Daily Overview: Today’s highlights focus on deepening the understanding of the electronic structure of hybrid Ruddlesden-Popper nickelates. In [1], effective regulation of hole carriers was achieved through Na doping of La₃Ni₂O₇₊δ, significantly enhancing metallicity and suppressing the density wave transition, providing key experimental evidence for understanding the role of element substitution in stabilizing the high-pressure superconducting phase. Using soft X-ray absorption spectroscopy combined with theoretical calculations, [2] systematically tracked the topological reduction process from perovskite to infinite-layer nickelates, revealing that even in the most reduced films, the Ni 3d hole count remains as high as 1.35, and O 2p holes persist throughout. This challenges the simple hole-injection model of superconductivity, highlighting a complex synergistic mechanism involving self-doping and oxygen non-stoichiometry. Additionally, [3] rigorously examined the common method of estimating superconducting volume fractions based on internal magnetic susceptibility, pointing out that non-uniform distribution of the demagnetization factor can lead to significant overestimation, offering an important methodological caution for determining bulk superconductivity in systems such as Pr₄Ni₃O₁₀. arXiv submission processing window: 2026-03-10 00:00 to 2026-03-10 00:00 UTC.

1. Enhancement of metallicity by Na doping in La$_3$Ni$_2$O$_{7+δ}$

Summary: Polycrystalline samples of La₃₋ₓNaₓNi₂O₇₊δ with various sodium doping concentrations were synthesized via a solid-state method, and their structural, thermal, magnetic, and electrical transport properties were systematically investigated using X-ray diffraction, thermogravimetric analysis, and measurements of magnetic susceptibility and electrical resistivity. X-ray diffraction analysis revealed that when the sodium doping level x ≥ 0.075, the samples undergo a structural transition from the ‘327’ Amam phase to the ‘4310’ Bmab phase, accompanied by gradual lattice expansion. Resistivity measurements indicated that sodium doping significantly enhances metallicity while slightly suppressing the density wave transition temperature; applying external pressure further suppresses the density wave transition, yet the low-temperature insulating behavior remains insensitive to pressure. These findings demonstrate that hole doping introduced by substituting sodium for lanthanum effectively modulates competing electronic phases in layered nickelates, providing crucial experimental evidence for understanding the roles of elemental substitution and carrier doping in stabilizing high-pressure superconducting phases.

2. From perovskite to infinite-layer nickelates: hole concentration from x-ray absorption

Summary: This study systematically analyzes the electronic structure evolution of PrNiOx thin films during stepwise topotactic reduction using soft X-ray absorption spectroscopy. Combined with single-cluster and double-cluster ligand-field calculations, the researchers found that at no intermediate stage of the transition from perovskite to infinite-layer nickelate did any sample exhibit a pure d⁹ electronic configuration. Quantitative analysis using the charge sum rule reveals that even in the most reduced films, the average number of Ni 3d holes reaches 1.35, while superconducting samples show higher hole concentrations, challenging previously assumed hole-doping limits. Meanwhile, oxygen K-edge absorption spectra exhibit significant changes during reduction, with oxygen 2p holes persisting even in the most reduced films. Integrating these results, the paper argues that the hole-doping mechanism is complex, arising from a combination of self-doping effects and oxygen non-stoichiometry, rather than simple hole injection.

3. On the estimating the superconducting volume fraction from the internal magnetic susceptibility

Summary: Zhang et al. [1] reported zero-field cooling (ZFC) and field cooling (FC) data for Pr₄Ni₃O₁₀ single crystals under high pressure, confirming bulk superconductivity in Ruddlesden-Popper nickelates and estimating a superconducting volume fraction f = 0.85 using the amplitude of the internal susceptibility χ_internal. This paper questions the key assumption of that method, namely f = |χ_internal|, and demonstrates through a counterexample that even when the calculated |χ_internal| for a sample reaches 0.82, the actual superconducting volume fraction f may be below 0.10. The authors point out that the calculation of the demagnetizing factor N depends on the distribution of non-superconducting regions within the sample, and the homogeneity assumption does not hold—if 90.2% of the volume is non-superconducting phase, the measured susceptibility can still match the original results as long as the geometry alters the demagnetizing factor. Therefore, χ_internal alone cannot uniquely determine f, and this approach requires reexamination across the entire field of superconductivity research.