论文 on arXiv Daily: Nickelate Superconductors

3dz2 orbital delocalization and magnetic collapse in superconducting (La,Pr)₃Ni₂O₇-δ films

Mon, 01 Jan 0001 00:00:00 +0000

该研究利用X射线吸收谱和共振非弹性X射线散射，独立调控(La,Pr)₃Ni₂O₇₋δ薄膜的应变和氧含量，追踪从非超导母相到超导相的微观演化。研究发现，两种调控方式均导致氧2p₂轨道与镍3d₂²轨道退局域化：氧K边吸收谱中“上哈伯德”峰向空穴峰的谱权重转移，伴随镍L边吸收谱和dd轨道激发展宽与减弱。同时，长程自旋密度波（SDW）序的强度和关联长度被显著抑制，表明其与超导直接竞争；而短程磁振子虽被阻尼，但带宽保持不变。这表明，在SDW序熔化过程中，氧2p₂和镍3d₂²轨道的退局域化以及短程磁振子的鲁棒性是实现超导的先决条件，为理论模型提供约束，并为设计镍基超导体指明了轨道选择性的途径。

A chemical avenue to manipulate field-reentrant superconducting rivalries in infinite layer nickelates

Mon, 01 Jan 0001 00:00:00 +0000

该研究开发了一种兆帕级高压氧辅助化学合成路线，成功实现了向较重稀土元素延伸的无限层镍酸盐（(RE₁₋ᵧRE’ᵧ)₁₋ₓEuₓNiO₂，RE/RE’：Pr、Nd、Sm、Gd、Dy）的有效生长。通过Eu²⁺/Eu³⁺变价实现空穴掺杂，在Nd₁₋ₓEuₓNiO₂和Pr₁₋ₓEuₓNiO₂体系的超导圆顶边界处，观察到源于Eu²⁺ 4f⁷磁矩与磁涨落竞争而产生的稳健单轴各向异性磁场重入超导，而在最佳掺杂区则表现为常规高温超导，临界电流密度高达~266 kA/cm²（2 K），超越传统Sr/Ca掺杂体系。进一步引入不同RE’磁性离子可调节交换场强度，从而调制量子临界性：既增强重入行为，也可将Tc提升至40.1 K。该工作揭示了稀土4f磁矩在调制配对强度与量子临界性中的关键作用，为利用无限层镍酸盐平台研究4f相关的非常规超导与量子相变奠定了合成基础。

A superconducting half-dome in bilayer nickelates

Mon, 01 Jan 0001 00:00:00 +0000

在压缩应变双层镍酸盐薄膜中，通过连续调控氧化学计量，研究人员发现了一个超导半圆顶。从最优超导态出发，增加氧含量会逐渐抑制超导，使其向金属相转变；而减少氧含量则驱动颗粒状超导体-绝缘体转变，但超导起始温度保持不变。这种半圆顶结构源于间隙氧与氧空位的不同作用：前者主要通过掺杂效应调控载流子浓度，后者则引入强散射导致电子不均匀性。实验表明，这一半圆顶在不同稀土组合及有无碱土掺杂的样品中一致出现，揭示了双层镍酸盐相图的普遍特征。该发现为理解相关电子体系中超导的涌现与消亡提供了新视角。

A unified theory of thin film and bulk bilayer nickelates

Mon, 01 Jan 0001 00:00:00 +0000

该研究基于两分量模型提出了一个统一理论，用以解释双层镍酸盐超导体在加压块材与薄膜中的一系列核心实验现象。理论以轨道镍原子的强关联局域电子与巡游电子间的层间超交换耦合和杂化为核心，预言超导转变温度随掺杂变化呈现两类行为：当层间超交换耦合较强时，电子或空穴掺杂分别形成两个超导穹顶，半填充附近出现非超导的层间价键态；当耦合较弱或适中时，两个穹顶合并为一个跨越半填充的单一穹顶，但最高温度较低。掺杂增加会使正常态从费米液体转变为非费米液体或弱绝缘态，并在最优掺杂附近出现准线性电阻散射率。氧空位或化学替代可破坏层间价键，同时抑制超导并诱发巡游电子的局域近藤散射，解释了非超导样品中观测到的电阻对数温度依赖及负磁阻。该框架统一说明了块材和薄膜中超导转变与正常态的差异、孔穴掺杂及氧计量对穹顶状的影响，以及超导与近藤效应的竞争关系。基于理论，作者提出通过掺杂或降低层间磁耦合可在常压下实现块材超导，并预言电子掺杂将获得更高的转变温度。

A Unified Understanding of the Experimental Controlling of the Tc of La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

该论文基于先前提出的有效d_{x^2-y^2}轨道双层t-J∥-J⊥模型，结合第一性原理计算输入的模型参数，通过奴隶玻色子平均场和密度矩阵重正化群方法，统一解释了通过氧化学计量、元素替代、压力或应变调控La₃Ni₂O₇超导转变温度（Tc）的一系列实验。模型发现，由于d_{x^2-y^2}轨道接近四分之一填充，其Tc调控行为与空穴掺杂的过掺杂铜氧化物类似。在掺杂依赖性上，系统呈现粒子-空穴不对称性：空穴掺杂使系统更过掺杂而抑制Tc，电子掺杂则相反，这解释了氧过量或Ca/Sr替代La导致的Tc抑制以及氧化学计量调控中的“半穹顶”行为。在相互作用依赖性上，Tc随层间反铁磁超交换相互作用J⊥变化，这解释了Sm/Nd替代La增强体材料Tc、体材料Tc-压力呈“直角三角形”关系以及薄膜中压应变增强Tc的实验现象。与弱耦合理论（Tc主要依赖于态密度）和d_{z^2}轨道主导的配对机制（Tc与d_{z^2}空穴密度成比例）相比，该模型提供了更自然统一的解释。论文进一步提出，通过不引入无序的电子掺杂（如用更高价元素替代La）可提高Tc。

Absence of Ni₂/Ni₃ charge disproportionation and possible roles of O₂ p holes in La₃Ni₂O₇−δ revealed by hard x-ray photoemission spectroscopy

Mon, 01 Jan 0001 00:00:00 +0000

Absence of Ni₂/Ni₃ charge disproportionation and possible roles of O₂ p holes in La₃Ni₂O₇−δ revealed by hard x-ray photoemission spectroscopy

Ambient pressure growth of bilayer nickelate single crystals with superconductivity over 90 K under high pressure

Mon, 01 Jan 0001 00:00:00 +0000

Recently, the Ruddlesden-Popper bilayer nickelate $La_3Ni_2O_7$ has emerged as a superconductor with a transition temperature ($T_c$) of ~ 80 K above 14 GPa$^{[1-4]}$. Efforts to search for nickelate superconductors with higher $T_c$$^{[5,6]}$, to grow reproducible high-quality single crystals$^{[2,7-10]}$, and to eliminate reliance on demanding high gas pressure synthesis conditions$^{[11]}$, remain significant challenges. Here we report superconductivity up to 92 K under high pressure in single crystals of bilayer nickelates synthesized at ambient pressure using flux methods. High quality $La_2SmNi_2O_{7-δ}$ single crystals with dimensions up to 220 μm on edge were successfully grown. At ~ 15 GPa, these crystals exhibit superconductivity with an onset transition temperature ($T_c^{onset}$) of 68 K and zero-resistance temperature ($T_c^{zero}$) of 47 K. Increasing pressure further enhances both transition temperatures, reaching record values for nickelates: $T_{c,max}^{onset}$ = 92 K and $T_{c,max}^{zero}$ = 73 K @ 21 GPa. Notably, higher $T_c$ correlates with larger in-plane lattice distortion at ambient conditions for bilayer nickelates. Furthermore, we observed a structural transition from monoclinic $P2_1/a$ to tetragonal $I4/mmm$ at ~ 18 GPa, indicating that tetragonal structure is not a prerequisite for superconductivity to appear in this bilayer nickelate. This study provides an easy-to-access method for growing reproducible high-quality bilayer nickelate single crystals and offers new insights into achieving higher Tc superconductivity.

Anisotropic Electronic Correlations in the Spin Density Wave State of La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

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。

Anomalous Behavior of the Ni₁+ moment and interstitial band in bi-infinite-layered La₃Ni₂O₅F

Mon, 01 Jan 0001 00:00:00 +0000

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。

Atomically resolved intrinsic superconducting gap in (La,Pr)₃Ni₂O₇ films

Mon, 01 Jan 0001 00:00:00 +0000

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。

Bosonic phases across the superconductor-insulator transition in infinite-layer samarium nickelate

Mon, 01 Jan 0001 00:00:00 +0000

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

Mon, 01 Jan 0001 00:00:00 +0000

Bosonic Phases across the Superconductor-Insulator Transitions in Infinite-Layer Samarium Nickelate

Bulk high-temperature superconductivity in pressurized tetragonal La₂PrNi₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

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.

Bulk superconductivity up to 96 K in pressurized nickelate single crystals

Mon, 01 Jan 0001 00:00:00 +0000

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.

Co-operating multiorbital and nonlocal correlations in bilayer nickelate

Mon, 01 Jan 0001 00:00:00 +0000

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。

Collective spin excitations in trilayer nickelate La₄Ni₃O₁₀

Mon, 01 Jan 0001 00:00:00 +0000

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。

Contrasting Momentum-Selective Spin-Density-Wave Gaps in Bilayer and Trilayer Nickelates

Mon, 01 Jan 0001 00:00:00 +0000

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。

Contrasting Spin Excitations in Octahedral and Square-Planar n=8 Ruddlesden-Popper Nickelates

Mon, 01 Jan 0001 00:00:00 +0000

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。

Controlling the Band Filling and the Band Width in Nickelate Superconductors

Mon, 01 Jan 0001 00:00:00 +0000

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。

Cooperation between Electron-Phonon Coupling and Electronic Interaction in Bilayer Nickelates La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

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.

Correlated Electronic Structure and Density-Wave Gap in Trilayer Nickelate La₄Ni₃O₁₀

Mon, 01 Jan 0001 00:00:00 +0000

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.

