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研究人员利用偏振超快泵浦-探测光谱，对比研究了常压下双层La₃Ni₂O₇和三层La₄Ni₃O₁₀单晶的正常态电子动力学。两种材料均表现出密度波转变并伴随准粒子弛豫瓶颈的打开，但其电子响应具有显著不同的对称性：三层La₄Ni₃O₁₀在整个温区内保持光学各向同性，而双层La₃Ni₂O₇在低温下则展现出明显的二重（C₂）旋转对称性破缺，即电子向列性。这种向列性体现在准粒子慢弛豫动力学和有效能隙尺度的各向异性中，并且其在115 K以下与一个次级各向同性序相互竞争，导致向列信号随温度呈非单调变化。该工作揭示了双层镍酸盐中电子向列涨落的存在，而三层体系中并未出现这一现象，暗示了电子向列性与La₃Ni₂O₇高压超导配对之间可能存在的紧密关联，为深入理解这类镍基超导体的微观机制提供了关键信息。

基于对三层镍酸盐La₄Ni₃O₁₀高压超导的发现，该研究利用密度泛函理论结合动力学平均场理论（DFT+DMFT）设计并计算了钴基类似物La₄Co₂NiO₈Cl₂。通过将高压相La₄Co₃O₁₀内层Co替换为Ni并掺入Cl实现电子掺杂，该化合物获得了与超导La₄Ni₃O₁₀相似的晶体结构及强关联电子特征：外层Co轨道呈现强有效质量增强和非费米液体行为，内层Ni则表现为弱关联费米液体；在M点附近出现源于外层Co轨道且接近费米能级的平带；同时具有显著的轨道选择性以及高自旋与低自旋态混合的局域自旋涨落。这些特性与La₄Ni₃O₁₀的关键电子态高度吻合，表明La₄Co₂NiO₈Cl₂有望成为钴基层状化合物中实现高温超导电性的候选材料，为后续实验探索提供了理论依据。

It has been recently shown that under pressure trilayer Ruddlesden-Popper nickelate La4⁢Ni3⁢O10 (LNO) becomes superconducting below a critical temperature ≈20 K, in addition to the infinite-layer and bilayer systems. Motivated by this observation, we explore the effects of electron correlations on its electronic structure and magnetic properties using the advanced density functional theory plus dynamical mean-field theory approach. Our results for the normal-state electronic structure and correlation effects in LNO show much in common with the infinite-layer and bilayer nickelates, with remarkable site- and orbital-dependent renormalizations of the Ni 3⁢𝑑 bands and notable incoherence of the Ni 𝑑3⁢𝑧2−𝑟2 states, caused by correlation effects. Our analysis of the Fermi surface and magnetic correlations suggests the emergence of competing spin and charge stripe states, implying the importance of in-plane spin fluctuations to explain superconductivity in this material.

Motivated by the recent observation of superconductivity in the pressurized trilayer Ruddlesden-Popper (RP) nickelate La4⁢Ni3⁢O10, we explore its structural, electronic, and magnetic properties as a function of hydrostatic pressure from first-principles calculations. We find that an orthorhombic (monoclinic)-to-tetragonal transition under pressure takes place concomitantly with the onset of superconductivity. The electronic structure of La4⁢Ni3⁢O10 can be understood using a molecular trimer basis wherein 𝑛 molecular subbands arise as the 𝑑𝑧2 orbitals hybridize strongly along the 𝑐 axis within the trilayer. The magnetic tendencies indicate that the ground state at ambient pressure is formed by nonmagnetic inner planes and stripe-ordered outer planes that are antiferromagnetically coupled along the 𝑐 axis, resulting in an unusual ↑, 0, ↓ stacking that is consistent with the spin density wave model previously suggested by neutron diffraction. Such a state is destabilized at the pressure where superconductivity arises. Despite the presence of 𝑑𝑧2 states at the Fermi level, the 𝑑𝑥2−𝑦2 orbitals also play a key role in the electronic structure of La4⁢Ni3⁢O10. This active role of the 𝑑𝑥2−𝑦2 states in the low-energy physics of the trilayer RP nickelate, together with the distinct electronic behavior of the inner and outer planes, resembles the physics of multilayer cuprates.

Signatures of superconductivity under pressure have recently been reported in the bilayer La3⁢Ni2⁢O7 and trilayer La4⁢Ni3⁢O10 Ruddlesden-Popper (RP) nickelates with the general chemical formula La𝑛+1⁢Ni𝑛⁢O3⁢𝑛+1 (𝑛 = number of perovskite layers along the 𝑐-axis). The emergence of superconductivity is always concomitant with a structural transition in which the octahedral tilts are suppressed, bringing the apical Ni-O-Ni angle to 180∘ and causing an increase in the out-of-plane 𝑑𝑧2 orbital overlap. Due to this strong interlayer coupling, a flat band of pure 𝑑𝑧2 character crosses the Fermi level. Here, using first-principles calculations, we explore biaxial strain (both compressive and tensile) as a means to mimic the electronic structure characteristics of RP nickelates (up to 𝑛=5) under hydrostatic pressure. Our findings highlight that strain enables the decoupling of the structural and electronic structure effects obtained under hydrostatic pressure: While compressive strain brings the apical Ni-O-Ni angle closer to 180∘, it shifts the 𝑑𝑧2 flat bands away from the Fermi energy, giving rise to a more cupratelike electronic structure. In contrast, tensile strain reduces the apical Ni-O-Ni angle (to values of ∼160∘), but it recovers the flat 𝑑𝑧2 band at the Fermi level appearing in the bilayer and trilayer RP nickelates under pressure. Overall, strain represents a promising way to tune the electronic structure of RP nickelates and could be an alternative route to achieve superconductivity at ambient pressure in this family of materials.

