Daily Overview: Today’s highlight focuses on an in-depth discussion of the microscopic mechanism and a unified theoretical framework for superconductivity in bilayer nickelate superconductors at ambient pressure. On the experimental front, epitaxial strain engineering has successfully stabilized the high-pressure phase in La₃Ni₂O₇ thin films and induced ambient-pressure superconductivity, yet the Fermi surface topology revealed by angle-resolved photoemission spectroscopy remains controversial, possibly related to growth conditions and elemental diffusion. Pathways to enhance the transition temperature include optimizing film quality and chemical substitution with rare-earth elements. From a theoretical perspective, studies have further elucidated the spin-fluctuation enhancement mechanism driven by Fermi surface nesting, pointing out that nesting between the δ pocket and γ pocket composed of (d_{z^2}) antibonding orbitals can significantly enhance (s^\pm)-wave pairing and form a superconducting dome in the low-hole-doping region. More importantly, a unified theory based on a two-component model has been proposed. Through interlayer superexchange coupling and hybridization, it successfully explains the two distinct doping phase diagrams in pressurized bulk samples and thin films, the anomalous normal-state transport behavior, and the competition between superconductivity and the Kondo effect, and it predicts that reducing interlayer magnetic coupling can realize bulk superconductivity at ambient pressure, with the electron-doped system possibly achieving a higher transition temperature. arXiv submission processing window: times are unavailable (UTC).

1. Progress of ambient-pressure superconductivity in bilayer nickelate thin films

Summary: Utilizing epitaxial strain engineering, superconductivity has been achieved in bilayer nickelate La₃Ni₂O₇ thin films at ambient pressure, where the compressive strain imposed by a SrLaAlO₄ substrate plays a crucial role in stabilizing the high-pressure phase and inducing superconductivity. Experimental characterizations such as angle-resolved photoemission spectroscopy have revealed controversial results regarding the Fermi surface topology, with some measurements indicating the presence of three pockets while others do not observe specific electron pockets, possibly related to film growth conditions and elemental diffusion. In enhancing the superconducting transition temperature, the onset temperature can be raised to about 63–68 K through optimized thin-film growth techniques or the application of hydrostatic pressure, and the chemical pressure effect from rare-earth substitution in bulk materials also provides a strategy for thin-film tuning. Theoretical calculations employing density functional theory, dynamical mean-field theory, and random phase approximation methods have constructed electronic structure models of bilayer nickelate thin films and analyzed pairing symmetry, with most studies supporting an s-wave dominated pairing mechanism, although the detailed symmetry remains debated. These advances demonstrate that bilayer nickelate thin films constitute a highly tunable platform for high-temperature superconductivity research.

2. Possible Enhancement of Superconductivity in Ambient-Pressure La$_3$Ni$_2$O$_7$ Thin Film

Summary: Using the fluctuation exchange approximation, this study systematically analyzes the superconducting properties of ambient-pressure La₃Ni₂O₇ thin-film superconductors in the weak correlation regime, with a focus on the influence of hole doping on the pairing mechanism. Based on a two-site two-orbital model, it is found that when a δ pocket composed of the d_{z^2} antibonding orbital appears near the Γ point on the Fermi surface, its nesting with the γ pocket and the nesting between the α and β pockets can mutually enhance s±-wave pairing via spin fluctuations at specific wave vectors. Numerical results show that in the low hole-doping regime, the eigenvalue of s±-wave pairing increases significantly and forms a dome-like structure, primarily attributed to enhanced spin fluctuations at those wave vectors. This work reveals that Fermi surface nesting-driven spin fluctuation-enhanced pairing can serve as an effective pathway to boost superconductivity, providing a theoretical basis for understanding and optimizing nickel-based thin-film superconductors.

3. A unified theory of thin film and bulk bilayer nickelates

Summary: This study proposes a unified theory based on a two-component model to explain a series of key experimental phenomena in pressurized bulk and thin-film bilayer nickelate superconductors. Centered on the interlayer superexchange coupling and hybridization between strongly correlated localized electrons and itinerant electrons at the nickel orbitals, the theory predicts two distinct behaviors of the superconducting transition temperature with doping: when the interlayer superexchange coupling is strong, electron or hole doping respectively produce two superconducting domes, with a non-superconducting interlayer valence-bond state appearing near half-filling; when the coupling is weak or moderate, the two domes merge into a single dome that spans half-filling but has a lower maximum temperature. Increasing doping drives the normal state from a Fermi liquid to a non-Fermi liquid or weakly insulating state, with a quasi-linear resistivity scattering rate emerging near optimal doping. Oxygen vacancies or chemical substitutions can disrupt the interlayer valence bond, simultaneously suppressing superconductivity and inducing local Kondo scattering of itinerant electrons, which explains the logarithmic temperature dependence of resistivity and the negative magnetoresistance observed in non-superconducting samples. This framework uniformly accounts for the differences in superconducting transition and normal state between bulk and thin films, the effects of hole doping and oxygen stoichiometry on the dome shape, and the competitive relationship between superconductivity and the Kondo effect. Based on the theory, the authors propose that bulk superconductivity at ambient pressure can be achieved through doping or by reducing the interlayer magnetic coupling, and predict that electron doping will yield higher transition temperatures.