Correlation between superfluid density and transition temperature in infinite-layer nickelate superconductor Nd₁₋ₓSrₓNiO₂

Mon, 01 Jan 0001 00:00:00 +0000

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。

Correlation-Driven Orbital-Selective Fermiology and Superconductivity in the Bilayer Nickelate La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

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。

Correlation-renormalized spin-fluctuation pairing and the stabilization of s± superconductivity in pressurized La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

该研究针对加压La₃Ni₂O₇中超导配对对称性尚未定论的问题，采用四轨道Wannier哈密顿量，将单格点双轨道动力学平均场理论（DMFT）的自能引入随机相近似（RPA），构建自能重整化粒子－空穴气泡以替代裸气泡，保持相同的局域Slater-Kanamori相互作用顶点。普通RPA计算显示主导配对属于B₂g的d_xy通道，而一旦包含DMFT自能，配对层级发生反转：A₁g符号改变的s_±态成为主导，B₁g d_x²-y²通道次之，原先的B₂g不稳定性被强烈抑制。口袋对分解和轨道分辨磁化率表明，该反转源于d_3z²-r²轨道的选择性重整化，它滤除了有利于d_xy配对的γ口袋散射过程，同时保留了有利于s_±的分布性口袋间散射。进一步利用双Bethe-Salpeter方程结合局域DMFT顶点计算静态自旋磁化率，得到宽广的有限动量磁响应且在Γ点附近较弱，从两粒子层面强化了s_±态的自旋涨落背景。结果表明，在La₃Ni₂O₇中强关联效应并非次要修正，恰当处理关联重整化的准粒子对于准确预测超导配对对称性至关重要。

Counterintuitive inverse superconducting transition beyond 4He-cooling limit

Mon, 01 Jan 0001 00:00:00 +0000

本文报道了在Eu基无限层镍酸盐（EuxNd1-xNiO₂和EuxPr1-xNiO₂）中实现的超过液氦冷却极限的逆超导转变。通过磁场调控，在过掺杂和欠掺杂区域均观察到超导零电阻态被限制在较低临界温度（Tc-inv约2.6–5.4 K）与较高正常Tc之间，升温或增加电流密度可从电阻态驱动进入超导态，并在更高温度和电流下再次消失。系统变温输运测量表明，这一Kelvin区间的逆超导转变源于温度变化引起的Eu²⁺4f⁷相关补偿有效磁场与上临界场的交替主导，并得到了磁场下约300 mK处超导重现所形成的温度诱导重入超导现象的支持。该工作建立了磁相互作用重构的高温超导体系作为探索逆转热退相干范式的量子现象的平台，也为量子相变器件的反向设计开拓了应用场景。

Decoding Superconductivity in La₃Ni₂O₇-δ Thin Films via Ozone-Driven Structure and Oxidation Tuning

Mon, 01 Jan 0001 00:00:00 +0000

该研究通过扫描透射电子显微镜结合电子能量损失谱，对外延生长的La₃Ni₂O₇₋δ薄膜进行了详细结构分析。薄膜采用脉冲激光沉积法在SrLaAlO₄衬底上制备，经不同臭氧退火处理后表现出显著差异的超导特性。研究发现，超导相的稳定与氧化学计量均匀性、外延应变以及特定堆叠结构基序（如双层和多型体）密切相关。通过关联堆叠多型体的丰富形貌与输运行为，建立了理解双层镍酸盐薄膜中亚稳态超导相的理论框架。结果揭示了氧含量、晶格应变和结构有序性在实现环境压力超导中的关键作用，为设计新型镍基超导材料提供了明确途径。

Delafossites as an unexpected competing phase to infinite-layer oxides

Mon, 01 Jan 0001 00:00:00 +0000

通过高通量第一性原理模拟，该研究系统比较了delafossite（D1）、有序岩盐变体（D2）及无限层（IL）氧化物在ABO₂化学计量下的热力学稳定性，并构建了涵盖2346种元素组合的相图。结果表明，对于镍酸盐、钯酸盐和铂酸盐，delafossite结构的稳定性与无限层相相当甚至更优，且这两种相与钙钛矿相之间存在竞争关系。电子结构分析显示，delafossite化合物具有反转的阳离子顺序，费米面以d_{z^2}轨道贡献为主，显著区别于无限层相的d_{x^2-y^2}特征。在所有候选体系中，La-Ni组合是稳定无限层结构的热力学最优选择。此外，通过Ca、Sr、Ba进行空穴掺杂可系统提高三种过渡金属家族中无限层相的相对稳定性。这些结果揭示了合成无衬底体相无限层氧化物的根本挑战，并为实验探索新型超导化合物提供了指导。

Density waves in low-pressure bilayer nickelates

Mon, 01 Jan 0001 00:00:00 +0000

本文利用无约束Hartree-Fock方法，基于多轨道Hubbard-Hund模型，研究了低压双层镍酸盐La₃Ni₂O₇的密度波相图。计算表明，在正交相中，电子体系首先在约150 K形成波矢为Q_Y=(0,π)的双条纹自旋密度波序；随后在约130 K，纯双条纹自旋态对共度电荷密度波不稳定，出现自旋调制的双条纹有序态，表现为面内Ni1和Ni2位点的磁矩与电荷密度发生调制，形成低自旋位点。该电荷序参数比磁序参数小一个数量级，并在能带中诱导额外的能隙和折叠费米面。研究确立了La₃Ni₂O₇中自旋密度波和电荷密度波的层级关系，为理解常压有序相与高压超导相之间的关联提供了重要线索，并提出了进一步实验检验的建议。

Density-wave order enhances the phonon thermal Hall effect in a trilayer nickelate

Mon, 01 Jan 0001 00:00:00 +0000

在三层Ruddlesden-Popper镍酸盐La₄Ni₃O₁₀的常压正常态中，研究人员观测到由密度波序增强的声子热霍尔效应。该材料在约140 K发生密度波相变，热霍尔响应在转变温度以下急剧增强，热霍尔角从160 K时的1.5‰升至100 K附近的6‰，并在70 K达到约7‰的峰值，同时在热霍尔电阻率上出现两个明显平台。纵向热导率几乎不随磁场变化且电子贡献极小，证明纵向与横向热输运均以声子为主。从热霍尔数据提取的特征能量约4.1 meV，与磁振子-声子色散交叉跨越能量3.2 meV高度吻合，表明自旋密度波序诱导的磁振子-声子杂化是增强热霍尔效应的核心机制。该工作揭示了镍酸盐中自旋-晶格耦合对声子输运的显著调控，并指出这种动态耦合可能通过软化光学声子参与压制反铁磁序、促进高压下超导电性的出现，为理解非常规超导体中电荷、自旋与晶格自由度的交织提供了新视角。

Detecting pairing symmetry of bilayer nickelates using electronic Raman scattering

Mon, 01 Jan 0001 00:00:00 +0000

本研究利用双轨道双层模型，通过多轨道和能带加和两种方法系统计算了不同拉曼通道下的电子拉曼响应，以区分双层镍酸盐超导体La₃Ni₂O₇中争议的配对对称性。通过对比多轨道与能带加和近似的拉曼磁化率，发现拉曼响应能有效区分不同配对对称性，并识别完全开缝和节状超导态中依赖于费米口袋的能隙大小。具体地，节状dₓ²⁻ᵧ²/dₓᵧ波配对在低能下表现出稳健的幂律行为，与完全开缝配对截然不同；而对于s±波配对，则可以确定β口袋上的能隙各向异性细节。研究还强调了多轨道效应对拉曼谱的关键塑造作用，并指出电子拉曼散射作为一种具有对称分辨能力的探针，能够为确定非常规超导体的超导能隙结构提供有力手段，对理解双层镍酸盐的超导机制具有重要实验意义。

Dichotomous electronic system in a bilayer Ni₁+ nickelate

Mon, 01 Jan 0001 00:00:00 +0000

本文利用密度泛函理论计算了双层无限层镍酸盐La₃Ni₂O₅F的电子结构，揭示其具有理想二维特性且存在两种截然不同的准粒子行为。采用虚拟晶体近似处理氧/氟无序后，能带计算显示常规Ni dpσ带形成空穴型费米面，而源自间隙密度的E*带则构成圆柱状电子费米面，导致体系自掺杂0.18个电子。该间隙密度分布在无顶位氧的La层间，与Ni dxz/dyz带以平行线性色散趋近M点，耦合形成近非解析的狄拉克点，表现出奇异的间隙-轨道带耦合效应。同时，dz2带通过与间隙密度相互作用产生约1 eV的对称性劈裂。这种电子与空穴行为的二相性将主导正常态输运和远红外特性，并可能影响镍酸盐的超导态，为理解无限层镍酸盐物理提供了新的视角。

Dimensionality of vortex matter in superconducting infinite-layer nickelates

Mon, 01 Jan 0001 00:00:00 +0000

该研究通过绘制超导Pr0.8Sr0.2NiO2薄膜的涡旋相图，从不同角度探究了无限层镍氧化物的超导态维度。实验发现，低无序薄膜存在准二维性质的涡旋液体-玻璃转变，而增加无序度则会驱动体系进入纯二维态。这一结果表明，纯二维性并非本征属性，而是由NiO2层间因无序增强而解耦所导致的外在现象。该工作确立了无序度作为无限层镍氧化物超导的关键调控参数，并指出无序主要存在于NiO2层内，为理解该类材料提供了两个基础性见解。

Disorder-Induced Suppression of Superconductivity in Infinite-Layer Nickelates

Mon, 01 Jan 0001 00:00:00 +0000

Disorder-Induced Suppression of Superconductivity in Infinite-Layer Nickelates

Dissecting superconductivity in the Ruddlesden-Popper nickelates: The role of electron correlation and interlayer magnetic exchange

Mon, 01 Jan 0001 00:00:00 +0000

本研究利用共振非弹性X射线散射（RIXS）直接对比了三层镍酸盐La₄Ni₃O₁₀与双层La₃Ni₂O₇的电子和磁激发性质。结果显示，La₄Ni₃O₁₀表现出更强的巡游性，如更宽的Ni d-d轨道激发和更强的荧光背景，表明其电子关联弱于双层体系。尽管关联较弱，仍可观测到清晰的集体自旋激发，包括色散的声学和光学磁振子支以及非公度自旋密度波（SDW）。通过线性自旋波理论分析，提取出其层间超交换相互作用Jz约为22 meV，远小于La₃Ni₂O₇中的值。较弱的电子关联和减弱的层间磁交换共同解释了三层化合物超导转变温度（约30 K）显著低于双层（约80 K）的原因。该研究确立了层间磁耦合和电子关联作为层状镍酸盐超导性的关键参数，为理解这类新兴家族的超导配对机制提供了重要约束。

Doping evolution of spin excitations in La₃₋ₓSrₓNi₂O₇/SrLaAlO₄ superconducting thin films

