Daily Overview: Today’s highlights center on an in-depth understanding of the electronic structure of hybrid Ruddlesden–Popper nickelates. A theoretical study on 1313-La₃Ni₂O₇ thin films reveals the critical role of strain in tuning superconductivity: compressive strain suppresses superconductivity, whereas tensile strain induces band reconstruction in the trilayer subsystem, generating a new hole-like γ pocket that can drive an s±-wave pairing state with a sign-reversed order parameter, thereby establishing design principles for achieving superconductivity at ambient pressure through strain engineering. Complementarily, another work addresses the long-standing challenge of severe spectral overlap between La 3d and Ni 2p signals in nickelate spectroscopy. By innovatively employing Ni 1s core-level hard X-ray photoelectron spectroscopy, the study successfully extracts interference-free information on charge-transfer excitations in La-based nickelates, uncovering differences in nickel–ligand hybridization strength between La₃Ni₂O₇ and Nd₃Ni₂O₇ induced by tensile strain. This advance provides a sensitive and reliable spectroscopic method for systematically disentangling the effects of strain, doping, and other factors on the electronic structure of nickelates. arXiv submission processing window: 2026-06-17 00:00 to 2026-06-17 00:00 UTC.

1. Tunable Superconductivity in 1313-La$_3$Ni$_2$O$_7$: Suppressed under Compression and Possible $s^{\pm}$ Pairing under Tension

Summary: By combining density functional theory and random phase approximation, the effects of compressive and tensile strain on the superconductivity of 1313-La3Ni2O7 thin films are systematically investigated. A self-doping effect is found between the monolayer and trilayer blocks regardless of compressive or tensile strain, and it is most pronounced under tensile strain. Under compressive strain imposed by an LSAO substrate, even considering hole doping from strontium ion migration in the substrate, superconductivity is difficult to appear, consistent with experiments. However, under tensile strain from a KTO substrate, a band in the trilayer subsystem that originally did not cross the Fermi level shifts downward, giving rise to a small hole-type γ pocket at the M point, which is connected to a small electron-type σ pocket at the Γ point by a near-(π,π) wavevector. Random phase approximation calculations reveal that the trilayer subsystem can then form a stable s±-wave pairing state, with the order parameter reversing sign between these two pockets. Further analysis indicates that the size of the γ pocket is crucial for pairing, and an excessively large γ pocket suppresses superconductivity. This work predicts a strain-driven electronic structure reconstruction and proposes a design principle to realize superconductivity in 1313-La3Ni2O7 under ambient pressure through tensile strain engineering.

2. Probing La-based nickelates with Ni 1$s$ core-level photoelectron spectroscopy

Summary: This study investigates the electronic structures of La₃Ni₂O₇, Nd₃Ni₂O₇, and LaNiO₃ by comparing Ni 2p and Ni 1s core-level photoelectron spectra. Owing to the severe overlap of La 3d with Ni 2p levels and the presence of La high-energy satellite peaks, conventional Ni 2p spectra fail to reliably extract the intrinsic signal from La-based nickelates. Using hard X-ray photoelectron spectroscopy to probe the deeper Ni 1s core level, which is free of spin–orbit coupling and has negligible multiplet interactions, provides a clean perspective on charge-transfer excitations. The results show that the Ni 1s spectra can clearly distinguish the perovskite LaNiO₃ from the bilayer Ruddlesden–Popper phases and reveal that compared to Nd₃Ni₂O₇, La₃Ni₂O₇ exhibits a broadened main peak with reduced intensity and an enhanced satellite peak. Combined with DFT+DMFT calculations, these spectral changes are attributed to alterations in the charge-transfer energy and hybridization strength, where the tensile strain in La₃Ni₂O₇ weakens the Ni–ligand hybridization. This approach demonstrates the sensitivity of Ni 1s core-level spectroscopy to subtle electronic-structure variations and offers an effective means for systematically characterizing nickelates with different strains, doping levels, or layer numbers.