Daily Overview: Today’s highlighted work focuses on a deeper understanding of the electronic structure of mixed-type Ruddlesden-Popper nickelates. On the theoretical side, [1] combines variational Monte Carlo calculations within an effective five-layer Hubbard model and finds that the spin-singlet d_{x^2-y^2}-wave state of La6Ni5O12 is energetically favored while extended s-wave pairing is strongly suppressed; based on the optimized superconducting state, [1] predicts signatures of a clean nodal superconductor, including a V-shaped tunneling density of states, linear depletion of the low-temperature superfluid density, a decrease in the Knight shift below Tc, and 1/(T1T)∝T^2. Within the same framework, LaNiO2 shows a robust d-wave tendency, and La4Ni3O8 lies close to the boundary between d-wave superconducting and non-superconducting states; on this basis, [1] proposes that a common d_{x^2-y^2}-wave pairing tendency may exist in the reduced RP nickelate family but emphasizes that this remains a theoretical prediction rather than an experimental establishment. On the experimental side, [2] uses 139La NQR spectra of La3Ni2O7.0 at ambient pressure combined with DFT+U calculations, observes two La(2)a/La(2)b sites with near-1:1 occupancy, points out that the two intrinsic inequivalent La(2) sites and two inequivalent Ni sites in the low-symmetry Am2m (No.38) structure naturally explain this phenomenon, and rules out defect models attributing the signal to stacking faults, impurity phases, or oxygen vacancies; based on the Am2m framework, [2] identifies the commensurate SDW state as “large-small spin stripe” order, in which different internal fields are associated with the small magnetic moment at Nia and the larger magnetic moment at Nib, respectively, and the small-spin Ni site has higher electron occupancy but a smaller magnetic moment, inconsistent with the local-moment picture, indicating that the ambient-pressure SDW order is driven by Fermi-surface nesting and is itinerant in nature. On the model side, [3] performs finite-temperature determinant quantum Monte Carlo simulations of a single-orbital bilayer Hubbard model under conditions of strong interlayer coupling and comparable intralayer and interlayer antiferromagnetic correlation strengths, showing that spin stripes form spontaneously with increasing hole doping and that the dominant pairing symmetry crosses from d-wave to s±-wave; this crossover tracks changes in the relative strengths of the two types of spin correlations, and the odd-channel dynamic spin structure factor further exhibits momentum-dependent reorganization of magnetic excitations. On this basis, [3] considers the spin-fluctuation structure to be a key link connecting spin stripes and pairing competition, providing a minimal-model perspective for understanding La3Ni2O7, although this result is a model-calculation prediction rather than an established experimental mechanism. arXiv submission processing window: 2026-11-05 00:00 to 2026-11-05 00:00 UTC.

1. $d$-wave superconductivity in quintuple-layer bulk superconductor La$_6$Ni$_5$O$_{12}$

Summary: Using an effective five-layer Hubbard model combined with variational Monte Carlo calculations, the study investigates the pairing symmetry of the five-layer Ruddlesden–Popper bulk nickelate La6Ni5O12. The calculations indicate that the spin-singlet d_{x^2-y^2}-wave state is energetically favored, while extended s-wave pairing is strongly suppressed; based on the optimized superconducting state, the study predicts a V-shaped low-energy tunneling density of states, linear depletion of the superfluid density at low temperatures, a decrease in the Knight shift below T_c, and a spin-lattice relaxation rate satisfying 1/(T1T)∝T^2, which together constitute the characteristics of a clean nodal superconductor. Extending the same framework to LaNiO2 reveals a robust d-wave tendency, whereas La4Ni3O8 at nominal filling lies close to the boundary between d-wave superconducting and non-superconducting states. On this basis, the study proposes that the reduced RP nickelate family may share a common d_{x^2-y^2}-wave pairing tendency and positions bulk La6Ni5O12 as a candidate platform for testing the pairing symmetry of nickelates, but the associated pairing mechanism remains a theoretical prediction rather than an experimentally established result.

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: The study re-examines the crystal structure and spin-density-wave (SDW) spin configuration of the bilayer nickelate La3Ni2O7.0 under ambient pressure using ¹³⁹La nuclear quadrupole resonance (NQR) spectroscopy combined with DFT+U calculations; two La(2) sites, La(2)a and La(2)b, are observed experimentally with an occupancy ratio close to 1:1, and it is pointed out that the two inequivalent La(2) sites and two inequivalent Ni sites intrinsically present in the low-symmetry Am2m structure (No. 38) naturally explain this phenomenon, whereas the Amam structure is difficult to reconcile with it, thereby rejecting previous models that attributed this signal to defects such as La4Ni3O10 stacking faults, impurity phases, or oxygen vacancies. Based on the Am2m framework, the commensurate SDW state is identified as “large-small spin stripe” order: the different internal fields at La(2)a and La(2)b are associated with the tiny magnetic moment of Nia and the larger magnetic moment of Nib, respectively, and the previously so-called “spinless” sites are reassigned as small-spin sites. Further comparison of valence and magnetic moments shows that the small-spin Ni sites have higher electron occupancy but smaller magnetic moments, which is inconsistent with a local-moment picture, indicating that this ambient-pressure SDW order is driven by Fermi-surface nesting and is itinerant in nature, and providing a basis for understanding the emergence of high-temperature superconductivity under high pressure.

3. Interplay between Spin Stripes and Pairing Crossover in the Bilayer Hubbard Model

Summary: Motivated by the unresolved question of whether d-wave or s±-wave pairing dominates in the bilayer nickelate La3Ni2O7 and by the unclear connection between its spin order, magnetic excitations, and pairing mechanism, the authors use finite-temperature determinant quantum Monte Carlo simulations of a single-orbital bilayer Hubbard model to study pairing competition under conditions of relatively strong interlayer coupling and comparable intra- and interlayer antiferromagnetic correlation strengths. The results show that with increasing hole doping, spin stripes form spontaneously, accompanied by a crossover of the dominant pairing symmetry from d-wave to s±-wave; this crossover tracks changes in the relative strengths of the two types of spin correlation, with interlayer correlations exceeding intralayer correlations in magnitude. The odd channel of the dynamical spin structure factor further exhibits momentum-dependent reorganization of magnetic excitations, consistent with a fluctuating spin-stripe picture. On this basis, the authors argue that the spin-fluctuation structure is a key link connecting spin stripes and pairing competition and offers a minimal-model perspective for understanding La3Ni2O7; this is a prediction from model calculations rather than an established experimental mechanism.