Mon, 01 Jan 0001 00:00:00 +0000

利用Ni $L_3$边共振非弹性X射线散射（RIXS），该工作系统研究了在相干压应变La$_{3-x}$Sr$_x$Ni$_2$O$_7$/SrLaAlO$_4$薄膜中电子与自旋激发随载流子掺杂的演变，覆盖了超导（$x=0,0.09,0.21$）与过掺杂非超导（$x=0.38$）区间。在超导薄膜中，色散自旋激发沿$[H,H]$和$[H,0]$方向持续存在，色散几乎不随掺杂变化且阻尼很小，谱权重仅适度降低，表明双条纹自旋关联保持稳健。然而在$x=0.38$的非超导薄膜中，磁响应强烈展宽和减弱，阻尼显著增强，谱权重下降约50%，标志着相干双条纹自旋激发崩溃。磁相干性随超导电性的同步消失，直接建立了层状镍酸盐薄膜中掺杂控制的磁性与超导电性之间的关联。

Effect of Pressure and Oxygen-Isotope Substitution on Density-Wave Transitions in La₄Ni₃O₁₀

Mon, 01 Jan 0001 00:00:00 +0000

通过μ子自旋旋转/弛豫和电阻率测量，结合氧同位素替代，系统研究了三层Ruddlesden-Popper镍酸盐La₄Ni₃O₁₀中密度波转变的压力和同位素效应。常压下观察到两个不相称自旋密度波（SDW）转变，分别发生在132 K和80-90 K；磁结构显示外层两个Ni层呈反铁磁耦合SDW序，内层磁矩较小，且在T以下磁矩出现c轴分量。TSDW处内场突现表明SDW转变类似于一级相变，并与同一温度发生的电荷密度波（CDW）紧密交织。施加压力后，TSDW、T和TCDW均以约-13 K/GPa的速率均匀抑制，不同于双层La₃Ni₂O₇中压力增大SDW与CDW间距的行为。¹⁶O→¹⁸O替代使TCDW升高；在CDW与SDW交织区域，TSDW亦呈现显著同位素效应且与TCDW位移相近，而在T*处SDW独立演化时则无同位素效应。这些结果揭示了La₄Ni₃O₁₀中SDW与CDW的强交织本质，并指出压力诱导CDW序的抑制可能是Ruddlesden-Popper镍酸盐中高压超导的关键机制。

Electron Doping of La₃Ni₂O₇ Thin Films: Candidate Metal Dopants and Their Potential Impact on Superconductivity

Mon, 01 Jan 0001 00:00:00 +0000

通过第一性原理密度泛函理论计算，系统研究了双层Ruddlesden-Popper型镍酸盐La₃Ni₂O₇薄膜中四价元素替代的电子掺杂效应。研究发现，与铜氧化物不同，铈（Ce）掺杂难以有效向低能带引入电子载流子，而锆（Zr）、铪（Hf）和钍（Th）可作为高效电子掺杂剂。这些元素替代能显著增大Ni-dz²轨道间的层间跳跃积分t⊥，可能增强层间超交换耦合J⊥，进而潜在提高超导转变温度Tc。利用约束随机相位近似评估了相互作用参数，发现电子掺杂使得低能轨道（包括Ni-dx²-y²和dz²及其杂化氧轨道）的占据增加，并改变了层内与层间轨道的电子填充比例。结构分析显示，掺杂剂离子半径的差异导致Ni-O键长变化，其中Zr和Hf引起晶格收缩，而Th掺杂效果最强。结果表明，Zr、Hf、Th是实现La₃Ni₂O₇电子掺杂的候选材料，为澄清该体系中电子配对机制的争议提供了新途径。

Electron vs. hole doping in infinite-layer nickelates: electronic structure, magnetism and correlations

Mon, 01 Jan 0001 00:00:00 +0000

通过结合密度泛函理论和动力学平均场理论，研究了无限层镍酸盐LaNiO₂在电子掺杂和空穴掺杂下电子结构、磁性与关联效应的演化。结果显示，由于稀土5d态的存在，Ni-d_{x²-y²}带的自掺杂效应呈现显著不对称：空穴掺杂强烈抑制自掺杂，而电子掺杂虽增大稀土5d电子口袋尺寸，却未有效空穴掺杂Ni-d_{x²-y²}带。这一差异直接影响磁性响应——空穴掺杂迅速压制反铁磁序，电子掺杂则使反铁磁态保持为基态。尽管存在这些差异，两种掺杂下的电子关联均由Ni-d_{x²-y²}轨道主导，表明单带描述在电子和空穴掺杂区均可能适用。

Electronic and magnetic excitations in La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

High-temperature superconductivity was discovered in the pressurized nickelate La3Ni2O7 which has a unique bilayer structure and mixed valence state of nickel. The properties at ambient pressure contain crucial information of the fundamental interactions and bosons mediating superconducting pairing. Here, using X-ray absorption spectroscopy and resonant inelastic X-ray scattering, we identified that Ni 3

$${d}_{{x}^{2}-{y}^{2}}$$

, Ni 3

$${d}_{{z}^{2}}$$

, and ligand oxygen 2p orbitals dominate the low-energy physics with a small charge-transfer energy. Well-defined optical-like magnetic excitations soften into quasi-static spin-density-wave ordering, evidencing the strong electronic correlation and rich magnetic properties. Based on an effective Heisenberg spin model, we extract a much stronger inter-layer effective magnetic superexchange than the intra-layer ones and propose two viable magnetic structures. Our findings emphasize that the Ni 3

$${d}_{{z}^{2}}$$

orbital bonding within the bilayer induces novel electronic and magnetic excitations, setting the stage for further exploration of La3Ni2O7 superconductor.

Electronic correlations and Hund’s rule coupling in trilayer nickelate La₄Ni₃O₁₀

Mon, 01 Jan 0001 00:00:00 +0000

Trilayer Ruddlesden-Popper phase La4Ni3O10 has been observed with Tc of ∼30 K at high pressure in a recent experiment, which further expanded the family of nickelate superconductors. In this study, we explored the effects of electronic correlations in La4Ni3O10 using density functional theory plus dynamical mean-field theory at ambient and high pressures. Our derived spectral functions and Fermi surface of the ambient pressure phase are nicely consistent with the experimental results by angle-resolved photoemission spectroscopy, which emphasized the importance of electronic correlations in La4Ni3O10. We also found the electronic correlations in pressurized La4Ni3O10 are both orbital-dependent and layer-dependent due to the presence of Hund’s rule coupling. There is a competition between the Hund’s rule coupling and the crystal-field splitting, and therefore, the Ni–O layers with weaker crystal-field splitting energy would have stronger electronic correlations.

Electronic correlations and partial gap in the bilayer nickelate La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

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.

Electronic correlations, layer distinction, and electron doping in the alternating single-layer--trilayer La₃Ni₂O₇ polymorph

Mon, 01 Jan 0001 00:00:00 +0000

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.

Electronic Nematicity Revealed by Polarized Ultrafast Spectroscopy in Bilayer La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

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。

Electronic structure and correlation of La₄Co₂NiO₈Cl₂: a theoretical proposal for a La₄Ni₃O₁₀-like high-temperature superconductor

Mon, 01 Jan 0001 00:00:00 +0000

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。

Electronic structure and magnetic correlations in the trilayer nickelate superconductor La₄Ni₃O₁₀ under pressure

Mon, 01 Jan 0001 00:00:00 +0000

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.

Electronic structure and magnetic tendencies of trilayer La₄Ni₃O₁₀ under pressure: Structural transition, molecular orbitals, and layer differentiation

Mon, 01 Jan 0001 00:00:00 +0000

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.

Electronic structure of Ruddlesden-Popper nickelates: Strain to mimic the effects of pressure

Mon, 01 Jan 0001 00:00:00 +0000

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.

Electronic structure trends in La₂RNi₂O₇ (R= Pr, Nd, Sm) from first-principles

Mon, 01 Jan 0001 00:00:00 +0000

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。

Electronic structure, quasiparticle renormalizations, and magnetic correlations in the alternating single-layer bilayer nickelate La₅Ni₃O₁₁

Mon, 01 Jan 0001 00:00:00 +0000

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。

Electronic theory for scanning tunneling microscopy spectra in bilayer nickelate thin films

Mon, 01 Jan 0001 00:00:00 +0000

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。

Emergence of Kugel-Khomskii physics in quarter-filled bilayer correlated systems

Mon, 01 Jan 0001 00:00:00 +0000

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。

Emergent quantum phenomena via phase-coherence engineering in infinite-layer nickelate superconductors

Mon, 01 Jan 0001 00:00:00 +0000

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。

Emergent s+id Superconductivity from the Interplay between Electronic Correlations and Electron-Phonon Coupling in R₁₋ₓSrₓNiO₂

Mon, 01 Jan 0001 00:00:00 +0000

通过结合第一性原理计算与涨落交换-米格达-埃利阿什伯格理论，本文研究了无限层镍酸盐超导体中电子关联与电子-声子耦合的相互作用。研究发现，自旋涨落驱动Ni d_{x^2-y^2}轨道产生稳健的d波超导，而电子-声子耦合则在间隙轨道上诱导s波配对，二者协同形成s+id混合超导态。s波成分的涌现强烈依赖于载流子密度：中等电声耦合强度（λ=0.4）仅在电子密度n=0.9时稳定该混合态，在n=0.8时则无法实现。热力学极限下稳定s波所需的临界耦合约为0.6，但在有限尺寸体系中可降低至0.4。这些结果揭示，局部氧缺陷通过调制局域电子密度，可形成具有不同配对对称性的有限尺寸畴,从而为实验中观测到的空间非均匀超导能隙提供了微观解释，并强调了关联效应与电声耦合协同作用对镍酸盐超导配对对称性的关键影响。

Enhanced s^±-wave superconductivity in electron-doped La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

通过第一性原理计算和大规模动态簇量子蒙特卡洛模拟，该工作系统研究了电子掺杂对三种代表性体系（常压与15 GPa下的块体La₃Ni₂O₇，以及异质结La₃Ni₂O₇:La₃Al₂O₇）的双轨道双层模型超导性质的影响。结果表明，电子掺杂普遍增强了s±波配对超导，其中异质结在欠掺杂区表现出最高的超导转变温度，甚至超过15 GPa加压下的块体样品。进一步分析发现了一种轨道间协同机制：d_{z²}轨道上的配对诱导了d_{x²-y²}轨道上的配对，后者在低温下逐渐占据主导，形成双轨道协作的超导不稳定性。这一结论在两种不同簇尺寸的模拟中均得到验证。该研究为电子掺杂Ruddlesden-Popper相镍酸盐中增强超导性提供了理论预测，并提出了异质结这一可行的实验实现路径，期待未来实验验证。

