Daily Overview: Today’s highlight focuses on deepening the understanding of the electronic structure of mixed Ruddlesden-Popper nickelates. In [1], X-ray absorption spectroscopy systematically probed the unoccupied states of infinite-layer La₁₋ₓCaₓNiO₂, revealing that Ni–O covalency undergoes an orbital-selective crossing near the superconducting dome. This crossing coincides with the sign reversal of the Hall coefficient and the decline in superconducting critical temperature, providing key experimental evidence for establishing a unified orbitally resolved phase diagram. [2] used polarized ultrafast spectroscopy to uncover significant electronic nematicity in the normal state of bilayer La₃Ni₂O₇, while the trilayer La₄Ni₃O₁₀ exhibits isotropy, suggesting that the breaking of two-fold rotational symmetry may promote superconducting pairing in bilayer systems. [3] first-principles calculations show that the superconducting transition temperature of freestanding infinite-layer Nd₀.₈₅Sr₀.₁₅NiO₂ films increases monotonically under pressure. The physical origin lies in lattice compression increasing the Ni-3d orbital bandwidth and weakening relative electronic correlations, thereby alleviating the overly strong correlation effects induced by the low valence state of Ni. These three works elucidate the intrinsic relationship among electronic structure, correlation effects, and superconductivity in nickel-based superconductors from different perspectives. arXiv submission processing window: 2026-06-01 00:00 to 2026-06-01 00:00 UTC.

1. Ni-O hybridization-driven electronic reconstruction across the superconducting dome in an infinite-layer nickelate

Summary: This study systematically characterizes the evolution of unoccupied states in the infinite-layer nickel oxide La₁₋ₓCaₓNiO₂ as a function of doping and temperature using O K-edge and Ni L-edge X-ray absorption spectroscopy. Superconductivity emerges in the doping range of x = 0.18 to 0.27. Near x ≈ 0.20–0.23, a redistribution of low-energy spectral weight occurs: Ni 3d-dominated states decrease while O 2p hybridized states increase, indicating an orbital-selective crossover in Ni–O covalency. This crossover coincides precisely with the sign reversal of the Hall coefficient and precedes the suppression of the superconducting critical temperature at higher doping levels. By directly linking transport anomalies and the superconducting dome to measurable Ni–O orbital reorganization, these results represent a critical step toward establishing a unified orbital-resolved phase diagram for infinite-layer nickelates and offer a practical route for designing superconductivity through hybridization engineering.

2. Electronic Nematicity Revealed by Polarized Ultrafast Spectroscopy in Bilayer La$_3$Ni$_2$O$_7$

Summary: Through polarization-resolved ultrafast pump-probe spectroscopy, the normal-state electron dynamics of ambient-pressure bilayer La₃Ni₂O₇ and trilayer La₄Ni₃O₁₀ single crystals have been investigated. Both nickel oxides exhibit a density-wave (DW) transition accompanied by the opening of a quasiparticle relaxation bottleneck, yet their electronic responses display markedly different symmetries: La₄Ni₃O₁₀ remains optically isotropic across the entire temperature range, whereas La₃Ni₂O₇ shows pronounced twofold (C₂) rotational symmetry breaking—namely, electronic nematicity—at low temperatures, manifested both in the slow relaxation dynamics and in the polarization-dependent effective bottleneck energy scale. This anisotropy is strongly modulated below 115 K, suggesting coupling or competition with a secondary DW-like instability. The presence of macroscopic electronic anisotropy in the bilayer system and its absence in the trilayer system imply that electronic nematic correlations may play a crucial role in the superconducting normal state of La₃Ni₂O₇. This finding provides important clues for understanding the superconducting mechanism in this new material and suggests that electronic nematicity could be a common factor enhancing superconductivity in nickel-based superconductors.

3. Theoretical study of superconductivity in freestanding infinite-layer nickelate membranes under pressure: mitigation of excess correlation enhances $T_c$

Summary: Using first-principles calculations, this study constructs a seven-orbital effective model to theoretically analyze the superconductivity of free-standing infinite-layer Nd₀.₈₅Sr₀.₁₅NiO₂ thin films under pressure. By solving the linearized Eliashberg equation within the fluctuation exchange approximation, it is found that the superconducting transition temperature (Tc) increases monotonically with pressure, consistent with recent experimental results. This enhancement arises from the reduction in lattice constants under pressure, which broadens the Ni-3d orbital bandwidth while the Coulomb interaction parameter U remains nearly unchanged, leading to a significant decrease in the U/t ratio and thereby alleviating excessively strong electronic correlations. Phonon calculations confirm that the crystal structure remains dynamically stable below 90 GPa. The study demonstrates that a relatively large U value (approximately 5.1 eV) is required to quantitatively explain the experimentally observed Tc enhancement, whereas artificially reducing U results in premature Tc saturation or even dome-like behavior, which contradicts experiments. Therefore, the key to enhancing superconductivity in infinite-layer nickelates lies in mitigating the excessively strong electronic correlations caused by the abnormally low valence state of Ni atoms.