Daily Overview: Today’s highlighted work focuses on deeper understanding of the electronic structure of mixed Ruddlesden–Popper nickelates. Experimentally, [1] grew four-layer Ruddlesden–Popper nickelate La5Ni4O13 bulk single crystals by a high-pressure high-temperature method and reproduced the synthesis of La6Ni5O16, then topochemically reduced La5Ni4O13 single crystals with CaH2 to obtain the square-planar phase La5Ni4O10; this reduced phase exhibits bulk superconductivity at ambient pressure, with Tc^onset ≈18 K and a zero-field-cooled shielding fraction exceeding 60%, a specific-heat jump supports a bulk pairing transition, and in all crystals superconductivity coexists with a density-wave or metal–insulator transition; the upper-critical-field anisotropy γ≈2.4 remains finite as T→Tc, indicating three-dimensional superconducting characteristics and significant coupling between four-layer blocks, on which basis the authors propose that R_{n+1}Ni_nO_{2n+2} square-planar nickelates can enter the superconducting region solely by tuning the Ni valence through the layer number, without chemical substitution, applied pressure, or epitaxial constraint. Theoretically, [2] addresses the contradiction between the nodeless bulk gap and d-wave-like point-contact transport in La3Ni2O7 by assuming that strong interlayer antiferromagnetic coupling drives interlayer pairing as the dominant pairing channel and introducing interlayer and intralayer pairing amplitudes in a two-orbital tight-binding model to reproduce the ARPES/STM multiple-gap structure; the analysis shows that when the point contact couples unequally to the two layers, a contact potential odd under layer-exchange mirror symmetry can act as an effective Zeeman field for layer pseudospin, bind in-gap Andreev states in the barrier region, and in the mirror eigenbasis mix mirror sectors with opposite pairing signs, opening Andreev channels with reversed pairing sign; simulations of the top-layer barrier in La3Ni2O7 reproduce the orientation-dependent zero-bias conductance peaks and side peaks usually regarded as d-wave fingerprints, showing that a nodeless interlayer pairing state can mimic d-wave Andreev signals. Another theoretical work [3] gives spin-fluctuation-induced dominant d_{x^2-y^2}-wave pairing within a five-layer Hubbard model and RPA framework for bulk La6Ni5O12 and examines two phenomenological electron-phonon couplings inspired by the oxygen breathing mode: when present alone, both tend to favor s-wave, and attractive scattering between regions where the spin-fluctuation-induced gap has opposite signs suppresses d-wave pairing; after electron charge screening is included, this suppression is significantly weakened, the boundary between s-wave and d-wave pairing strength shifts toward the d-wave-favorable side, and momentum-resolved projection further locates where the unfavorable scattering processes are weakened, with d-wave pairing remaining dominant under representative electron interactions, thus emphasizing the competing roles of the momentum structure of electron-phonon coupling and charge screening in spin-fluctuation d-wave pairing. arXiv submission processing window: 2026-11-10 00:00 to 2026-11-10 00:00 UTC.

1. Ambient-pressure superconductivity in bulk single crystals of La$_5$Ni$_4$O$_{10}$

Summary: In this study, bulk single crystals of the four-layer Ruddlesden–Popper nickelate La5Ni4O13 were grown by a high-pressure high-temperature method, and the synthesis of the five-layer La6Ni5O16 was reproduced; La5Ni4O13 single crystals were then topotactically reduced with CaH2 to yield the square-planar phase La5Ni4O10. This reduced phase behaves as a bulk superconductor at ambient pressure, with an onset transition temperature Tc^onset of approximately 18 K and a zero-field-cooled shielding fraction exceeding 60%, while a specific-heat jump confirms a bulk pairing transition; superconductivity coexists with density-wave or metal–insulator transitions in all crystals. The upper critical field anisotropy γ≈2.4 remains finite as T→Tc, indicating three-dimensional superconducting characteristics and significant coupling between the four-layer blocks. On this basis, the authors propose that square-planar nickelates R_{n+1}Ni_nO_{2n+2} can enter the superconducting regime solely by tuning the Ni valence through the layer number, without chemical substitution, applied pressure, or epitaxial constraint.

2. Mimicry of $\textit{d}$-wave Andreev Signatures by Interlayer Pairing

Summary: Addressing the contradiction in the bilayer nickelate La3Ni2O7 between a bulk nodeless energy gap and d-wave-like features in point-contact transport, the paper theoretically assumes that interlayer pairing driven by strong interlayer antiferromagnetic coupling is the dominant pairing channel and introduces interlayer and intralayer pairing amplitudes in a two-orbital tight-binding model to reproduce the multi-gap structure seen by ARPES/STM. It is pointed out that when point-contact coupling to the two layers is inhomogeneous, a contact potential that is odd under layer-exchange mirror symmetry can act as an effective Zeeman field on the layer pseudospin, thereby binding in-gap Andreev states in the barrier region that shift with barrier width and strength; in the mirror eigenbasis, this barrier mixes mirror sectors with opposite pairing sign and opens a sign-reversal Andreev channel. Simulations for a top-layer barrier on La3Ni2O7 show that its subgap conductance can reproduce the orientation-dependent zero-bias conductance peaks and side peaks usually taken as a d-wave fingerprint, indicating that a nodeless interlayer pairing state can mimic d-wave Andreev signals and providing a natural picture for reconciling conflicting experimental evidence.

3. Momentum-dependent phonon effects on spin-fluctuation $d$-wave pairing in bulk La$_6$Ni$_5$O$_{12}$

Summary: This study addresses bulk La6Ni5O12 by employing a five-layer Hubbard model and, within the random phase approximation (RPA) framework, examines the influence of electron-phonon coupling on spin-fluctuation-induced d_{x^2-y^2}-wave pairing; the dominant pairing state given by this framework is d_{x^2-y^2}-wave. Two phenomenological couplings inspired by oxygen breathing modes, when present alone, favor s-wave pairing, but their attractive scattering between regions where the spin-fluctuation-induced gap has opposite signs suppresses d-wave pairing. Upon inclusion of electronic charge screening, this suppression is significantly weakened; comparison of the s-wave and d-wave pairing strengths with and without screening shows that screening shifts their boundary toward the side favorable to d-wave pairing, and momentum-resolved projections further identify where the unfavorable scattering processes are weakened. Under representative electron interactions, d-wave pairing remains dominant; these results emphasize the competing roles of the momentum structure of electron-phonon coupling and charge screening in spin-fluctuation d-wave pairing.