Enhancement of metallicity by Na doping in La₃Ni₂O₇+δ

Mon, 01 Jan 0001 00:00:00 +0000

本研究通过固相法合成了不同钠掺杂浓度的La₃₋ₓNaₓNi₂O₇₊δ多晶样品，利用X射线衍射、热重分析、磁化率和电阻率测量系统研究了其结构、热、磁和电输运性质。X射线衍射分析表明，当钠掺杂浓度x≥0.075时，样品会发生从‘327’ Amam相到‘4310’ Bmab相的结构转变，且晶格逐渐膨胀。电阻率测量显示，钠掺杂显著增强了金属性，同时密度波转变温度略微降低；施加外部压力可进一步抑制密度波转变，但低温绝缘行为对压力不敏感。这些结果揭示了钠替代镧引入的空穴掺杂能有效调控层状镍酸盐中竞争电子相，为理解元素替代和载流子掺杂在稳定高压超导相中的作用提供了关键实验依据。

Evidence for charge and spin density waves in single crystals of La₃Ni₂O₇ and La₃Ni₂O₆

Mon, 01 Jan 0001 00:00:00 +0000

Evidence for charge and spin density waves in single crystals of La₃Ni₂O₇ and La₃Ni₂O₆

Evidence for Clean d-wave Superconductivity in Samarium Nickelates

Mon, 01 Jan 0001 00:00:00 +0000

利用超快太赫兹光谱技术，对Tc为20 K的无限层钐镍氧化物薄膜进行了光学泵浦-太赫兹探测实验，测量光致电导随温度的变化。在弱激发下，光致破坏的超流密度与平衡态超流密度成正比，其随温度升高呈线性下降，符合干净极限d波配对特征。从该线性关系提取出超导能隙为2.5 meV，2Δ/kTc约等于3，表明体系处于弱耦合区间。同时，独立估算得到平均自由程与相干长度之比(l/ξ)约为1.5，进一步确认干净极限行为。这些结果证明镍氧化物超导体可实现干净超导态，并揭示其配对机制与铜氧化物高温超导体存在密切相似性。

Evidence of Spin Density Waves in La₃Ni₂O_7-δ

Mon, 01 Jan 0001 00:00:00 +0000

The recently discovered superconductivity with critical temperature 𝑇𝑐 up to 80 K in the double-layer Nickelate La3⁢Ni2⁢O7−𝛿 under pressure has drawn great attention. Here, we report the positive muon spin relaxation (𝜇+⁢SR) study of polycrystalline La3⁢Ni2⁢O6.92 under ambient pressure. Zero-field 𝜇+⁢SR experiments reveal the existence of magnetic order in La3⁢Ni2⁢O6.92 with 𝑇𝑁=154 K. The weak transverse field 𝜇+⁢SR measurements reveal the bulk nature of magnetism. In addition, a small quantity of oxygen deficiencies can greatly broaden the internal magnetic field distribution sensed by muons.

Evidence of Spin Density Waves in La₃Ni₂O₇−δ

Mon, 01 Jan 0001 00:00:00 +0000

Evidence of Spin Density Waves in La₃Ni₂O₇−δ

Evidence of universal spectral collapse at a marginal dynamical regime

Mon, 01 Jan 0001 00:00:00 +0000

该研究提出，强关联材料中非相干电子态并非源于无序或材料特定机制，而是由竞争涨落导致的自生动力学无序所产生。在这一边际动力学区域中，电子动力学与时间依赖散射自然耦合，导出谱函数形式ρ(z)=exp(-z²/4)D_ν(z)，其中z为标度能量，D_ν为抛物柱面函数，且ν=-1/2固定。通过对铜氧化物Nd₂₋ₓCeₓCuO₄和Bi₂Sr₂CaCu₂O₈₊δ、Kagome金属CsCr₃Sb₅以及双层镍酸盐La₃Ni₂O₇的角分辨光电子能谱（ARPES）能量分布曲线进行独立标度后，所有数据集坍塌至同一普适曲线，仅振幅与能量标度因材料而异。该光谱坍塌表明低能区域中晶格几何、能带结构与化学成分等微观细节不再相关，呈现固定点般的动力学行为。这一结果建立了跨多种强关联材料的连续主导型ARPES谱的统一定量框架。

Evolution of structure and density wave order in La₃Ni₂O₇−δ single crystals at ambient pressure

Mon, 01 Jan 0001 00:00:00 +0000

The recent discovery of high-temperature superconductivity in bilayer nickelate La3Ni2O7−δ single crystals under high pressure has sparked significant interest in the correlated electronic physics and unconventional pairing mechanisms within Ruddlesden-Popper phase nickelates, where the oxygen content plays a crucial role in both structure and superconductivity. Here we report the evolution of the structure and density wave order in La3Ni2O7−δ single crystals at ambient pressure under various oxygen annealing conditions. Structural analysis reveals that with the increase of annealing oxygen pressure, La3Ni2O7−δ undergoes a structural phase transition from an orthorhombic to a tetragonal phase. Magnetic torque measurements indicate that the density wave order, present in the orthorhombic phase, vanishes upon transition to the tetragonal structure, accompanied by an abrupt change in carrier concentration. A phase diagram illustrating the dependence of both the crystal structure and density wave order on the annealing oxygen pressure is mapped. Our findings suggest a promising pathway for studying the interplay between structure, density wave order, and superconductivity in bulk nickel-based materials.

Evolution of the Superfluid Density in Infinite-Layer Nickelates

Mon, 01 Jan 0001 00:00:00 +0000

该论文通过互感法系统测量了无限层镍基超导体Nd1-xSrxNiO2在掺杂超导穹顶内的超流密度。研究发现，超流刚度较弱，且与超导转变温度Tc呈近似平方根关系。同时，观察到Nd的4f磁矩与超流之间存在强烈的相互作用，导致低温下超流密度显著被抑制，其效应远超简单的顺磁解释，暗示了磁有序与超导相的耦合。这些结果表明，超导相位涨落在限制Tc中起重要作用，并揭示了稀土磁性离子与超流之间意想不到的强耦合。

Experimental evidence of Tc enhancement above 50 K and diode and paramagnetic-Meissner effects, in Nickelate films on highly reduced SrTiO₃

Mon, 01 Jan 0001 00:00:00 +0000

本研究在高度还原且导电的SrTiO₃衬底上制备了缺氧镍酸盐薄膜，通过迈斯纳效应和输运测量观察到超导起始温度达到50–70 K，并在20–25 K实现零电阻，表明超导存在于薄膜中的岛状区域。在约48 K处出现巨大的顺磁迈斯纳效应峰，进一步支持了该温度附近的超导转变。此外，还观测到非互易、无磁滞的超导二极管效应，其极性可完全偏振并反转。薄膜由多种Ruddlesden-Popper相（包括无限层相）混合构成。这些增强的超导特性归因于缺氧薄膜与高度还原的SrTiO₃衬底的协同作用。

Experimental Progress in Ambient-Pressure Superconducting Bilayer Nickelate Films

Mon, 01 Jan 0001 00:00:00 +0000

双层Ruddlesden-Popper镍酸盐在高压下呈现近80 K的超导电性，而近期通过外延应变在常压下稳定了RA₃Ni₂O₇（RA为稀土或碱土元素）超导薄膜，使输运、光谱、显微及器件测量成为可能。本综述总结了常压超导双层镍酸盐薄膜的实验进展，涵盖合成路线、氧计量比、衬底诱导应变、正常态输运、超导性质、掺杂相图以及动量分辨电子结构。当前未解决的关键问题包括：纯相超薄膜的可重复性、两步超导转变的微观起源、氧缺陷和衬底掺杂的作用、Ni 3dz₂衍生γ带的位置以及配对对称性。文章最后指出，未来需建立晶体结构、轨道重建与超导电性之间更定量的联系，以深入理解此类非常规高温超导体系。

First-Principles Evidence for Strongly Correlated Superconductivity Driven by Structural Variations in La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

本研究通过第一性原理模拟，结合约束随机相位近似和从头算分子动力学，系统分析了La₃Ni₂O₇在超导压力范围内的电子关联行为。结果显示，伴随结构相变，Ni的e_g轨道有效在位排斥显著增强，归因于轨道局域化与竞争屏蔽通道（特别是spacer层La能带）之间的动态平衡。该增强区域与实验观测的右三角形超导穹顶高度吻合，且在18 GPa处关联强度达到峰值，对应最高超导临界温度。有限温度模拟进一步澄清了结构相图边界，而Ac₃Ni₂O₇的计算则确认A位阳离子在压力驱动电子关联演化中的关键作用。这些结果直接揭示了结构变化如何通过调控电子关联强度驱动非常规超导电性。

From perovskite to infinite-layer nickelates: hole concentration from x-ray absorption

Mon, 01 Jan 0001 00:00:00 +0000

该研究利用软X射线吸收光谱系统研究了PrNiOₓ薄膜在拓扑还原各中间阶段（x=2-3）的变化。通过比较Ni-L边实验光谱与单团簇及双团簇配位场模型计算，发现所有样品均未呈现纯d⁹电子构型。基于电荷和规则的定量分析表明，即便在最大程度还原的薄膜中，镍3d空穴的平均数仍为1.35，而超导样品的空穴数更高，这质疑了此前关于空穴掺杂极限的假设。还原过程中氧K边吸收光谱的同步变化表明，即使在最还原的薄膜中也存在氧2p空穴。综合结果指出，自掺杂效应与氧非化学计量共同导致了复杂的空穴掺杂机制。

Granular Superconductivity in La₂PrNi₂O₇-δ Thin Films

Mon, 01 Jan 0001 00:00:00 +0000

研究显示，La₂PrNi₂O₇₋δ薄膜中的两步超导转变源于其颗粒超导性质，即两种不同临界温度的超导颗粒相通过约瑟夫森结网络共存耦合。通过脉冲激光沉积制备的薄膜在臭氧退火后，输运测量发现即使在30 K附近残余电阻极小的情况下，仍出现明显的次级低温转变，导致零电阻温度仅约10 K。磁电阻滞后现象及对弱磁场的敏感响应与颗粒超导体的有效场模型一致，排除了自旋玻璃相的可能性。结构表征表明，氧不均匀性和局部结构紊乱（如单层相插层）是产生两相分离的主要原因。该发现揭示了双层镍酸盐薄膜中复杂超导行为的微观机制，并强调改善氧均匀性是实现更高零电阻温度体超导的关键，为后续光谱学研究奠定基础。

