Daily Overview: Today’s highlights focus on deepening the understanding of the electronic structure of hybrid Ruddlesden-Popper nickelates. [1] Through theoretical predictions, several cobalt-based La₃Ni₂O₇ analogs (such as LaTh₂Co₂O₇, etc.) have been proposed, in which the cobalt 3d orbitals exhibit half-filled occupancy and strong Hund coupling akin to those in nickelates, with local magnetic moments falling precisely within the optimal window for nickel-based superconductivity. This suggests the possibility of realizing s-wave superconductivity via spin fluctuation mechanisms, providing a theoretical basis for expanding the nickelate superconductor family to cobalt-based systems. [2] In contrast, [2] raises significant doubts about the previously reported method for calculating the superconducting volume fraction in pressurized Ruddlesden-Popper nickelates (e.g., La₄Ni₃O₁₀). It finds that the nonstandard equations used earlier, due to geometric parameter errors, led to nearly twofold overestimations. After correction, the actual superconducting phase fraction is only 51%–59%. This finding necessitates a re-evaluation of all published superconducting volume fraction data for this system. arXiv submission processing window: 2026-02-26 00:00 to 2026-02-26 00:00 UTC.

1. Prediction of several Co-based La$_3$Ni$_2$O$_7$-like superconducting materials

Summary: By electronic doping of the high-pressure bilayer cobaltate La₃Co₂O₇, cobalt-based La₃Ni₂O₇ analogs such as LaTh₂Co₂O₇, La₃Ni₂O₅Cl₂, and La₃Ni₂O₅Br₂ are predicted, which possess similar crystal structures and strongly correlated electronic states. Calculations based on density functional theory plus dynamical mean-field theory (DFT+DMFT) and random phase approximation (RPA) indicate that the cobalt 3d orbitals exhibit half-filled or near-half-filled occupancy, analogous to the nickel orbitals in La₃Ni₂O₇, and display strong Hund coupling and mass enhancement. The local magnetic moments (approximately 0.64 μ_B) of these cobalt-based compounds fall precisely within the optimal window for nickelate superconductivity (0.63–0.68 μ_B), strongly suggesting the possibility of high-temperature superconductivity via a similar spin fluctuation mechanism. RPA calculations further reveal that the leading pairing symmetry in electron-doped cobaltates is s-wave, belonging to the A₁ᵍ irreducible representation. This work provides a theoretical basis for achieving high-temperature superconductivity in cobalt-based systems and encourages further experimental synthesis and characterization.

2. Nearly twofold overestimation of the superconducting volume fraction in pressurized Ruddlesden-Popper nickelates

Summary: This study points out that Zhu et al., when measuring the superconducting volume fraction of pressurized Ruddlesden-Popper nickelate La₄Ni₃O₁₀, employed a previously unreported calculation equation, leading to a significant overestimation of the results. By reanalyzing the original data published by Zhu et al. using standard methods for calculating superconducting magnetic moments, the authors found that the superconducting volume fraction is only 51% to 59%, rather than the 81% to 86% reported by Zhu et al. Upon examining the equation and its derivation provided by Zhu et al., the authors discovered that the equation mistakenly used sample geometry parameters in its calculation, resulting in an approximately two-fold overestimate of the volume proportion occupied by the superconducting phase. Using a hypothetical sample as an example, the authors demonstrate that even if the superconducting phase actually accounts for only 50%, this equation would still yield a result close to 100%. Consequently, this error affects all previously reported superconducting volume fraction data for Ruddlesden-Popper nickelates, necessitating a re-evaluation of these conclusions.