本文利用极化分辨电子拉曼散射测量了高质量La₃Ni₂O₇单晶，在150 K以下的密度波转变中观察到显著的、对称性依赖的谱权重重新分布：B₁g通道呈现不对称峰，B₂g通道呈现对称宽峰，分别对应布里渊区X/Y点附近和对角线方向的电子激发。定量分析提取出两组SDW能隙值，B₁g通道能隙约为37.5–40.4 meV（2Δ/kBTc≈5.5–5.9），B₂g通道能隙约为23.0 meV（2Δ/kBTc≈3.4），表明前者属中等至强耦合，后者属弱耦合。这种动量选择的各向异性耦合强度无法用简单的弱耦合嵌套理论解释，揭示了非常规SDW源于各向异性电子关联。能隙的温度依赖性明显弱于平均场预期，且B₂g通道各向同性与B₁g通道弱各向异性也支持两种不同耦合机制的共存。该工作确立了La₃Ni₂O₇中SDW的电子特性，为理解压力下镍酸盐高温超导的涌现提供了微观基础。
本研究利用第一性原理密度泛函理论（GGA及GGA+U）探讨了具有双层NiO2无限层结构的La3Ni2O5F的电子与磁性。La(O/F)La阻挡层实现了NiO2双层的严格隔离，形成纯粹二维电子与磁系统。计算发现一个由间隙区域电子密度构成的E单带，该带不与任何原子轨道关联，沿M-A方向下穿费米能级，提供0.09个穴/Ni的自掺杂，使Ni实际价态变为+1.09。该E带费米面呈柱状，占布里渊区9%面积。dpσ带近乎半满但受自掺杂偏移至范霍夫奇点附近。磁响应显示出不同于以往镍酸盐的反常特性，磁化率在较大磁场下趋于消失。无磁相变的现象可归因于强烈的二维自旋涨落及偏离半填充的自掺杂效应。这些结果揭示了Ni^{1+}离子在该体系中独特的行为，以及界面阻挡层对间隙带形态的关键影响。
本研究利用原子分辨率扫描隧道显微镜和谱学技术，对生长在SrLaAlO4上的1.5个单胞(La,Pr)3Ni2O7超薄膜进行了表征。通过低温超高真空样品转移，保持了有序的√2×√2表面重构，并在隧道谱中观测到具有两个能隙尺度（约14和20 meV）且零偏压电导平坦的U形谱，表明超导能隙无节点。相比之下，若转移过程中样品在超高真空中暴露时间较长且未冷却，虽仍保持表面重构和40 K以上的输运超导转变起始温度，但隧道谱呈现V形，且宽能谱显示氧缺失会使密度波相关的谱权重混合。通过对比不同转移时间的样品，确定了获得本征超导能隙需控制氧含量，提供了双层镍酸盐超薄膜本征无节点超导能隙的原子尺度观测证据。
该研究通过将无限层钐镍氧化物超导薄膜制备成空间周期性网络结构，调控库珀对的相位相干性，实现了超导-绝缘体相变。实验中观测到周期为h/2e的磁阻振荡，直接证明了镍氧化物中2e库珀对的存在。相变主要受增强的超导涨落驱动，且库珀对在整个相变过程中参与电荷输运。研究还发现了两种异常金属态：一种出现在有限磁场下，另一种在零磁场下即可出现；这两种态均可由玻色子激发表征，暗示涡旋在基态中的动态角色。该工作确立了镍氧化物作为研究库珀对相位相干性调控下丰富玻色子相的关键平台。
Bosonic Phases across the Superconductor-Insulator Transitions in Infinite-Layer Samarium Nickelate
The Ruddlesden–Popper (R–P) bilayer nickelate, La3Ni2O7, was recently found to show signatures of high-temperature superconductivity (HTSC) at pressures above 14 GPa (ref. 1). Subsequent investigations achieved zero resistance in single-crystalline and polycrystalline samples under hydrostatic pressure conditions2–4. Yet, obvious diamagnetic signals, the other hallmark of superconductors, are still lacking owing to the filamentary nature with low superconducting volume fraction2,4,5. The presence of a new 1313 polymorph and competing R–P phases obscured proper identification of the phase for HTSC6–9. Thus, achieving bulk HTSC and identifying the phase at play are the most prominent tasks. Here we address these issues in the praseodymium (Pr)-doped La2PrNi2O7 polycrystalline samples. We find that substitutions of Pr for La effectively inhibit the intergrowth of different R–P phases, resulting in a nearly pure bilayer structure. For La2PrNi2O7, pressure-induced orthorhombic to tetragonal structural transition takes place at Pc ≈ 11 GPa, above which HTSC emerges gradually on further compression. The superconducting transition temperatures at 18–20 GPa reach $${T}_{{\rm{c}}}^{{\rm{onset}}}=82.5\,{\rm{K}}$$and $${T}_{{\rm{c}}}^{{\rm{zero}}}=60\,{\rm{K}}$$, which are the highest values, to our knowledge, among known nickelate superconductors. Importantly, bulk HTSC was testified by detecting clear diamagnetic signals below about 75 K with appreciable superconducting shielding volume fractions at a pressure of above 15 GPa. Our results not only resolve the existing controversies but also provide directions for exploring bulk HTSC in the bilayer nickelates.