H-linear magnetoresistance in the T₂ resistivity regime of overdoped infinite-layer nickelate La₁₋ₓSrₓNiO₂

Mon, 01 Jan 0001 00:00:00 +0000

本研究对高结晶度的过掺杂无限层镍酸盐La₁₋ₓSrₓNiO₂薄膜（x=0.20-0.24）进行了系统的磁输运测量，在高达62 T的脉冲磁场中揭示了正常态的两个显著特征：磁电阻不遵循Kohler定则，在高H/T极限下表现为H线性行为；同时，正常态电阻率在30 K以下始终呈现T²依赖关系。结果表明，在该模型非常规超导体中，H线性磁电阻与T²电阻率共存，为理解无限层镍酸盐中承载超导电性的正常基态的输运特性提供了新信息。

Hall Coefficient Sign Reversal Driven by Orbital-Selective Oxygen-Vacancy Scattering in Nickelate Films

Mon, 01 Jan 0001 00:00:00 +0000

该研究结合DFT+CDMFT导出的关联多轨道准粒子模型与T矩阵方法，在半经典玻尔兹曼输运框架中处理氧空位散射，揭示了双层镍酸盐薄膜中霍尔系数符号反转的微观起源。多带补偿本身并不足以解释这一现象；面内氧空位通过轨道选择性散射强烈压制由d_x²⁻ᵧ²轨道主导的输运通道，驱动霍尔系数穿越零值而变为正，而顶点氧空位却使霍尔系数更趋向负值。这一口袋分辨和轨道选择性的氧空位散射机制表明，氧空位不仅是电子掺杂源，更是活跃的散射中心，其空间分布直接调控正常态输运行为，为统一理解实验中随氧化学计量比变化的多样霍尔响应提供了理论框架。

Heterostructuring as Gateway to Electron Doping of Nickelate Superconductors

Mon, 01 Jan 0001 00:00:00 +0000

该研究提出通过异质结构实现镍酸盐超导体电子掺杂的新途径。第一性原理计算表明，在La₂NiO₄中插入宽带隙绝缘层LaXO₃（X=Al, Ga, Sc）时，额外的(LaO)⁺层作为电子供体向Ni-3d轨道释放载流子，从而实现对Ruddlesden-Popper型镍酸盐的无序电子掺杂。该掺杂使La₂NiO₄:La₂AlO₄自然处于d_(x²-y²)波超导的最佳区域，动态顶点近似、涨落交换和动态团簇近似等多体方法预测其超导临界温度超过50 K，最高可达127 K。这种策略避免了传统化学掺杂引入的无序问题，且同样适用于La₃Ni₂O₇等镍酸盐，为全面探索镍酸盐电子掺杂相图及拓展至其他过渡金属氧化物体系提供了可行方案。

Hierarchical structure of primary and hybridization-induced superconducting correlations in bilayer nickelates

Mon, 01 Jan 0001 00:00:00 +0000

该研究采用变分蒙特卡洛方法对双层双轨道哈伯德模型进行非微扰计算，揭示了双层层状镍酸盐La₃Ni₂O₇中超导配对的层次结构。研究发现，主要配对相互作用源自Ni 3d({z^2})轨道的成键-反成键劈裂，而轨道杂化将超导关联重新分配到3d({x^2-y^2})通道，尽管该通道自身的内禀配对相互作用很弱。这种配对起源与超导关联来源的区别解释了为何两个轨道通道表现出相当的长程超导关联，且所得s±态对费米面拓扑变化（如α费米面片的消失）具有鲁棒性。该结果调和了此前关于配对机制的不同理论观点，表明超导关联的强度主要由低能态密度的轨道特征决定，而配对相互作用则源于双层结构中的轨道能级分离，突出了轨道杂化在多层层状超导体中稳定超导的关键作用。

High oxygen pressure floating zone growth and crystal structure of the metallic nickelates R₄Ni₃O₁₀ ( R = La, Pr )

Mon, 01 Jan 0001 00:00:00 +0000

High oxygen pressure floating zone growth and crystal structure of the metallic nickelates R₄Ni₃O₁₀ ( R = La, Pr )

High temperature transitions in Ruddlesden-Popper nickelates Lan+1NinO₃n+1

Mon, 01 Jan 0001 00:00:00 +0000

本研究系统性地分析了Ruddlesden-Popper镍酸盐La_{n+1}Ni_nO_{3n+1}（n=1,2,3,∞）系列单晶及粉末样品，结合粉末与单晶X射线衍射、热容、差示扫描量热等多种实验手段，在2–1000 K宽温域内揭示了此前被忽视的高温相变。对n=2和n=3化合物，在约560 K处观测到明显的晶格参数异常，例如双层2222相中面外晶格常数突增、面内收缩，表明八面体倾斜的突然释放；单层-三层1313多形体则表现为各向同性的体积骤缩；三层相n=3在该温度附近单斜角β出现清晰拐折，热容与DSC数据也证实该转变的热力学特征。该转变与已知的高温四方相变及低温密度波相变截然不同，且n=∞钙钛矿LaNiO₃未呈现类似行为。研究确认该高温相变是镍酸盐RP系列的普遍特性，强调在探索超导电性时必须重视此高温结构不稳定性对低温物性的潜在影响。

High-energy electronic excitations in La₃Ni₂O₇ by time-resolved optical spectroscopy

Mon, 01 Jan 0001 00:00:00 +0000

该研究利用时间分辨光学光谱技术，在环境压力下10 K至室温范围内探索了双层镍氧化物La₃Ni₂O₇中高能电子激发的超快动力学。研究识别出源于不同带间跃迁的两个高能电子激发，能量分别位于约1.8 eV和2.4 eV，并揭示它们具有不同的密度波（DW）能隙，分别为约54 meV和67 meV。这两个激发态的弛豫动力学可由Rothwarf-Taylor模型很好地描述。此外，实验观测到四个相干拉曼活性声子模式，这些模式与不同的电子激发表现出不同的耦合强度。升温引起的声子软化在约100 K至室温之间可用包含热膨胀和非谐声子-声子耦合的半定量模型解释；而在低温下，测量到的声子频率与模型拟合的偏差暗示了电子-声子耦合的额外贡献。该工作直接证明了该材料中复杂的能隙结构和声子动力学，为理解其密度波机制及多体效应提供了关键见解。

High-field-stabilized reentrant superconductivity in infinite-layer nickelate thin films

Mon, 01 Jan 0001 00:00:00 +0000

本研究在(Sm,Eu,Ca,Sr)NiO₂无限层镍酸盐薄膜中发现了高磁场稳定的重入超导电性。通过电阻和射频感应测量，在低场超导态外，于高场区域观察到另一个电阻急剧下降的超导态，其转变温度在低Tc样品中约9.6–11.7 K，高Tc样品（最高31.7 K）中则表现为低场与高场超导相融合，上临界磁场远超泡利顺磁极限。利用修正了内交换场的Werthamer-Helfand-Hohenberg（WHH）模型可精确描述相图，表明该现象源于Eu²⁺磁矩产生的内交换场抵消外加磁场的Jaccarino-Peter补偿机制。这项发现首次在具有较高超导转变温度的材料中实现场诱导重入超导，为开发可在数十特斯拉磁场下工作的超导磁体与器件提供了新路径。

High-pressure crystal growth and investigation of the metal-to-metal transition of Ruddlesden–Popper trilayer nickelates La₄Ni₃O₁₀

Mon, 01 Jan 0001 00:00:00 +0000

Single crystals of Ruddlesden–Popper nickelates La4Ni3O10 were grown by means of the floating-zone technique at oxygen pressure of 20 bar. Our results reveal the effects of the annealing process under pressure on the crystal structure. We present the requirements for crystal growth and show how a reported ferromagnetic impurity phase can be avoided. The different growth and post-annealing processes result in two distinct phases 𝑃 21∕𝑎 and Bmab in which the metal-to-metal transitions occur at 152 K and 136 K, respectively.

High-temperature superconductivity in Nd₀.85Sr₀.15NiO₂ membranes under pressure

Mon, 01 Jan 0001 00:00:00 +0000

研究人员开发了一种将自由站立无限层Nd₀.₈₅Sr₀.₁₅NiO₂薄膜集成到金刚石对顶砧中的技术，从而克服了薄膜在高压环境中测量的困难。通过对该薄膜施加高达约90 GPa的压力，他们观测到超导转变温度（T_c）从常压下的约17 K单调线性提升至约74.2 K，增强速率约为0.65 K/GPa，且未出现饱和迹象。这一线性的、非饱和的T_c压力演化趋势与大多数铜氧化物超导体及双层镍酸盐中观察到的压力诱导过掺杂导致的T_c下降行为显著不同，表明无限层镍酸盐的配对强度可被提升至出乎意料的高水平。此外，上临界场和相干长度的测量进一步证实了超导态随压力增强。该研究提供了通过晶格压缩持续增强超导电性的新途径，且所开发的自由薄膜高压技术有望广泛应用于其他二维材料。

High-temperature superconductivity with zero resistance and strange-metal behaviour in La₃Ni₂O₇−δ

Mon, 01 Jan 0001 00:00:00 +0000

Recent experimental observations have showed some signatures of superconductivity close to 80 K in La3Ni2O7 under pressure and have raised the hope of achieving high-temperature superconductivity in bulk nickelates. However, a zero-resistance state—a key characteristic of a superconductor—was not observed. Here we show that the zero-resistance state does exist in single crystals of La3Ni2O7−δ using a liquid pressure medium at up to 30 GPa. We also find that the system remains metallic under applied pressures, suggesting the absence of a metal–insulator transition proximate to the superconductivity. Moreover, analysis of the normal state T-linear resistance reveals a link between this strange-metal behaviour and superconductivity. The association between strange-metal behaviour and high-temperature superconductivity is very much in line with other classes of unconventional superconductors, including the cuprates and Fe-based superconductors. Further investigations exploring the interplay of strange-metal behaviour and superconductivity, as well as possible competing electronic or structural phases, are essential to understand the mechanism of superconductivity in this system.

