Daily Overview: Today’s highlighted work focuses on deepening the understanding of the electronic structure of hybrid Ruddlesden-Popper nickelates. On the experimental side, Michon et al. used high-pressure, high-temperature Raman spectroscopy and synchrotron infrared reflectivity measurements to establish the pressure-temperature phase diagram of La₃Ni₂O₇. They found that the transition from the weakly metallic Amam phase to a high-carrier-density, untilted metallic phase is accompanied by an increase in carrier density of nearly two orders of magnitude, and that superconductivity appears near the boundary of the two-phase coexistence region. This suggests that although high-symmetry structure and metallicity are prerequisites, they are not sufficient alone to induce superconductivity, and density-wave-related fluctuations may play a role in the pairing mechanism. On the theoretical side, Zhang et al. employed a minimal bilayer two-orbital model and the random phase approximation to study the doping evolution of pairing symmetry in pressurized La₃Ni₂O₇. They found that the most favorable pairing state in the undoped system is s±-wave, that heavy hole doping can drive a transition to dxy-wave, and that s±-wave remains robust under electron doping, indicating that the γ Fermi pocket is not a necessary condition for superconductivity in bilayer nickelates. In addition, Lesser et al. developed a machine-learning model, GP-Tc, which uses the electron affinity difference distribution as a core descriptor. The model successfully reproduced the experimental Tc range of the infinite-layer nickelate Nd₀.₈Sr₀.₂NiO₂, predicted superconductivity in PtPb₃Bi, which was subsequently confirmed experimentally, and identified high-priority candidate materials such as SrNiO₂, providing a new tool for the prediction and screening of nickelate superconductors. arXiv submission processing window: 2026-09-01 00:00 to 2026-09-01 00:00 UTC.

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

Summary: Using high-pressure, high-temperature Raman spectroscopy and synchrotron infrared reflectivity measurements, this study investigates the transition from the tilted Amam phase to the untilted phase in the pressure–temperature phase diagram of La₃Ni₂O₇ and the associated changes in electronic properties. Raman measurements confirm the pressure-driven structural transition, and the phonons exhibit Fano asymmetric line shapes, indicating enhanced electron–phonon coupling; high-temperature data show similar spectral features above 544 K, establishing the upper temperature limit of the Amam phase at ambient pressure. Infrared reflectivity measurements show that the transition is accompanied by a marked enhancement of metallicity, a substantial increase in the plasma frequency, and an increase in carrier density by nearly two orders of magnitude, indicating that the system crosses from a weak metallic state to a high-carrier-density metallic state. The combined results establish a T–P phase diagram in which the Amam phase corresponds to the weak metallic state and the untilted phase corresponds to the high-carrier-density metallic state, with a coexistence region between them; superconductivity appears at the boundary of the coexistence region above about 6 GPa and may evolve from filamentary superconductivity in untilted domains to bulk superconductivity. The authors point out that high-symmetry structure and metallicity, although necessary for superconductivity, are not sufficient by themselves to induce superconductivity, and density-wave-related fluctuations near the structural transition may provide a pathway for the pairing mechanism.

2. Doping-driven evolution of pairing symmetry in pressurized La$_3$Ni$_2$O$_7$

Summary: Using a minimal bilayer two-orbital model and the random phase approximation, we investigate the superconducting pairing symmetry and its doping evolution in pressurized La₃Ni₂O₇. In the undoped system, the most favorable pairing state is an s± wave whose gap function changes sign between different Fermi pockets; the analysis indicates that this unconventional pairing arises from repulsive interactions mediated by the magnetic odd mode of the bilayer nickelate. Hole doping enlarges the γ Fermi pocket, and the intrapocket repulsion driven by the magnetic even mode gradually strengthens and dominates the pairing interaction, eventually driving the pairing symmetry from s± wave to dxy wave in the heavily hole-doped regime. In contrast, under electron doping the s±-wave pairing persists and remains stable even in the deeply electron-doped region where a Lifshitz transition occurs, indicating that the γ Fermi pocket is not a necessary condition for superconductivity in bilayer nickelates; owing to favorable nesting between the α and β pockets that enhances spin fluctuations, the s± wave becomes even more robust in the absence of the γ pocket. This study reveals the doping control of pairing symmetry and provides a new route for testing the superconducting pairing mechanism in pressurized La₃Ni₂O₇.

3. Electron-affinity difference distributions as an organizing principle for superconductivity, enabling the discovery of PtPb$_3$Bi

Summary: Predicting superconducting transition temperature (Tc) remains challenging. This study proposes an interpretable, structure- and chemistry-aware Gaussian process model, GP-Tc, that encodes local bonding environments via graphlet histograms and constructs an effective kernel using Earth mover’s distance, enabling Tc prediction with uncertainty. Analysis shows that Tc across different superconducting families can be predicted using only the distribution of electron affinity differences between adjacent atoms, interatomic distances, and a small number of elemental features, revealing that electron affinity difference is a key chemical parameter linking local bonding to macroscopic superconductivity in a mechanism-agnostic manner. The model reproduces the experimental Tc range of the infinite-layer nickelate Nd0.8Sr0.2NiO2 and predicts superconductivity in stoichiometric PtPb3Bi, which is experimentally confirmed with Tc≈3 K. In addition, GP-Tc is openly accessible through a web interface and identifies high-priority candidate materials such as SrNiO2 and K(PRh)2.