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The recent discovery of superconductivity in La3Ni2O7−δ under high pressure with a transition temperature around 80 K (ref. 1) has sparked extensive experimental2–6 and theoretical efforts7–12. Several key questions regarding the pairing mechanism remain to be answered, such as the most relevant atomic orbitals and the role of atomic deficiencies. Here we develop a new, energy-filtered, multislice electron ptychography technique, assisted by electron energy-loss spectroscopy, to address these critical issues. Oxygen vacancies are directly visualized and are found to primarily occupy the inner apical sites, which have been proposed to be crucial to superconductivity13,14. We precisely determine the nanoscale stoichiometry and its correlation to the oxygen K-edge spectra, which reveals a significant inhomogeneity in the oxygen content and electronic structure within the sample. The spectroscopic results also reveal that stoichiometric La3Ni2O7 has strong charge-transfer characteristics, with holes that are self-doped from Ni sites into O sites. The ligand holes mainly reside on the inner apical O and the planar O, whereas the density on the outer apical O is negligible. As the concentration of O vacancies increases, ligand holes on both sites are simultaneously annihilated. These observations will assist in further development and understanding of superconducting nickelate materials. Our imaging technique for quantifying atomic deficiencies can also be widely applied in materials science and condensed-matter physics.

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本文综述了加压双层镍酸盐 La₃Ni₂O₇ 超导电性的弱耦合理论研究，主要采用随机相近似（RPA）、涨落交换近似（FLEX）和泛函重整化群（FRG）三种方法，基于包含 Ni d_{x²-y²} 和 d_{3z²-r²} 轨道的双层双轨道 Hubbard 模型。研究表明，压力下费米面由电子型 α 口袋和空穴型 β、γ 口袋构成，其中主要由 d_{3z²-r²} 轨道构成的 γ 口袋与其他口袋存在强嵌套，显著增强反铁磁自旋涨落并介导吸引配对。不同方法普遍预言主导配对对称性为 s± 波：γ 与 α 口袋超导能隙同号，β 口袋反号，且与 d_{3z²-r²} 主导的层间配对通道密切相关。RPA 高效描述自旋和电荷磁化率，FLEX 通过自洽修正准粒子与自旋涨落，FRG 可无偏处理超导、自旋密度波和电荷密度波等竞争序。综述认为弱耦合理论能有效关联费米面几何与自旋涨落配对机制，并指出未来需与强耦合图像及更精确的第一性原理参数相结合。

本研究通过定制金刚石对顶砧旋转器，对Pr₄Ni₃O₁₀单晶进行原位高压角度依赖电输运测量，证实了其超导各向异性。在50.2 GPa压力下，样品出现超导转变，临界温度约31 K。通过测量垂直和平行于ab面的上临界场，得到各向异性参数γ约为1.6，该值随温度升高而减小，在接近超导临界温度时趋近于1。根据Ginzburg‑Landau模型拟合，零温上临界场平行和垂直于ab面分别为89.9 T和57.3 T，并计算出ab面内和c轴方向的相干长度分别为2.4 nm和1.5 nm。与铜氧化物及铁基超导体对比发现，Pr₄Ni₃O₁₀的各向异性行为符合双带模型，其中层内量子限域诱导了层间相干性，从而呈现出三维超导特性。该研究不仅证实了块体Ruddlesden‑Popper镍酸盐中存在各向异性超导，而且为理解维度对高温超导机制的影响提供了关键见解。

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针对双层镍酸盐La₃Ni₂O₇中配对机制的争议，本文利用角分辨光电子能谱（ARPES）和扫描隧道显微镜（STM）揭示的低各向异性无节点全间隙作为约束，提出以布里渊区对角线上的配对能隙为关键探针。对称性分析表明，沿该对角线上dx2-y2与dz2轨道间的杂化消失，使得γ口袋和α/β口袋的能隙分别反映两个轨道的本征配对强度。基于dz2轨道主导的杂化驱动配对机制会导致α/β口袋在对角线方向出现能隙节点，与实验观测的U形dI/dV谱相矛盾；而dx2-y2轨道主导的洪特规则驱动配对机制则产生整个费米面上的均匀全间隙，与ARPES和STM结果一致。弱耦合随机相近似计算也因dz2轨道的态密度优势在对角线附近给出节点或近节点行为，与实验冲突。因此，该工作澄清了dx2-y2轨道在配对中的主导地位，并确立洪特规则驱动的配对机制为La₃Ni₂O₇中最相关的超导配对图像。

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