Hubbard-U-corrected electron-phonon interactions in strongly correlated materials via the finite-displacement method

Mon, 01 Jan 0001 00:00:00 +0000

该论文将密度泛函理论加哈伯德U修正（DFT+U）与有限位移法相结合，实现了对强关联材料中声子谱和电子-声子矩阵的全哈伯德修正计算。作者将该方法应用于两种典型体系：无限层镍酸盐LaNiO₂和二氧化钌RuO₂。结果表明：在20%空穴掺杂的LaNiO₂中，哈伯德U修正微弱增强其电子-声子相互作用，但总耦合强度仍很小，不足以解释实验中约10-30 K的超导转变温度；这与近期GW修正预言的结果相反，差异源于DFT+U与GW方法所得费米面拓扑不同。在RuO₂中，哈伯德U修正消除了TiO₂衬底应变下的虚声子模，并大幅降低了电子-声子耦合，缓解了理论上过大的电子-声子耦合与实验观测到低超导转变温度之间的矛盾。该工作提供了完整纳入哈伯德U修正的电子-声子性质计算方案，并凸显了费米面形状及关联效应对声子谱和电子-声子矩阵的关键影响。

Identification of superconductivity in bilayer nickelate La₃Ni₂O₇ under high pressure up to 100 GPa

Mon, 01 Jan 0001 00:00:00 +0000

Identification of superconductivity in the Ruddlesden-Popper phases of nickelates under high pressure remains challenging. Here, we report a comprehensive study of the crystal structure, electrical resistance, and Meissner effect in single crystals of bilayer nickelate La3Ni2O7 under hydrostatic pressures up to 104 GPa. Using high-pressure X-ray diffraction, we observe a structural transition from an orthorhombic to a tetragonal phase above 40 GPa. Superconductivity emerges with a maximum onset transition temperature Tconset of 83 K at 18.0 GPa, accompanied by zero resistance. The superconducting phase is gradually suppressed and vanishes above 80 GPa, forming a right-triangle-like superconducting region. Direct-current magnetic susceptibility measurements demonstrate the Meissner effect and reveal a superconducting volume fraction of ∼41% at 22.0 GPa and 20 K, confirming the bulk nature of superconductivity in La3Ni2O7. Our results highlight the intricate relationship between superconductivity, oxygen content, and structural transitions in this material.

Identifying the structure of La₃Ni₂O₇ in the pressurized superconducting state

Mon, 01 Jan 0001 00:00:00 +0000

该工作利用高压变温拉曼光谱和偏振分析，在低至3 K、高达32.7 GPa的条件下系统追踪了La₃Ni₂O₇单晶的结构演化。基于严格的对称性选择定则，光谱中多个声子模式的消失与重整化在约14.5 GPa处指示了一个从正交Amam相到正交Fmmm相的一级结构相变，且体超导电性恰好在这一压力点出现。偏振拉曼测量进一步表明，在1.92 GPa以上，样品因退孪晶而恢复本征D₂h对称性；在超导态（3 K, 19.45 GPa）下，极化通道中仍可观测到声子模式，直接排除了四方I4/mmm相。研究结果确认，低于19.45 GPa的加压超导态本征晶体结构为正交Fmmm，而非此前争议的四方相，并揭示沿c轴呈180°的Ni–O–Ni键角是实现高超导转变温度的关键结构前提，为理解双层镍氧化物超导机制奠定了重要的结构基础。

Imaging stripe dynamics in trilayer nickelate La₄Ni₃O₁₀

Mon, 01 Jan 0001 00:00:00 +0000

该研究利用自旋极化扫描隧道显微镜对三层镍酸盐La₄Ni₃O₁₀中的条纹序进行了实空间成像，揭示了其局域磁性和电荷分布。实验发现，该条纹序具有四倍单胞的周期性，与铜氧化物高温超导体中的条纹序高度相似，并在费米能级附近打开了一个约66 meV的近完全能隙。更重要的是，当隧穿电子能量超过约20 meV的阈值时，可以触发离散的相位滑移，从而实现了原子尺度上的条纹动力学成像。这些结果强调了关联物理在驱动镧镍酸盐中类似条纹序中的关键作用，并显示出其与铜氧化物超导体惊人的相似性，为理解镍基超导体的配对机制提供了重要线索。

Impact of multiband effects on non-Fermi-liquid transport phenomena in bilayer nickelates

Mon, 01 Jan 0001 00:00:00 +0000

本研究采用多轨道紧束缚模型分析薄层双层镍酸盐La₃Ni₂O₇中的非费米液体输运现象，重点考察多带效应对霍尔系数的影响。通过格林函数方法推导了包含准量子度量（qQM）项的严格霍尔系数公式，发现该qQM项的温度依赖性在强关联多带体系中至关重要。计算表明，Ni d₂²轨道自旋涨落导致更强的准粒子阻尼，而Ni dₓ²₋ᵧ²轨道形成冷点。La₃Ni₂O₇中霍尔系数的显著温度依赖源于空穴带正贡献与电子带负贡献的竞争，qQM项使得低温下正霍尔系数增强，并解释了实验中观察到的T线性电阻率和霍尔系数随温度降低而上升的行为。此外，该qQM项对描述Nernst系数及其他涉及速度二阶导数的输运现象也起关键作用。研究揭示了自旋涨落诱导的轨道选择性重整化在非费米液体输运中的核心机制，为理解该体系的奇异输运性质提供了理论框架。

Impact of pressure and apical oxygen vacancies on superconductivity in La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

The bilayer nickelate La3Ni2O7 under pressure has recently emerged as a promising system for high-Tc superconductivity. In this work, we investigate the fate of the superconducting properties in La3Ni2O7 under pressure, focusing on the effects of structural deformation and apical oxygen vacancies. Employing a low-energy effective t-J∥-J⊥ model for the

$$3{d}_{{x}^{2}-{y}^{2}}$$

orbitals within the slave-boson mean-field approach, we demonstrate that the pairing strength is significantly enhanced in the high-pressure tetragonal I4/mmm phase compared to the ambient pressure orthorhombic Amam phase. Furthermore, by simulating random configurations of apical oxygen vacancies, we show that oxygen vacancies suppress both pairing strength and superfluid density. These results underscore the critical role of pressure and oxygen stoichiometry in tuning the SC of La3Ni2O7, providing key insights into optimizing its high-Tc behavior.

Incommensurate spin fluctuations and competing pairing symmetries in La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

The recent discovery of superconductivity in the bilayer Ruddlesden-Popper nickelate

 La
3
Ni
2
O
7
under high pressure has generated much interest in the superconducting pairing mechanism of nickelates. Despite extensive work, the superconducting pairing symmetry in
La
3
Ni
2
O
7
remains unresolved, with conflicting results even for identical methods. We argue that different superconducting states in
La
3
Ni
2
O
7
are in close competition and highly sensitive to the choice of interaction parameters as well as pressure-induced changes in the electronic structure. Our study uses a multiorbital Hubbard model, incorporating all Ni
3
d
and O
2
p
states. We analyze the superconducting pairing mechanism of
La
3
Ni
2
O
7
within the random phase approximation and find a transition between
d
-wave and sign-changing
s
-wave pairing states as a function of pressure and interaction parameters, which is driven by spin fluctuations with different wave vectors. These spin fluctuations with incommensurate wave vectors cooperatively stabilize a superconducting order parameter with
d
x
2
−
y
2
symmetry for realistic model parameters. Simultaneously, their competition may be responsible for the absence of magnetic order in
La
3
Ni
2
O
7
, demonstrating that magnetic frustration and superconducting pairing can arise from the same set of incommensurate spin fluctuations.

Insulator-to-metal transition in Co-doped La₃Ni₂O₇−δ with high oxygen pressure annealing

Mon, 01 Jan 0001 00:00:00 +0000

The effects of Co doping in a series of La3Ni2−xCoxO7−δ samples before and after high oxygen pressure annealing have been investigated. The structural refinemen

Interlayer electronic coherence links magnetism and superconductivity in Ruddlesden-Popper nickelates

Mon, 01 Jan 0001 00:00:00 +0000

本研究采用六端法对Ruddlesden-Popper镍酸盐双层与三层单晶进行高精度输运测量，自洽提取了面内与面外电阻率。结果发现，样品呈现强本征电子各向异性，面外电阻率表现出非单调温度依赖，揭示了普遍的层间相干至非相干渡越。在压力下，最大超导转变温度与环境压强下的电阻率各向异性成反比，表明更强的层间电子相干性有利于超导。此外，面外电阻率是磁性与密度波序的灵敏探针，而面内电阻率响应较弱。这些结果突出层间相干性作为关键调控参数，既追踪磁性关联，又与超导电性紧密相关，为镍基高温超导的微观理论提供了严格约束。

Interlayer Five-Spin Polaron in Superconducting Bilayer Nickelates

Mon, 01 Jan 0001 00:00:00 +0000

利用共振X射线散射和光谱学方法，研究双层镍氧化物La₂PrNi₂O₇薄膜中自旋密度波（SDW）序与超导电性的关系。发现超导仅出现在无SDW且氧化学计量完整的区域，而氧缺失则促进SDW序，表明两者发生相分离。进一步通过Ni-L₃和O-K边光谱揭示，超导相具有金属性基态，主要呈现Ni d⁸与氧配体空穴特征；氧缺失导致电子局域化并出现低能激发。结合理论分析，提出配体空穴主要位于层间顶角氧上，形成稳定的层间五自旋极化子态，作为超导双层镍氧化物的基态。研究表明氧化学计量是控制层间耦合及电子结构的关键参数，SDW序并非超导的内在母态。

Interlayer hybridization enables superconductivity in bilayer nickelates

Mon, 01 Jan 0001 00:00:00 +0000

通过稳定双层镍酸盐（La,Pr）₃Ni₂O₇超导薄膜并采用保护覆盖层，结合X射线吸收和共振非弹性X射线散射光谱，本研究直接探测了绝缘体、超导体和金属态下的电子结构演化。实验与理论分析表明，面内d_{x²-y²}态构成巡游电子骨架，而超导电性仅在面外d_{z²}-p_z-d_{z²}层间杂化形成相干时出现，此时静态自旋序被抑制，自旋激发呈现强阻尼特征。氧化学计量和外延应变共同调控该层间通道，使超导态局限于一个较窄的层间相干性与关联强度窗口内。这些结果揭示了双层镍酸盐中超导所需的微观要素，并提供了多轨道图像来描述其涌现机制。

