yet the s±-wave pairing symmetry remains robust. This robustness primarily arises because pairing is mediated by finite-energy spin fluctuations, which are insensitive to details of the Fermi surface topology and yield a nearly momentum-independent interlayer d_{3z²−r²} pairing gap function in the orbital representation. On the other hand, the superconducting transition temperature of the thin films (approximately 40 K) is about half that of bulk material under pressure (approximately 80 K). Within the FLEX framework, this can only be understood by adopting a model with a small interlayer hopping parameter |t⊥| derived from experimentally determined crystal structures, although other contributing factors cannot be ruled out.

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By employing a low-temperature ultrahigh-vacuum dark-box transfer technique to preserve sample surface quality, this study systematically resolves the three-dimensional electronic band structure of superconducting (La,Pr,Sm)₃Ni₂O₇/SrLaAlO₄ thin films using angle-resolved photoemission spectroscopy (ARPES) with multiple photon energies. The experiments reveal orbital-dependent dimensionality: the dx²-y²-dominated band exhibits quasi-two-dimensional character, while the dz²-dominated γ band displays clear kz dispersion. Finite gaps are observed along high-symmetry directions for all detected bands, with temperature-dependent analysis of the γ band indicating a superconducting gap of approximately 18 meV and a ratio 2Δ/kBTc ~ 8, far exceeding the weak-coupling BCS limit. Moreover, suppression of spectral weight near the Fermi level persists above the superconducting transition temperature, and ubiquitous waterfall-like spectral features are observed, indicating a significant influence of electron correlations. These findings underscore the critical role of the third dimension and the dz² orbital in the nickelate superconducting mechanism, imposing important constraints on theoretical models.

This paper points out that Li et al. made three errors in calculating the superconducting volume fraction from zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements on single crystals of the high-pressure nickelate superconductor La₂SmNi₂O₇. First, due to the paramagnetic Meissner effect (Wohlleben effect), the magnetic moment in FC mode can be either positive or negative, and thus cannot be used to calculate the superconducting volume fraction. Second, reanalysis of Li et al.’s ZFC data reveals that, according to their own calculation method, the superconducting volume fraction should be 22.8%, not the reported 62.1%—a discrepancy of about a factor of three, primarily arising from differences in the demagnetization factor (Li et al. used 0.849, while the authors calculated 0.81548 using Brandt’s formula). Third, even if the ZFC magnetic moment value is correctly calculated, it cannot be directly used to determine the superconducting volume fraction, because there are infinitely many shapes and distributions of superconducting regions smaller than the actual sample size that can produce the same measured magnetic moment. The authors emphasize that they agree with Li et al.’s experimental confirmation of bulk superconductivity in pressurized nickelates, but argue that these calculation errors need to be corrected to avoid misleading future research.

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The research team conducted electrical transport measurements on (La, Pr, Sm)₃Ni₂O₇ double-layer nickelate thin films and discovered a time-reversal symmetry breaking superconducting state accompanied by electronic glass behavior. This superconducting state emerges in the low-temperature regime near zero resistance, exhibiting three prominent features: first, an unconventional magnetoresistance hysteresis that directly evidences time-reversal symmetry breaking and remains robust under different magnetic field orientations, fundamentally distinct from vortex pinning or long-range magnetic order; continuous oxygen reduction simultaneously weakens both superconductivity and the hysteresis, revealing their connection to specific Ni 3d electronic orbitals. Second, the current–voltage response demonstrates magnetic history dependence and non-reciprocity under zero field, further confirming spontaneous intrinsic time-reversal symmetry breaking. Third, the resistance exhibits slow logarithmic relaxation after removing the magnetic field, a hallmark of glassy dynamics. These phenomena reveal for the first time in nickel-based superconductors a superconducting state that simultaneously possesses spontaneous time-reversal symmetry breaking and intrinsic glassy characteristics, providing significant phenomenological and conceptual breakthroughs for understanding the mechanism of high-temperature superconductivity.

Layered perovskites─including the Dion–Jacobson, Ruddlesden–Popper, and Aurivillius families─exhibit a wide range of correlated electron phenomena, from high-temperature superconductivity to multiferroicity. Here, we report a new family of layered perovskites realized through topochemical oxidation of Lan+1NinO3n+1+δ (n = 1–4) Ruddlesden–Popper nickelate thin films. Postgrowth ozone annealing induces a substantial c-axis expansion─17.8% for La2NiO4+δ (n = 1)─that monotonically decreases with increasing n. Surface synchrotron X-ray diffraction and coherent Bragg rod analysis (COBRA) reveal that this structural expansion arises from the intercalation of approximately δ ≈ 0.7–1.0 oxygen atoms into interstitial sites within the rock salt spacer layers, far exceeding the previous record of δ ≈ 0.3 for any Ruddlesden–Popper oxide. These oxygen-intercalated phases form a new class of layered perovskites with a spacer layer composition intermediate between the Ruddlesden–Popper and Aurivillius phases. Furthermore, oxygen intercalation induces metallicity, enhances nickel–oxygen hybridization, and suppresses oxygen octahedral rotations, a feature associated with high-temperature superconductivity in Ruddlesden–Popper nickelates. Our work establishes topochemical oxidation as a powerful approach to accessing highly oxidized, metastable phases across a broad range of layered oxide systems, offering new platforms to engineer electronic properties via intercalation chemistry.

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Topotactical Hydrogen Induced Single-Band d -Wave Superconductivity in La₂ NiO 4