Daily Overview: Today’s highlights focus on the experimental verification of the superconducting pairing symmetry in Ruddlesden–Popper nickelates and the theoretical elucidation of the correlation-enhanced mechanism. A team from the University of Science and Technology of China, using angle-resolved photoemission spectroscopy, directly observed for the first time a nodeless superconducting gap opening at the transition temperature in pseudogap-free La₂PrNi₂O₇ thin films. The order parameter is approximately 16 meV, and the corresponding specific-heat jump was extracted, providing the previously missing thermodynamic evidence for the superconducting phase transition in nickelates. They also found no one-to-one correspondence between the γ pocket on the Fermi surface and superconductivity, but that Fermi surface topology and strain effects profoundly influence the emergence of superconductivity. The experiment unambiguously supports s-wave pairing. On the theoretical side, for La₃Ni₂O₇ under high pressure, studies combining dynamical mean-field theory with self-energy renormalized random phase approximation reveal that only when the selective correlation renormalization of the d₃z²−r² orbital is included does the dominant pairing instability reverse from a d-wave channel to a sign-changing s± state. This resolves the ambiguity in the pairing symmetry that plagued bare weak-coupling calculations and highlights the decisive role of strong correlation effects in determining the superconducting pairing symmetry in nickelates. arXiv submission processing window: 2026-08-05 00:00 to 2026-08-05 00:00 UTC.

1. Thermodynamic phase transition, pairing symmetry and Fermi surface topology in Ruddlesden-Popper nickelate films

Summary: Using angle-resolved photoemission spectroscopy, the electronic structure of the Ruddlesden-Popper phase nickelate heterostructure La₂PrNi₂O₇/NdAlO₃ was investigated, and for the first time a superconducting gap opening at Tc was observed in the absence of a pseudogap, accompanied by pronounced coherent peaks, directly yielding a nodeless superconducting order parameter of approximately 16 meV. By reconstructing the electronic density of states from momentum-integrated spectra, a sharp drop and an electronic specific heat jump near Tc were revealed, providing the previously missing thermodynamic evidence for the superconducting phase transition in nickelates. The Fermi surface consists of three multiorbital pockets, α, β, and γ; while the γ pocket persists under various epitaxial strains and thus shows no one-to-one correspondence with superconductivity, its steep dispersion and the significant changes of the β pocket suggest a link between Fermi surface topology and the onset of superconductivity. These results establish a nodeless pairing symmetry (supporting s-wave rather than d-wave) and clarify the interplay among Fermi surface topology, strain, and superconductivity, offering direct experimental insight into the mechanism of high-temperature superconductivity in RP nickelates.

2. Correlation-renormalized spin-fluctuation pairing and the stabilization of $s_{\pm}$ superconductivity in pressurized La$_3$Ni$_2$O$_7$

Summary: The superconducting pairing symmetry in La₃Ni₂O₇ under high pressure has remained unsettled, as conventional weak-coupling calculations often place the system near sign-changing s-wave and d-wave instabilities. In this study, based on a four-orbital Wannier model, we combine single-site two-orbital dynamical mean-field theory (DMFT) with a self-energy-renormalized random phase approximation (RPA), where the core idea is to replace the bare RPA particle–hole bubble with the DMFT Green’s function while retaining the same Slater–Kanamori interaction vertex. In the bare RPA benchmark, the dominant pairing eigenvalue belongs to the B₂g (dxy) channel; upon introducing the DMFT self-energy, the hierarchy reverses, with the sign-changing A₁g s± state becoming dominant and the B₁g (dx²-y²) channel secondary, while the original B₂g instability is strongly suppressed. Pocket-pair decomposition and orbital-resolved susceptibility analyses reveal that this reversal originates from a selective renormalization of the d₃z²-r² orbital, which filters out the γ-pocket scattering processes that stabilize dxy pairing while preserving the dispersive inter-pocket processes favorable for s± pairing. As an independent verification, the spin susceptibility computed via the dual Bethe–Salpeter equation maintains a broad finite-momentum magnetic response and is weak near the Γ point, providing a spin-fluctuation background for the correlation-stabilized s± state. These findings demonstrate that strong correlations are not minor corrections; properly treating correlation-renormalized quasiparticles is essential for predicting the superconducting pairing symmetry of La₃Ni₂O₇.