Interlayer pairing mechanism for bilayer nickelate superconductors

Mon, 01 Jan 0001 00:00:00 +0000

该综述系统梳理了双层镍酸盐超导体中基于强关联效应的层间配对机制理论进展。文章从关键实验观测出发，提炼出核心物理要素，包括杂化的Ni-3d_(x²-y²)与3d_(z²)轨道电子结构、轨道依赖的电子关联、洪德耦合以及强层间磁耦合，并引入了双层两轨道哈伯德模型及其t-J变体等基本理论框架。重点阐述了基于半填充d_(z²)轨道原子极限下层间价键图像的强关联配对机理，尤其强调杂化机制：即d_(z²)局域单重态配对提供凝聚能，其与巡游d_(x²-y²)轨道的杂化促进超导相位相干。文章进一步分析了配对对称性、临界温度对各种内外部参数的依赖关系，以及正常态的费米液体、非费米液体、弱绝缘和赝能隙等非平庸行为，并讨论了压力调控、氧含量及氧空位诱导的近藤散射的影响。全文核心结论指向一种由局域配对与巡游性协同作用驱动的非传统超导图像，并简要提及了基于费米面嵌套自旋涨落的弱关联理论。

Interstitial oxygen order and its competition with superconductivity in La₂PrNi₂O₇+δ

Mon, 01 Jan 0001 00:00:00 +0000

High-temperature superconductivity in pressurized La3Ni2O7 has attracted considerable interest, yet the superconducting phase is rather fragile. Although bulk superconductivity can be achieved by Pr substitution for La, the underlying mechanism is still unclear. A further puzzle is the role of oxygen content: moderate oxygenation enhances superconductivity, whereas high-pressure oxygen annealing suppresses it. Here combining multislice electron ptychography and electron energy-loss spectroscopy, we show that Pr doping mitigates oxygen vacancies and stabilizes a near-stoichiometric La2PrNi2O7 structure. Strikingly, high-pressure oxygen annealing introduces interstitial oxygen atoms that arrange into a stripe-ordered superstructure, which generates excess hole carriers and alters the electronic structure, ultimately suppressing superconductivity under pressure. This contrasts sharply with cuprates, where similar oxygen ordering is known to induce superconductivity. Our findings reveal a competition between interstitial oxygen ordering and superconductivity in bilayer nickelates, providing key insights into the pairing mechanism and guiding principles for engineering more robust superconducting phases.

Ionic-Bond-Driven Atom-Bridged Room-Temperature Cooper Pairing in Cuprates and Nickelates: a Theoretical Framework Supported by 32Experimental Evidences

Mon, 01 Jan 0001 00:00:00 +0000

针对铜氧化物和镍氧化物中高温超导的配对机制这一长期难题，本文基于离子键在eV量级的主导作用、O⁻和O²⁻的电子亲和能（分别为1.46 eV和-8.08 eV）以及金属原子较大的双电子电离能（约15–28 eV），提出了氧（O）桥连电子对（e⁻-O-e⁻）或金属（M）桥连空穴对（h⁺-M-h⁺）的巡游库珀对图像。该配对在赝隙温度T*高于Tc时形成，遵循化学键→结构→性质的关系，并适用于铜氧化物、镍氧化物、铁基及其他离子型超导体。作者通过32项不同的实验证据，尤其是CuO₂面内的STM图像与极小配对尺寸，证实了该机制的正确性与普适性，并指出任何亚eV或共价键合配对机制均不可靠。该理论揭示了离子键与超导电性之间的缺失环节，解决了长达40年的困惑，并验证了在离子键超导体中实现室温载流子配对的可行性。在此基础上，作者建立了以最强配对强度和玻色-爱因斯坦凝聚为核心的新理论框架，为理解高温超导机理开辟了新路径，并使室温超导的梦想更近一步。

Itinerant Nature of Spin-Density-Wave Order in Ruddlesden-Popper Nickelates

Mon, 01 Jan 0001 00:00:00 +0000

本文针对Ruddlesden-Popper镍酸盐中自旋密度波（SDW）序和磁激发的本质，发展了一个统一的巡游电子描述。核心要素是NiO₂多层块的镜面对称结构，它将低能电子态组织成镜偶和镜奇两个扇区。研究表明，镜相反扇区之间的主导带间嵌套驱动了镜选择性的巡游SDW不稳定性，其集体模式自然重现了实验中观察到的类似自旋波的谱。在La₄Ni₃O₁₀中，SDW进一步诱导出次级镜偶电荷密度波，产生交织的自旋和电荷纹理。这些结果证明，多层镍酸盐的磁性本质上是巡游而非局域矩起源，并确立了镜选择性带间SDW序作为这些体系中磁关联的统一组织原则。

Jahn-Teller distortion on strained La₃Ni₂O₇ thin films

Mon, 01 Jan 0001 00:00:00 +0000

该研究通过密度泛函理论计算系统分析了应变状态下La₃Ni₂O₇薄膜的电子结构，发现双轴压缩应变主要拉伸外层顶角Ni-O键，而内层顶角Ni-O键几乎不变，进而显著增强Jahn-Teller分裂能Δ_JT，但层间d_z²轨道跳跃参数t_⊥^z变化微弱。由于超导仅出现在面内晶格常数低于临界值时，该结果将应变增强的Δ_JT识别为关键微观调控参数。计算的费米面结构和霍尔响应与LaAlO₃和SrLaAlO₄衬底上的ARPES及霍尔测量结果吻合，证实Jahn-Teller畸变在优化双层镍酸盐超导中起核心作用。

Lattice-Charge Coupling in a Trilayer Nickelate with Intertwined Density Wave Order

Mon, 01 Jan 0001 00:00:00 +0000

Lattice-Charge Coupling in a Trilayer Nickelate with Intertwined Density Wave Order

Layer-resolved Electronic Structure and Correlation of Low-n Square-planar Nickelates: A DFT+DMFT Prediction of Superconducting Candidates

Mon, 01 Jan 0001 00:00:00 +0000

该研究采用密度泛函理论结合动力学平均场理论（DFT+DMFT），系统分析了低 n 层四方镍酸盐的层分辨电子结构和关联效应。结果表明，未掺杂体系中 Ni-d 轨道的电子关联强度随层数增加而增强，且内层 NiO₂ 平面普遍比外层具有更强的关联性，这种层间差异源于空间电荷分布的不均匀性。针对因过度空穴掺杂而不超导的 n=2 和 n=3 化合物，提出在间隔层氧位进行 Cl 替代的电子补偿策略，通过虚拟晶体近似模拟将其名义 Ni 价调控至与最佳超导 n=6 体系相同的水平。计算显示，Cl 掺杂可显著提升低层数体系的 Ni-d 质量增强因子，使其进入强关联金属区，同时低能电子结构得以保持。该工作揭示了层分辨电子关联在超导机制中的关键作用，并预测间隔层 Cl 掺杂是将低 n 四方镍酸盐转化为潜在超导候选材料的可行路径。

Magnetic configurations and excitations in high-Tc multilayer nickelates

Mon, 01 Jan 0001 00:00:00 +0000

该研究采用多轨道巡游电子框架，结合 Hartree-Fock 和随机相近似方法，系统分析了双层与三层镍酸盐的磁基态及横模自旋激发。对于双层体系，虽双条纹序能量略低，但单条纹态的激发谱在波矢 Q_BL 处呈现各向异性低能锥形色散，在 Γ 点附近显示各向同性高能激发，与共振非弹性 X 射线散射及中子散射实验定性吻合；还发现 Q_BL 处的镜面偶数层间光学模能量与 Γ 处镜面奇数模一致。对于三层体系，镜面奇数和镜面偶数自旋密度波均可稳定，镜面奇数态能量更低，并携带由中间层主导的近零能隙模，而镜面偶数态仅含一支声学模和两支有能隙光学模；与实验对比支持镜面奇序图像。结果表明，磁激发可作为灵敏探针区分磁有序构型，并支撑多层镍酸盐磁性具有共同巡游起源的结论。

Magnetic Order in bilayer Ruddlesden-Popper Nickelates

Mon, 01 Jan 0001 00:00:00 +0000

基于轨道选择性电子关联的实验证据，本文针对双层镍酸盐La₃Ni₂O₇提出了一种新的磁性描述框架。在正常态的坏金属区中，体系邻近轨道选择性Mott相，电子谱分解为相干的d_(x²-y²)轨道准粒子和非相干的d_(z²)轨道局域磁矩。通过整合局域磁矩间的超交换作用与相干电子介导的RKKY相互作用，构建有效自旋模型，其中RKKY贡献主导了第三近邻耦合并引发磁阻挫。该模型自然稳定了一个波矢接近(π/2, π/2)的非共面反铁磁基态，层间呈反铁磁堆叠，与中子散射等实验一致。计算的自旋波谱包含低频声学支和高频光学支，声学支在序波矢处软化，总带宽约80 meV，与共振非弹性X射线散射实验相符。这一结果揭示轨道选择性关联是决定双层镍酸盐磁性的核心要素，并指出低能磁涨落及短程交换作用可能为非常规超导配对提供驱动力。

Metallic crossover through the tilt-free transition in La₃Ni₂O₇ at high pressure and temperature

Mon, 01 Jan 0001 00:00:00 +0000

该研究通过高压高温拉曼光谱和同步辐射红外反射光谱，系统研究了双层镍氧化物La₃Ni₂O₇在压力与温度下的结构相变与电子性质演变。拉曼测量证实了压力驱动的结构相变（从倾斜的Amam相到无倾斜的Fmmm或I4/mmm相），并观察到Fano线型出现，表明电子-声子耦合增强。高温数据揭示了该相变在常压下存在544 K的上限温度，完善了温压相图。红外反射率测量显示，相变伴随载流子密度提升近两个数量级，标志着从不良金属到良金属的交叉。实验建立了结构相变与电子性质强耦合的统一图像，超导性在约6-7 GPa处与无倾斜相的出现紧密相关，但高对称性与金属性本身不足以诱发超导，可能还需考虑密度波涨落等强电子关联效应。

Microscopic Evidence of Charge- and Spin-Density Waves in La₃Ni₂O₇–δ Revealed by 139La-NQR

Mon, 01 Jan 0001 00:00:00 +0000

The recent discovery of superconductivity in La3Ni2O7–δ with a transition temperature Tc close to 80 K at high pressures has attracted significant attention, due particularly to a possible density wave (DW) transition occurring near the superconducting dome. Identifying the type of DW order is crucial for understanding the origin of superconductivity in this system. However, owing to the presence of La4Ni3O10 and other intergrowth phases in La3Ni2O7–δ samples, extracting the intrinsic information from the La3Ni2O7 phase is challenging. In this study, we employed 139La nuclear quadrupole resonance (NQR) measurements to eliminate the influence of other structural phases in the sample and obtain microscopic insights into the DW transition in La3Ni2O7–δ. Below the DW transition temperature TDW ∼ 153 K, we observe a distinct splitting in the ±5/2 ↔ ±7/2 transition of the NQR resonance peak at the La(2) site, while only a line broadening is seen in the ±3/2 ↔ ±5/2 transition peak. Through further analysis of the spectra, we show that the line splitting is due to a unidirectional charge modulation. A magnetic line broadening is also observed below TDW, accompanied by a large enhancement of the spin-lattice relaxation rate, indicating the formation of magnetically ordered moments in the DW state. Our results suggest a simultaneous formation of charge- and spin-density wave orders in La3Ni2O7–δ, thereby offering critical insights into the electronic correlations in Ni-based superconductors.

