Zhang等人[1]报道了高压下Pr₄Ni₃O₁₀单晶的零场冷却(ZFC)和场冷却(FC)数据，证实了Ruddlesden-Popper镍酸盐中的体超导电性，并利用内部磁化率χ_internal的幅度估算超导体积分数f=0.85。本文对该方法的关键假设f=|χ_internal|提出质疑，并通过反例说明：即使样品计算出的|χ_internal|达0.82，实际超导体积分数f可能低于0.10。作者指出，退磁因子N的计算依赖于非超导区域在样品中的分布，而均匀性假设并不成立——若90.2%的体积为非超导相，只要其几何形状改变退磁因子，磁化率测量值仍可匹配原始结果。因此，仅凭χ_internal无法唯一确定f，该方法在整个超导研究领域需要重新审视。
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.
该研究以新发现的镍基超导体La₃Ni₂O₇为例，揭示了其一级结构相变背后的“轨道二聚化”机制。作者指出，标准密度泛函理论（DFT）及考虑原子内关联的LDA+DMFT方法均无法重现该相变的突然性，因为这些方法忽略了关键的原子间关联。通过构建包含低能活性轨道的多体有效哈密顿量并进行精确对角化，他们发现当Ni-O-Ni键角趋于平直时，层间Ni轨道之间会产生强烈的反铁磁超交换作用，形成自旋单态键合，从而引起总能量急剧下降。这种能量降低在特定构型处产生新的局域能量极小值，与倾斜构型共存，完美解释了实验中观测到的一级相变特征，以及高低压结构的共存现象。该机制不仅改变了晶格键合性质，还导致低能电子结构定性变化（如超导电性的出现）。该工作建立的DFT加多体修正计算框架具有普适性，适用于含开壳层d/f电子的离子材料，为理解此类体系的结构与电子性质关联提供了关键微观机理。
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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.
利用位点选择性17O-NMR对内顶端O(1)、外顶端O(2)和平面O(3,4)位点的测量研究了双层镍酸盐La3Ni2O7的自旋密度波序。在T_SDW=150 K以下，所有平面O(3,4)位点因内部磁场出现而显著展宽，而O(2)位点几乎无内部场，这与具有单自旋-无自旋（或大-小自旋）条纹的共格自旋密度波序一致；桥接NiO2平面的O(1)位点内部场几乎抵消，表明相邻平面间为反平行自旋构型。然而在T_A约115 K以下，尽管面内自旋密度波序仍保持强健，O(1)谱却消失，说明通过Ni–O(1)–Ni键的反平行自旋排列并不特别稳定。尤其与d_{3z^2-r^2}轨道强共价的O(2)位点局域自旋磁化率极小，表明桥接NiO2平面的Ni-d_{3z^2-r^2}轨道已形成良好的层间自旋单态。这些结果揭示了层间自旋单态形成及通过Ni–O(1)–Ni键合轨道的异常自旋重构，体现了该双层镍酸盐的轨道选择性本质。