Recently, the Ruddlesden–Popper bilayer nickelate La3Ni2O7 has emerged as a superconductor with a transition temperature (Tc) of approximately 80 K above 14 GPa (refs. 1–3). Achieving a higher Tc in nickelate superconductors, along with the synthesis of reproducible high-quality single crystals without relying on high-oxygen-pressure growth conditions, remains a significant challenge4–7. Here we report superconductivity up to 96 K under high pressure in bilayer nickelate single crystals synthesized at ambient pressure. Energy-dispersive spectroscopy, single-crystal X-ray diffraction, nuclear quadrupole resonance and scanning transmission electron microscopy evidenced high crystal quality of the flux-grown La2SmNi2O7−δ single crystals. La2SmNi2O7 exhibits clear bulk superconductivity, including zero resistivity ( $${T}_{{\rm{c}},\max }^{{\rm{onset}}}$$ = 92 K and $${T}_{{\rm{c}},\max }^{{\rm{zero}}}$$ = 73 K at 21.6 GPa) and the Meissner effect (Tc = 60 K at 20.6 GPa). A low-temperature high-pressure structural study indicates that both monoclinic and tetragonal structures can support superconductivity in this bilayer nickelate. Furthermore, we established a correlation between higher Tc under high pressures and larger in-plane lattice distortion under ambient conditions, corroborated by observing even higher $${T}_{{\rm{c}}}^{{\rm{onset}}}$$of 96 K in La1.57Sm1.43Ni2O7−δ. This study overcomes key limitations in growing nickelate superconductor crystals, resolves the crystal structure in the superconducting state and demonstrates an effective pathway towards achieving higher Tc.
本研究基于有效三轨道模型，采用超越动力学平均场理论的D-TRILEX多体框架，系统分析了高压超导双层镍酸盐La₃Ni₂O₇正常态中多轨道与非局域自能效应的相互作用。结果表明，低能物理场景高度依赖于轨道间相互作用强度：当该强度较弱时，著名的γ准粒子平坦带位于费米能级以下；当强度增强时，该平坦带穿越费米能级，导致电子与铁磁性的顺磁子激发发生散射，形成自旋极化子束缚态。这些束缚态表现为费米能级下方出现的非相干谱权重阴影带。研究的发现揭示了双层镍酸盐中存在额外的竞争电子态，这为解释近期角分辨光电子能谱实验中关于费米面附近谱结构的争议提供了理论依据。
通过Ni L边共振非弹性X射线散射（RIXS）对三镍酸盐La4Ni3O10单晶的研究，发现其集体自旋激发具有约60 meV的带宽，与双层镍酸盐La3Ni2O7相当，但谱权重显著降低，表明三镍酸盐中电子关联较弱。同时观察到约100和200 meV的局域自旋激发，源自局域偶极和四极激发。分散的磁激发表现出三维特征，利用线性自旋波理论拟合得到层内与层间交换耦合参量相当，其中层间耦合最强。结果表明La4Ni3O10具有更强的三维磁性，其自旋动力学与自旋密度波序一致，而电子关联的减弱和三维多轨道特性是导致其磁激发谱与双层镍酸盐差异的关键因素，为理解Ruddlesden-Popper镍酸盐家族中磁性的演化及其与超导性的关联提供了重要见解。
利用偏振分辨电子拉曼散射技术，本研究系统测绘了三层镍氧化物La4Ni3O10中自旋密度波（SDW）能隙的动量选择性。实验观察到SDW引起的谱权重重分布同时出现在布里渊区中心的α口袋和靠近区边界的一部分β口袋上，对应的能隙能量约为55 meV；而β口袋沿对角线方向的区域未观察到可比的谱权重抑制，表明该区域几乎没有能隙打开。这一动量空间能隙拓扑与双层镍氧化物La3Ni2O7中仅β口袋出现各向异性SDW能隙的特征形成鲜明对比。上述结果确立了双层与三层镍氧化物间不同的动量空间能隙拓扑结构，为密度波不稳定性的有序波矢和与超导电性相关的机理提供了新的约束。