Daily Overview: Today’s highlight work focuses on an in-depth understanding of the electronic structure of mixed Ruddlesden-Popper nickelates. On one hand, [1] identified triple calculation errors in the zero-field-cooled magnetization data of the high-pressure nickelate superconductor La₂SmNi₂O₇ single crystals, emphasizing that the superconducting phase fraction should be 22.8% instead of the previously reported 62.1%, and warned about the misleading influence of the selection of demagnetization factor and magnetization mode on the estimation of volume fraction, though the authors acknowledge the experimental confirmation of bulk superconductivity. On the other hand, [2] starting from an itinerant perspective, based on tight-binding fitting and random phase approximation analysis of compressively strained bilayer nickel oxide thin films, revealed the decisive role of Hund coupling strength on superconducting pairing symmetry and spin density wave type — under strong Hund coupling, s-wave superconductivity and (π/2,π/2) spin density wave dominate, while weak Hund coupling tends to d-wave pairing and (π,π) spin density wave, qualitatively consistent with density matrix renormalization group results. These two works advance the understanding of the superconducting phase and magnetism in nickelates from the perspectives of data correction and theoretical mechanism, respectively. arXiv submission processing window: 2026-02-27 00:00 to 2026-02-27 00:00 UTC.

1. Threefold error in the reported zero-field cooled magnetic moment of single crystal $La_2SmNi_2O_7$

Summary: This paper points out that Li et al. made three errors in calculating the superconducting volume fraction from zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements on single crystals of the high-pressure nickelate superconductor La₂SmNi₂O₇. First, due to the paramagnetic Meissner effect (Wohlleben effect), the magnetic moment in FC mode can be either positive or negative, and thus cannot be used to calculate the superconducting volume fraction. Second, reanalysis of Li et al.’s ZFC data reveals that, according to their own calculation method, the superconducting volume fraction should be 22.8%, not the reported 62.1%—a discrepancy of about a factor of three, primarily arising from differences in the demagnetization factor (Li et al. used 0.849, while the authors calculated 0.81548 using Brandt’s formula). Third, even if the ZFC magnetic moment value is correctly calculated, it cannot be directly used to determine the superconducting volume fraction, because there are infinitely many shapes and distributions of superconducting regions smaller than the actual sample size that can produce the same measured magnetic moment. The authors emphasize that they agree with Li et al.’s experimental confirmation of bulk superconductivity in pressurized nickelates, but argue that these calculation errors need to be corrected to avoid misleading future research.

2. Superconductivity and magnetism in bilayer nickelates: itinerant perspective

Summary: This study investigates the superconductivity and magnetism of bilayer nickelates from an itinerant perspective. Based on tight-binding fitting of angle-resolved photoemission spectroscopy data from compressively strained films, the authors introduced standard on-site repulsive interactions (including intra-orbital U, inter-orbital U′, Hund’s coupling JH, and pair-hopping JP) and renormalized these bare interactions through the random phase approximation (RPA) by considering particle-hole fluctuations, thereby obtaining an effective pairing interaction. The results show that in the strong Hund’s coupling regime, s-wave superconductivity and (π/2, π/2) spin density wave (SDW) order are the dominant ground states, while under weak Hund’s coupling, d-wave pairing and (π, π) SDW become the leading ground states. These findings are qualitatively consistent with previous density matrix renormalization group (DMRG) studies, underscoring the critical role of Hund’s coupling in determining the superconducting pairing symmetry and magnetic type of the system.