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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.

The orbital character of the so-called $A$-pocket in infinite-layer nickelates remains under debate. We investigate bulk NdNiO$_2$ using density-functional theory combined with dynamical mean-field theory and identify the $A$-pocket as a strongly hybridized state with substantial interstitial-$s$, Nd-$d_{xy}$ and Ni-d_${xz/yz}$ contributions. Our angle-resolved photoemission spectroscopy simulations reproduce the main polarization-dependent features measured experimentally. Removing the interstitial-$s$ matrix element leads to a clear disagreement with the measured Fermi surface maps, even though the interstitial-$s$ contribution is not dominant. Suppressing instead the Nd-d$_{xy}$ or Ni-$_{dxz/yz}$ matrix elements leaves the maps almost unchanged, because these are small at the zone corner. More generally, our results demonstrate that assigning the microscopic origin of spectral features in infinite-layer nickelates requires treating orbital hybridizations and photoemission matrix elements together.

This study investigates the newly discovered nickel-based superconductor La₃Ni₂O₇ and reveals the “orbital dimerization” mechanism underlying its first-order structural phase transition. The authors note that standard density functional theory (DFT) and the LDA+DMFT method incorporating intra-atomic correlations fail to reproduce the abruptness of this transition because they neglect crucial inter-atomic correlations. By constructing a many-body effective Hamiltonian that includes low-energy active orbitals and performing exact diagonalization, they find that when the Ni-O-Ni bond angle approaches linearity, strong antiferromagnetic superexchange interactions emerge between interlayer Ni orbitals, forming spin-singlet bonds that cause a sharp drop in total energy. This energy reduction creates a new local energy minimum at a specific configuration that coexists with the tilted configuration, perfectly explaining the first-order transition characteristics observed in experiments as well as the coexistence of high- and low-pressure structures. This mechanism not only alters the lattice bonding properties but also leads to qualitative changes in the low-energy electronic structure, such as the emergence of superconductivity. The computational framework of DFT plus many-body corrections established in this work is universal and applicable to ionic materials containing open-shell d/f electrons, providing key microscopic insights into understanding the structure–electronic property relationships in such systems.

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The report of superconductivity (SC) with Tc~80 K in bilayer Ruddlesden-Popper (RP) nickelate La3Ni2O7-delta have sparked considerable investigations on its normal state properties and SC mechanism under pressure and at low temperature. It is believed that the density wave (DW) at ~150 K plays an important role in SC emergence, but its nature remains largely underexplored. Here, we utilized temperature-dependent in-situ Ni K-edge X-ray Absorption Near-edge Spectroscopy (XANES) to probe the Ni-3d/4p electronic states of La3Ni2O7-delta and La2PrNi2O7-delta samples down to 4.8 K, enabling us to witness the evolution of both in-plane d_(x^2-y^2)/p_x (p_y) and out-of-plane d_(3z^2-r^2)/p_z orbitals of NiO6 octahedron across the DW transition. Main edge energy associated with Ni 4p orbital shows an anomalous decline near DW transition, signifying the occurrence of lattice distortions as a hallmark of charge density wave. Below DW transition, the enlarged crystal field splitting (CFS) indicates an enhanced NiO6 octahedral distortion. Intriguingly, magnetic Pr substituents could activate the mutual interplay of d_(x^2-y^2) and d_(3z^2-r^2) orbitals. We discussed its relevance to the favored bulk SC in the pressurized polycrystalline La2PrNi2O7-delta than pristine.

Site-selective (^{17})O-NMR measurements of the inner apical O(1), outer apical O(2), and planar O(3,4) sites were used to investigate the spin density wave order in the bilayer nickelate La(3)Ni(2)O(7). Below (T{\rm SDW}=150) K, all planar O(3,4) sites broaden significantly due to the appearance of internal magnetic fields, while the O(2) sites show almost no internal field, consistent with coherent spin density wave order with single-spin–no-spin (or large–small spin) stripes; the internal field at the O(1) sites bridging the NiO(2) planes nearly cancels, indicating an antiparallel spin configuration between adjacent planes. Below (T{\rm A}\sim 115) K, however, although the in-plane spin density wave order remains robust, the O(1) spectrum disappears, indicating that the antiparallel spin arrangement through the Ni–O(1)–Ni bonds is not particularly stable. In particular, the O(2) sites, which are strongly covalently bonded to the (d{3z^2-r^2}) orbitals, exhibit extremely small local spin susceptibility, suggesting that the Ni-(d{3z^2-r^2}) orbitals bridging the NiO(_2) planes already form a well-developed interlayer spin singlet. These results reveal interlayer spin singlet formation and an anomalous spin reconstruction through the Ni–O(1)–Ni bonding orbitals, reflecting the orbital-selective nature of this bilayer nickelate.