本文采用第一性原理DFT+U方法，研究了不同稀土元素（Pr、Nd、Sm）掺杂La₃Ni₂O₇的晶体结构和电子性质趋势。计算表明，掺杂原子优先占据岩盐层中的La位，随掺杂离子半径减小（Pr→Sm），化学压力效应使晶胞体积单调递减，单斜到四方的结构转变压力依次升高，且该转变与超导电性的出现基本吻合，与实验观测一致。在高压四方相中，d_{z²}能带平坦化并穿越费米能级，出现d_{z²}特征的空穴型费米面，被视为超导电性的关键电子标志。随稀土离子尺寸减小，面内跳跃积分增强，而由于顶端Ni-O键长缩短，面外跳跃积分反而减弱。这些发现为理解稀土掺杂对双层Ruddlesden-Popper镍酸盐电子结构的影响及其与超导转变温度的关联提供了微观机理见解。

本研究采用DFT+DMFT方法，系统探讨了交替单层-双层Ruddlesden-Popper型镍酸盐La₅Ni₃O₁₁（1212-LNO）在正常态的电子结构和磁相关性。结果表明，结构上不同的单层和双层Ni离子表现出显著差异：双层Ni离子中的e_g态形成了强烈重整化的准粒子能带，其中Ni的x²-y²和3z²-r²轨道有效质量增强因子分别达到约3.5和4.2；而单层Ni离子的e_g态则呈现轨道选择莫特绝缘态，其中Ni的3z²-r²轨道具有窄能隙，x²-y²轨道则表现为金属性且强非相干（非费米液体）行为。磁相关分析表明，双层NiO₆板中可能形成自旋与电荷密度波交织的条纹，主要不稳定性对应波矢Q=(1/3,1/3)的“上-下-0”自旋图案，并与(1/4,1/4)的“上-上-下-下”双列条纹态竞争。单层Ni的3d电子则倾向于形成奈尔型磁序。在压力下，1212-LNO经历轨道选择莫特绝缘体-金属相变，伴随单层Ni e_g态的金属化，其近费米能级处呈现强非相干的非费米液体行为。总体而言，关联效应显著重构了磁相关性，使其从DFT预测的单层主导转变为双层主导，强调了层间约束和轨道依赖关联的关键作用。

本文采用基于第一性原理瓦尼尔函数的二轨道双层模型和连续格林函数方法，理论分析了超导双层镍酸盐薄膜的扫描隧道显微镜谱。研究发现，多轨道特性和瓦尼尔函数的空间各向异性导致局域态密度对针尖位置高度敏感：随着针尖高度增加，不同能带相干峰的相对权重发生显著变化，从而可通过距离依赖测量区分处于争议中的γ带与β带相干峰的轨道来源。此外，在含杂质的系统中，准粒子干涉图样能清晰分辨s波和d波超导序参量的对称性。该工作为实验上识别超导能隙的能带归属及配对对称性提供了明确的理论指导。

该研究针对受过渡金属双层体系启发的四分之一空穴填充双轨道双层哈伯德模型，通过分子轨道基矢显式处理dz2轨道的强层间成键，并投影高能电子态，推导出描述电子自旋与层赝自旋耦合的低能有效库格尔-霍姆斯基哈密顿量。结合外斯平均场理论与广义味道波理论，研究揭示了丰富的基态相图，包括铁磁与反铁磁伴随层交错电荷密度相、具有自发层间量子相干性的层相干相，以及一种新颖的最大自旋-层纠缠相。该纠缠相源于自发破缺至O(3)的涌现O(4)对称性，其激发谱呈现三个纠缠的无隙戈德斯通模式。结果揭示了强关联双层体系中通过几何驱动机制实现复合纠缠的可能，并为理解双层镍酸盐超导体及其他多组分关联材料提供了具体的理论框架。

通过制备周期性纳米孔阵列将无限层镍酸盐超导薄膜（Nd0.8Sr0.2NiO2）构建为约瑟夫森结阵列，研究人员系统地增强了系统的相位涨落。在纳米图案化的薄膜中，宏观相位相干的减弱驱动了超导转变呈现两阶段特征，并最终趋向于电阻饱和的反常金属基态。电荷2e量子振荡的出现表明了阵列间的相干性，而零场磁电阻反常峰则标志着极端量子相位涨落持续至极低温度。值得注意的是，通过纳米图案化与磁场的协同增强，在Nd-镍酸盐中观测到超导各向异性反转，即面内临界场低于面外临界场，该各向异性的演化可能揭示了与集体电子态耦合的内禀交换塞曼场。这些结果阐明了超导如何响应相位涨落而演化，并确立了纳米图案化作为揭示强关联系统中隐藏交织序的有效范式。