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The discovery of superconductivity with a critical temperature of about 80 K in La3Ni2O7 single crystals under pressure has received enormous attention. La3Ni2O7 is not superconducting under ambient pressure but exhibits a transition at T ∗ ≃ 115 K. Understanding the electronic correlations and charge dynamics is an important step towards the origin of superconductivity and other instabilities. Here, our optical study shows that La3Ni2O7 features strong electronic correlations which significantly reduce the electron’s kinetic energy and place this system in the proximity of the Mott phase. The low-frequency optical conductivity reveals two Drude components arising from multiple bands at the Fermi level. The transition at T ∗ removes the Drude component exhibiting non-Fermi liquid behavior, whereas the one with Fermi-liquid behavior is barely affected. These observations in combination with theoretical results suggest that the Fermi surface dominated by the Ni- $${d}_{3{z}^{2}-{r}^{2}}$$orbital is removed due to the transition at T ∗. Our experimental results provide pivotal information for understanding the transition at T ∗ and superconductivity in La3Ni2O7.

We employ a density-functional theory plus dynamical mean-field theory framework to investigate the correlated electronic structure of the alternating single-layer–trilayer (1313) polymorph of La3⁢Ni2⁢O7, which becomes superconducting under pressure. At ambient pressure, the single layer is in a Mott-insulating regime and the low-energy physics is dominated by the trilayer block. Under pressure, the gap in the single-layer block closes due to orbital-selective physics, enabling charge transfer into the trilayer block. This change in effective doping of the trilayer block could be linked to the higher 𝑇𝑐 obtained in La3⁢Ni2⁢O7−1313 (∼80 K) when compared to the nominal trilayer La4⁢Ni3⁢O10 compound (∼30 K). We conclude that correlation-driven layer differentiation is crucial in the La3⁢Ni2⁢O7−1313 polymorph and that its low-energy physics aligns closely with the trilayer La4⁢Ni3⁢O10 compound (in spite of the apparent differences in nominal filling) rather than with the conventional bilayer La3⁢Ni2⁢O7.

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。

本文采用DFT+动力学平均场理论方法，研究了电子关联和外延应变对双层Ruddlesden-Popper镍酸盐La₃Ni₂O₇正常态电子结构、准粒子重整化、费米面及磁关联的影响。结果表明Ni 3d能带存在显著的轨道选择性重整化和强非相干性，Ni的x²−y²与3z²−r²态接近轨道选择性局域化。电子性质对面内应变高度敏感：拉伸应变和约−2%以内的中等压缩应变均能显著增强磁关联；而在约−4%的大压缩应变下，体系发生Lifshitz转变，γ费米面片消失，该费米面片与几乎完全占据的、浅的成键Ni 3z²−r²平带相关，并导致磁关联急剧下降，暗示超导被抑制。总体上，结果支持自旋/电荷密度波条纹不稳定性由费米面嵌套驱动的图像，并表明压力和应变可有效调控自旋-电荷密度波有序，从而增强自旋涨落。

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。