Daily Overview: Recent highlights in nickel-based superconductivity focus on advances in ambient-pressure bulk synthesis, ground-state magnetic structure resolution, and theoretical exploration of multi-orbital pairing mechanisms. In experimental synthesis, [1] employed high-temperature, high-pressure methods to synthesize the five-layer Ruddlesden-Popper phase La$_6$Ni$_5$O$_{16}$, followed by topological reduction to produce the five-layer square-planar nickelate La$_6$Ni$_5$O$_{12}$, achieving bulk superconductivity at ambient pressure near 18 K; heat capacity data indicate a superconducting volume fraction of approximately 25%, overcoming previous limitations that confined nickelate superconductivity to thin films or high-pressure environments. Regarding ground-state properties, [2] utilized $^{139}$La nuclear quadrupole resonance (NQR) spectroscopy to re-examine the crystal structure of the bilayer nickel oxide La$_3$Ni$_2$O$_7$ at ambient pressure, re-identifying the spin density wave (SDW) spin configuration within a low-symmetry $Am2m$ structural framework as a large-small spin stripe structure originating from two inequivalent Ni sites, and combined with DFT+U calculations to confirm that this SDW order is driven by Fermi surface nesting with itinerant character. In terms of theoretical mechanisms, [3] investigated the regulation of competition between interlayer $s^{\pm}$ and intralayer $d$-wave pairing in bilayer nickelates by vertical electric fields and electron correlations using a constrained path quantum Monte Carlo method based on a bilayer two-orbital Hubbard model, revealing how interlayer potential differences, crystal field splitting, and Hund coupling influence the relative competitiveness of pairing channels through charge redistribution and enhanced magnetic correlations, thereby providing a theoretical basis for understanding how layer polarization and multi-orbital interactions affect the relative pairing tendencies in bilayer nickelates. arXiv submission processing window: 2026-10-15 00:00 to 2026-10-15 00:00 UTC.

1. Bulk superconductivity in quintuple-layer square-planar nickelate La6Ni5O12

Summary: This study achieved bulk superconductivity at ambient pressure by synthesizing the five-layer Ruddlesden-Popper phase La6Ni5O16 under high temperature and high pressure conditions, followed by a topological reduction reaction to produce the five-layer square planar (SP) nickelate La6Ni5O12. The material exhibits a superconducting transition at approximately 18 K, a property confirmed by zero resistance, diamagnetic shielding, and specific heat measurements, with the latter revealing a superconducting volume fraction of about 25% that strongly supports the presence of bulk superconductivity. Magnetization and torque magnetometry measurements revealed distinct magnetic hysteresis loops, verifying the existence of vortex pinning effects and Type-II superconducting characteristics. This result overcomes the previous limitation that nickelate superconductivity was confined to thin films or high-pressure environments, thereby providing a critical platform for the in-depth investigation of the mechanisms underlying nickel-based high-temperature superconductivity in ambient-pressure bulk samples.

2. Large-Tiny Spin-Stripe SDW Order in La$_3$Ni$_2$O$_7$ via $^{139}$La-NQR Spectroscopy: Reconsideration Based on the $Am2m$ Structure

Summary: This study employs $^{139}$La nuclear quadrupole resonance (NQR) spectroscopy to re-examine the crystal structure and spin density wave (SDW) state of ambient-pressure bilayer nickelate La$_3$Ni$_2$O$_7$. The experimental observation of two distinct La(2) sites is naturally explained within the low-symmetry $Am2m$ structural framework, whereas previous defect models based on the $Amam$ structure fail to provide a self-consistent description. Within the $Am2m$ structure, the SDW spin configuration is re-identified as a “large-small” spin stripe structure, arising from the distinct large and small magnetic moments associated with two inequivalent Ni sites. Based on DFT+U calculations and NQR parameter analysis, the study confirms the itinerant nature of this SDW order, indicating that it is driven by Fermi surface nesting rather than local magnetic moment mechanisms, thereby providing a foundation for understanding the origin of high-$T_c$ superconductivity in this material under high pressure.

3. Interplay of electric field and electronic correlations in pairing competition in bilayer nickelates

Summary: Based on a bilayer two-orbital Hubbard model, this study employs the ground-state constrained path integral quantum Monte Carlo method to investigate the regulatory mechanisms of vertical electric fields (interlayer potential differences) and electron correlations on the competition between interlayer $s^{\pm}$ and intralayer $d$-wave pairing in bilayer nickelates. The results indicate that increasing the interlayer potential difference typically enhances the $d$-wave pairing vertex contribution and suppresses $s^{\pm}$ pairing, thereby shifting the pairing preference toward the $d$-wave channel; conversely, increasing the crystal field splitting enhances the relative competitiveness of $s^{\pm}$ pairing, while increasing the Hund coupling favors $s^{\pm}$ pairing under weak bias but enhances $d$-wave pairing under large bias. Microscopic analysis reveals that the electric field primarily induces interlayer charge redistribution within each orbital sector, accompanied by enhancements in the antiferromagnetic spin structure factor and local interorbital parallel spin correlations induced by Hund coupling. These findings elucidate the potential connections among charge redistribution, magnetic correlations, and pairing competition, providing a theoretical basis for understanding how layer polarization and multi-orbital interactions influence the relative pairing tendencies in bilayer nickelates.