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4 条关联记录

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。

7 条关联记录

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。

利用Ni L3边共振非弹性X射线散射（RIXS），本研究对比了八面体Ruddlesden-Popper（RP）相Nd9Ni8O25（非超导）及其还原后的平面相Nd9Ni8O18（具有约5 K的超导关联）中的低能自旋激发。结果显示，八面体相呈现自旋密度波（SDW）基态，有序波矢为（1/4,1/4），其低能谱由沿(0,π)和(π,π)方向的弱色散paramagnon主导；而平面相则在（1/3,0）处出现弹性峰，磁激发无色散且能量约65 meV。偏振RIXS进一步确认了两种相中磁激发的不同特性。这些发现系统揭示了两个结构家族在基态和自旋激发上的根本差异，为理解镍酸盐超导电性的机制提供了关键信息。

本研究采用高压合成与静水高压输运技术，系统调控双层镍酸盐La₃Ni₂O₇家族中的能带宽度和能带填充，以探究其对超导电性及非超导态性质的影响。通过将La部分替换为较小的Nd（增大NiO₆八面体倾斜、减小带宽），超导相所需压力显著升高；而同时引入Sr进行空穴掺杂则反转此趋势，使超导起始压力降低。在非超导态中，观察到多达三种特征电阻异常，其随压力演化，可能分别对应与超导相互竞争的电荷密度波和自旋密度波有序态。综合对比不同成分样品的相图，表明带宽和填充的独立控制是揭示该体系非常规超导机理及其竞争序的关键。

The recent observation of high-𝑇𝑐 superconductivity in the bilayer nickelate La3⁢Ni2⁢O7 under pressure has garnered significant interest. While researches have predominantly focused on the role of electron-electron interactions in the superconducting mechanism, the impact of electron-phonon coupling (EPC) has remained elusive and unexplored. In this Letter, we perform first-principles calculations to study the phonon spectrum and electron-phonon coupling within La3⁢Ni2⁢O7 under pressure and explore the interplay between EPC and electronic interactions on the superconductivity by employing functional renormalization group (FRG) approach. Our calculations reveal that EPC alone is insufficient to trigger superconductivity in La3⁢Ni2⁢O7 under pressure. We identify unique out-of-plane and in-plane breathing phonon modes which selectively couple with the Ni 𝑑𝑧2 and 𝑑𝑥2−𝑦2 orbitals, showcasing an orbital-selective EPC. Within the bilayer two-orbital model, it is revealed that solely electronic interactions foster 𝑠±-wave pairing characterized by notable frustration in the band space, leading to a relatively low transition temperature. Remarkably, we find that the out-of-plane EPC can act in concert with electronic interactions to promote the interlayer pairing in the 𝑑𝑧2 orbital, partially releasing the pairing frustration and thus elevating 𝑇𝑐. In contrast, the inclusion of in-plane EPC only marginally affects the superconductivity, distinct from the cuprates. Potential experimental implications in La3⁢Ni2⁢O7 are also discussed.

The discovery of pressurized superconductivity at 80 K in La3Ni2O7 officially brings nickelates into the family of high-temperature superconductors, which gives rise to not only new insights but also mysteries in the strongly correlated superconductivity. More recently, the sibling compound La4Ni3O10 was also shown to be superconducting below about 25 K under pressure, further boosting the popularity of nickelates in the Ruddlesden-Popper phase. In this study, combining high-resolution angle-resolved photoemission spectroscopy and ab initio calculation, we systematically investigate the electronic structures of La4Ni3O10 at ambient pressure. We reveal a high resemblance of La4Ni3O10 with La3Ni2O7 in the orbital-dependent fermiology and electronic structure, suggesting a similar electronic correlation between the two compounds. The temperature-dependent measurements imply an orbital-dependent energy gap related to the density-wave transition in La4Ni3O10. By comparing the theoretical pressure-dependent electronic structure, clues about the superconducting high-pressure phase can be deduced from the ambient measurements, providing crucial information for deciphering the unconventional superconductivity in nickelates.

该研究利用扫描超导量子干涉显微镜对无限层镍基超导体Nd₀.₈Sr₀.₂NiO₂薄膜进行局域磁化率与磁通成像，以揭示零温超流密度与超导转变温度之间的关联。由于样品存在微米尺度的不均匀性，研究通过空间统计分析发现：当局部T_c高于8K时，T_c与ρ_s0呈线性关系；而当T_c低于8K时，二者呈亚线性依赖（近似T_c∝ρ_s0^{1/2}）。这一整体关系与过掺杂铜氧化物超导体中的观测结果高度相似，暗示了无限层镍基超导体与铜氧化物超导体在超导机制上可能存在密切的内在联系。

该研究基于双层两轨道Hubbard模型，采用时间依赖变分原理团簇微扰理论（TDVP-CPT）与大尺度密度矩阵重正化群（DMRG）方法，系统探究了La₃Ni₂O₇中电子关联对费米面拓扑与超导配对对称性的影响。TDVP-CPT在包含多达16个物理位点的团簇上计算发现，电子关联驱动了显著的轨道选择性低能谱重构：d_{z²}轨道的谱权重逐渐耗尽，γ能带下沉至费米能级以下，而α和β能带则出现赝隙，最终在强耦合下形成以d_{x²-y²}轨道为主的费米弧。DMRG计算进一步揭示，主导的超导配对关联与该费米面重构一致演化，从弱耦合下的d_{z²}轨道主导的层间自旋单态配对转变为强耦合下的d_{x²-y²}轨道主导的配对，但始终保持s±波对称性。研究表明，γ费米面的消失非但不抑制超导电性，反而标志着关联驱动的配对通道转变，其关键中介机制包括层间反铁磁涨落、Hund耦合与轨道间杂化。