Daily Overview: Today’s highlighted work focuses on progress in nickel-based superconductivity across three areas: ambient-pressure bulk phase boundaries, the bilayer interlayer pairing mechanism, and infinite-layer spin/orbital excitations. In ambient-pressure bulk nickelates, [1] observed signatures of superconductivity in (La1-xPrx)4Ni3O8 and (La1-xYx)4Ni3O8 single crystals, where superconductivity appears within a narrow window near the insulator-metal transition and coexists with a charge stripe phase; the low superconducting volume fraction and nonpercolative character suggest that superconductivity may nucleate in filamentary form at phase boundaries, while the presence of intergrowth defects leaves its origin unresolved. This study further establishes spacer-layer engineering as an important means of tuning the electronic structure of nickel-oxide sandwich layers and exploring this competing regime. On the theoretical side, [2] systematically reviewed interlayer pairing mechanisms in bilayer nickelates, emphasizing Ni-3d_{x^2-y^2} and 3d_{z^2} hybridization, orbital-dependent correlations, Hund coupling, and strong interlayer magnetic coupling. They pointed out that half-filled d_{z^2} local singlet pairs provide pairing energy, and that their hybridization with itinerant d_{x^2-y^2} promotes phase coherence, with interlayer pairing regarded as central to understanding this class of superconductors. Spectroscopically, [3] used RIXS to compare nominally undoped yet superconducting infinite-layer PrNiO2 with insulating CaCuO2. They found that PrNiO2 has a smaller in-plane magnetic exchange integral but similar out-of-plane exchange; spin excitations remain relatively sharp and exhibit single-magnon dispersion, while orbital excitations show that the Ni-dxy peak has lower energy and a dispersion direction opposite to that of Cu-dxy. This indicates that infinite-layer nickelates and cuprates share most spin and orbital properties, but the larger charge-transfer energy and smaller hopping integrals in nickelates lead to lower-energy spin fluctuations and more localized doped charge. Overall, today’s work—from ambient-pressure phase-boundary superconductivity and interlayer pairing theory to infinite-layer magnetism and orbital excitations—collectively deepens the understanding of pairing mechanisms, phase competition, and materials-design boundaries in nickel-based superconductors. arXiv submission processing window: 2026-09-18 00:00 to 2026-09-18 00:00 UTC.

1. Superconductivity at the metal-insulator phase boundary in a bulk nickelate at ambient pressure

Summary: At ambient pressure, signatures of superconductivity were observed in bulk single crystals of the nickelates (La1−xPrx)4Ni3O8 and (La1−xYx)4Ni3O8, whose structures consist of alternating square-planar nickel-oxide sandwich layers and fluorite-like spacer layers. The parent La4Ni3O8 is an insulator in which electrons form intertwined charge/spin stripe order; substitution with smaller lanthanide ions disrupts this order and ultimately leads to a metallic ground state. Magnetotransport and magnetic susceptibility measurements indicate that superconductivity appears within a narrow window near the insulator-to-metal transition, where a metallic phase coexists with the charge-stripe phase. The observed superconducting volume fraction is low and non-percolative, suggesting that superconductivity may nucleate in filamentary form at phase boundaries; TEM also reveals intergrowth defects of a nearly infinite-layer nickelate, so it cannot be ruled out that superconductivity originates from these defects rather than the sandwich matrix. The electronic phase diagrams of the Y and Pr series coincide when plotted as a function of the fluorite-like spacer volume, indicating that this steric parameter strongly tunes the electronic structure of the nickel-oxide sandwich layers. A deeper understanding of the coexistence region between the metallic and charge-ordered phases is considered key to expanding superconductivity in bulk nickelates at ambient pressure, and spacer-layer engineering is established as a design approach for exploring this competing-phase region.

2. Interlayer pairing mechanism for bilayer nickelate superconductors

Summary: This review focuses on the theoretical progress triggered by the discovery of superconductivity in Ruddlesden-Popper bilayer nickelates under both high and ambient pressure, and summarizes the key experimental facts and effective models. The article identifies essential elements such as the hybridized electronic structure of Ni-3d_{x^2-y^2} and 3d_{z^2}, orbital-dependent electron correlations, Hund’s coupling, and strong interlayer magnetic coupling, and introduces the bilayer two-orbital Hubbard model together with its minimal t-J variant. It emphasizes the strongly correlated interlayer pairing mechanism, especially the hybridization mechanism: local singlet pairs formed by the half-filled d_{z^2} orbital provide the pairing energy, and their hybridization with itinerant d_{x^2-y^2} orbitals promotes superconducting phase coherence. The article also discusses pairing symmetry, the dependence of Tc on internal and external parameters, Fermi liquid, non-Fermi liquid, weak insulating, and pseudogap behavior in the normal state, as well as the effects of pressure, oxygen content, and oxygen-vacancy-induced Kondo scattering, and briefly mentions weak-coupling theories based on Fermi surface nesting spin fluctuations, emphasizing that interlayer pairing is central to understanding this class of superconductors.

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

Summary: This study uses momentum- and polarization-resolved resonant inelastic X-ray scattering to compare the nominally undoped yet superconducting infinite-layer nickelate PrNiO2 with the insulating infinite-layer cuprate CaCuO2 and to investigate their spin and orbital excitations. It finds that the in-plane magnetic exchange integral in PrNiO2 is smaller than that in CaCuO2, whereas the out-of-plane exchange is similar, indicating that both can support three-dimensional antiferromagnetic correlations. The orbital excitations can overall be described by a single-ion model, and their characteristics are similar; however, the Ni-dxy peak is not only significantly lower in energy but also disperses in the opposite direction to Cu-dxy. The authors attribute this to nearest-neighbor orbital superexchange coupling driving orbiton propagation. The spin excitation peaks in PrNiO2 remain relatively sharp and exhibit single-magnon dispersion similar to that of CaCuO2, indicating that self-doping and superconductivity disrupt magnetism much more weakly than chemical doping in cuprates. The study concludes that infinite-layer cuprates and nickelates share most spin and orbital properties, despite markedly different charge-transfer energies Δ; the larger Δ and smaller hopping integrals in nickelates imply smaller spin-fluctuation energies and stronger localization of doped charges at the metal sites.