Daily Overview: Today’s highlights focus on an in-depth understanding of the electronic structure of mixed Ruddlesden-Popper nickelates. [1] High-pressure high-temperature spectroscopy reveals the correlation between the disappearance of structural tilting and the metallicity transition in La₃Ni₂O₇, showing that the high-symmetry untilted phase, although accompanied by superconductivity, is not a sufficient condition. [2] Using a two-orbital model and DMRG calculations, it is clarified that the (π/2, π/2) spin stripe under ambient pressure originates from the synergy of Hund coupling and interlayer antiferromagnetic coupling in quasi-one-dimensional zigzag chains, and it is indicated that this model can enhance interlayer pairing in the high-pressure phase. Meanwhile, [3] focusing on the relationship between superconducting pairing and the filling of the 3d_{z²} orbital, numerical results suggest that the itinerancy of this orbital favors superconductivity, while charge order competes with it. In the field of infinite-layer nickelates, [4] based on first-principles and FLEX analysis, it is pointed out that pressure alleviates excessive electron correlation, thereby enhancing spin fluctuations, which is the key mechanism for the T_c enhancement in freestanding Nd₀.₈₅Sr₀.₁₅NiO₂ films. These works collectively advance the understanding of the microscopic origins of nickel-based superconductivity. arXiv submission processing window: 2026-05-26 00:00 to 2026-05-26 00:00 UTC.

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

Summary: This paper systematically investigates the structural transitions and electronic property changes of the bilayer nickel oxide La₃Ni₂O₇ under pressure and temperature using high-pressure high-temperature Raman spectroscopy and synchrotron infrared reflectivity measurements. Raman measurements confirm a pressure-driven structural transition from a tilted Amam phase to a untilted Fmmm (or I4/mmm) phase, with Fano line shapes observed above approximately 6 GPa, indicating enhanced electron-phonon coupling. High-temperature experiments further reveal a previously unreported upper limit temperature of 544 K for this structural transition at ambient pressure. Infrared reflectivity measurements show that the carrier density increases by nearly two orders of magnitude during the structural transition, marking a crossover from a weak metallic to a highly metallic state. These results not only establish a unified picture of the coupling between structural transition and electronic properties but also reveal the simultaneous occurrence of the untilted phase and superconductivity above 6–7 GPa, suggesting that high symmetry and high metallicity are important prerequisites for superconductivity, though not sufficient conditions.

2. Origin of Spin Stripes in Bilayer Nickelate La$_3$Ni$_2$O$_7$

Summary: This study proposes a microscopic Hamiltonian model that faithfully reflects the crystal symmetry for the (π/2, π/2) spin stripe order observed in bilayer nickelate La₃Ni₂O₇ under ambient pressure. The model includes the Ni dz² and dx²-y² orbitals, incorporates Hund coupling J_H and interlayer antiferromagnetic coupling J_⊥, and through large-scale density matrix renormalization group (DMRG) calculations reveals that in the ambient-pressure Amam phase, due to alternating strong and weak hoppings (t′ < t) in the dx²-y² orbital, the system forms hidden quasi-one-dimensional zigzag chains, where a large J_H induces ferromagnetic order within the chains via the double-exchange mechanism, while weak hopping gives rise to interchain antiferromagnetic coupling, ultimately yielding the (π/2, π/2) spin stripe consistent with experiments; in the high-symmetry Fmmm/I4/mmm phase (t′ = t) under high pressure or compressive strain, when J_⊥ is sufficiently large, the model exhibits an enhanced tendency for interlayer pairing. This study demonstrates that Hund coupling J_H and interlayer coupling J_⊥ are the key microscopic factors controlling the magnetic order and pairing tendency, respectively, providing a unified theoretical framework for understanding the origin of the ambient-pressure spin stripe and the high-pressure superconductivity mechanism in this material.

3. The evolution of pairing correlation with $3d_{z^{2}}$ electron filling in a bilayer two-orbital model for La$_3$Ni$_2$O$_7$

Summary: This study employs the density matrix renormalization group (DMRG) method to numerically simulate the two-orbital bilayer effective model of La₃Ni₂O₇ under a one-dimensional geometry, systematically tuning the filling of the Ni 3d_{z²} orbital from 1/12 doping to half-filling to investigate the effect of its itinerancy on superconducting pairing. The results show that when the 3d_{z²} orbital is near half-filling, the superconducting correlations are significantly suppressed, indicating that the itinerancy of this orbital is favorable for superconducting pairing. Furthermore, in parameter regions with large charge fluctuations, the pairing correlations are enhanced, and this region precisely corresponds to where the charge order pattern changes, suggesting a competition between charge order and superconductivity in the model. These results provide key numerical evidence for understanding the role of the 3d_{z²} orbital in La₃Ni₂O₇.

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

Summary: Based on first-principles calculations, this study constructs a seven-orbital effective model for a freestanding infinite-layer nickelate Nd₀.₈₅Sr₀.₁₅NiO₂ film and analyzes its superconducting properties using the fluctuation exchange (FLEX) approximation. Phonon calculations confirm that the crystal structure remains dynamically stable up to 90 GPa. The results reveal that pressure shortens the Ni–O bond length and increases the orbital bandwidth, while the Coulomb interaction parameter U of the Ni-3d orbital decreases significantly, though the Ni-4s orbital parameter shows little change. The eigenvalue λ of the Eliashberg equation, which measures the superconducting transition temperature Tc, monotonically increases with pressure, consistent with recent experimental findings. This enhancement of Tc is attributed to pressure alleviating the excessively strong electron correlation caused by the anomalously low valence state of Ni atoms: the reduction in U weakens quasiparticle damping and enhances spin fluctuations, thereby promoting d-wave superconductivity. Comparative analysis shows that models employing larger U values (≈5.1 eV) better explain experimental trends, whereas smaller U values lead to premature saturation of Tc. This work highlights that mitigating excessively strong electron correlation is a key mechanism for understanding the pressure-induced Tc enhancement in infinite-layer nickelates.