Multiband Metallic Ground State in Multilayered Nickelates La₃Ni₂O₇ and La₄Ni₃O₁₀ Probed by 139La-NMR at Ambient Pressure

Mon, 01 Jan 0001 00:00:00 +0000

We report a 139La-NMR study of polycrystalline samples of multi(n)-layered nickelates, La3Ni2O7−δ (n = 2) and La4Ni3O10−δ (n = 3), at ambient pressure. Measurements of the nuclear magnetic resonance (NMR) spectra and nuclear spin relaxation rate (1/T1) indicate the emergence of a density wave order with a gap below T* ∼ 150 K for La3Ni2O7−δ and ∼130 K for La4Ni3O10−δ. The finite value of 1/T1 below T* indicates metallic ground states with the remaining density of states at the Fermi level (EF) under the density wave order. These features are attributed to multiple d electron bands with different characteristics. Above T*, the gradual decrease in 1/T1T upon cooling implies the presence of a band with flat dispersion near EF. From our microscopic probes, we point out that these nickelates (n = 2 and 3) possess similar electronic states despite the difference in the formal valence of the Ni d electron states, which provides a basis for understanding the novel high-Tc superconductivity under high pressures.

Multimodal Terahertz Spectroscopy of the Pairing Symmetry and Normal-State Pseudogap in (La,Pr)₃Ni₂O₇ Films

Mon, 01 Jan 0001 00:00:00 +0000

该研究通过结合线性太赫兹时域光谱与三阶谐波产生技术，系统探测了压缩应变(La,Pr)₃Ni₂O₇薄膜的超导配对对称性及正常态赝能隙。线性太赫兹光谱观测到超导转变温度以下低频谱权重的显著抑制，伴随弱相干峰以及趋近零温时仍存在的较大剩余电导，与无序s±波配对图像一致。非线性三阶谐波信号在进入超导态后急剧增强，但其响应在超导转变温度以上持续存在，并在约100K处出现扭折，结合角分辨光电子能谱在同类薄膜中观测到的类似温度尺度，将其归因于正常态赝能隙。研究确立了(La,Pr)₃Ni₂O₇为具有s±波类配对的体超导体，且该超导态与另一有序态共存并可能相互竞争，为探索超越铜基和铁基的非常规超导机制提供了新平台。

Nature of charge density waves and metal-insulator transition in pressurized La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

Nature of charge density waves and metal-insulator transition in pressurized La₃Ni₂O₇

Nature of magnetism in bilayer nickelate La₃Ni₂O₇ single crystals

Mon, 01 Jan 0001 00:00:00 +0000

该研究利用中子散射技术解析了双层镍氧化物La₃Ni₂O₇单晶的自旋有序与动力学。在常压母体相中，于倒空间Q=(0,0.5,2.5)位置观测到清晰的自旋激发，具有约5 meV的自旋能隙和显著的面内各向异性色散——沿横向的带边软化揭示了竞争性交换相互作用。激发表现出双层周期性的面外调制，直接证实了反铁磁层间耦合。基于线性自旋波理论，实验色散可用包含强层间交换与竞争面内耦合的双层海森伯哈密顿量精确描述，磁结构为条纹型有序。将谱强度归一化为绝对单位后发现，尽管自旋波带宽仅为铜氧化物的25%，但在可比能量下局域动态磁化率显著增强，总涨落磁矩与铜氧化物相当。这些结果揭示了源自强电子关联的中能自旋激发是该体系的本质特征，构建了与铜氧化物截然不同的磁性框架，为理解该体系中超导的配对机制提供了直接依据。

Nearly perfect Fermi surface nesting in hole-doped La₃Ni₂O₇ enables bulk superconductivity without pressure or strain

Mon, 01 Jan 0001 00:00:00 +0000

通过结合密度泛函理论、动态平均场理论和随机相位近似求解超导能隙方程，研究者发现空穴掺杂的层状镍氧化物La₃₋ₓSrₓNi₂O₇在环境压力下可实现体超导电性。当掺杂浓度x接近0.4时，Ni-d₃z²₋r²轨道衍生的γ费米口袋从圆形演变为菱形并扩展至半个布里渊区，形成近乎完美的费米面嵌套，最佳嵌套矢量为Q=(π, π)。这一结构显著增强了反铁磁自旋涨落，将超导特征值提升至实验可观测的水平，无需施加高压或应变。该工作阐明了空穴掺杂调控费米口袋形状与大小的机制，为在常压下实现此类材料长期追求的体超导提供了理论依据和实验可行路径。

Nearly twofold overestimation of the superconducting volume fraction in pressurized Ruddlesden-Popper nickelates

Mon, 01 Jan 0001 00:00:00 +0000

本研究指出，Zhu等人在测量加压Ruddlesden-Popper镍酸盐La₄Ni₃O₁₀的超导体积分数时，采用了此前未见报道的计算方程，导致结果被显著高估。作者基于Zhu等人公布的原始数据，使用标准超导磁矩计算方法重新分析，发现超导体积分数仅为51%至59%，而非Zhu等人报告的81%至86%。通过检查Zhu等人提供的方程及其推导，作者发现该方程在计算中错误地使用了样品几何参数，导致对超导相所占体积比例的计算出现约两倍的高估。作者以假想样品为例，说明即使超导相实际只占50%，该方程仍会得出接近100%的结果。因此，该错误影响了迄今所有已报道的Ruddlesden-Popper镍酸盐超导体积分数数据，需要重新审视这些结论。

Ni-O hybridization-driven electronic reconstruction across the superconducting dome in an infinite-layer nickelate

Mon, 01 Jan 0001 00:00:00 +0000

该研究利用O K边和Ni L边X射线吸收谱，系统表征了无限层镍氧化物La₁₋ₓCaₓNiO₂中未占据态随掺杂和温度的演化。超导出现在x=0.18至0.27区间。在x≈0.20–0.23附近，低能谱权重发生重新分布：Ni 3d主导态减少而O 2p杂化态增加，表明Ni–O共价性出现轨道选择性交叉。该交叉与霍尔系数符号反转恰好重合，并先于更高掺杂时超导临界温度的下降。通过将输运异常和超导穹顶直接关联到可测量的Ni–O轨道重组，该结果向建立无限层镍酸盐统一的轨道分辨相图迈出了关键一步，并为通过杂化调控设计超导电性提供了实用途径。

Non-Fermi liquid behavior in La₃Ni₂O₇ thin films under hydrostatic pressure

Mon, 01 Jan 0001 00:00:00 +0000

该研究报道了在LaAlO₃(001)和SrLaAlO₄(001)基底上外延生长双层La₃Ni₂O₇薄膜的输运性质，并通过高压氧气退火和静水压进行调控。常压下，LaAlO₃基底上的薄膜表现出费米液体金属行为，伴随低温下的轻微Kondo-like上升；而施加0.53至1.41 GPa的静水压后，电阻温度依赖性逐渐演变为非费米液体行为，在1.41 GPa时趋近于~T¹⁴。值得注意的是，这一压力仅为单晶中通过金刚石对顶砧实现类似效应所需压力的6–8%，显示出薄膜形式下超乎预期的强可调性。此外，在YAlO₃基底上的薄膜中观察到自旋密度波（SDW）序的迹象，而在LaAlO₃基底上则被抑制。霍尔效应测量表明多带电子结构。这些结果揭示了La₃Ni₂O₇薄膜在适度压力下即可接近强涨落有序态，为非费米液体行为的起源和高压超导的薄膜化研究提供了新途径。

Nonthermal melting and density wave instability coupled to the lattice in La₄Ni₃O₁₀

Mon, 01 Jan 0001 00:00:00 +0000

研究人员使用超快光学光谱在研究La₄Ni₃O₁₀单晶时，于密度波转变温度约136 K处观察到准粒子弛豫动力学的突变，揭示出一个约52 meV的强耦合能隙打开。多个相干声子模式，包括3.88、5.28和2.09 THz附近的Ag模，在转变处表现出模式选择性的异常，尤其是3.88 THz声子的重整化行为从高温下的常规非谐衰减转变为低温下的急剧硬化，表明密度波不稳定性与晶格自由度之间存在强耦合，电子-声子相互作用可能起关键作用。在高激发通量下，密度波被非热抑制，得到的温度-通量相图与压力调谐行为相似，但能隙相对稳定，导致耦合比率增大。这些发现确立了La₄Ni₃O₁₀中的密度波是一种晶格纠缠的、涉及多轨道物理的不稳定性，并证实超快光激发可作为非平衡调控参数有效抑制镍酸盐中的密度波序。

Normal and Superconducting Properties of La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

This review provides a comprehensive overview of current research on the structural, electronic, and magnetic characteristics of the recently discovered high-temperature superconductor La3Ni2O7 under high pressures. We present the experimental results for synthesizing and characterizing this material, derived from measurements of transport, thermodynamics, and various spectroscopic techniques, and discuss their physical implications. We also explore theoretical models proposed to describe the electronic structures and superconducting pairing symmetry in La3Ni2O7, highlighting the intricate interplay between electronic correlations and magnetic interactions. Despite these advances, challenges remain in growing high-quality samples free of extrinsic phases and oxygen deficiencies and in developing reliable measurement tools for determining diamagnetism and other physical quantities under high pressures. Further investigations in these areas are essential to deepening our understanding of the physical properties of La3Ni2O7 and unlocking its superconducting pairing mechanism.

Observation of correlated plasmons in low-valence nickelates

Mon, 01 Jan 0001 00:00:00 +0000

使用共振非弹性X射线散射（RIXS）在氧K边观测到低化合价镍酸盐Pr4Ni3O8中的等离激元集体激发，并与过掺杂铜氧化物La₂₋ₓSrₓCuO₄进行对比。实验发现，镍酸盐的等离激元在面内动量下表现出色散行为，但相较铜氧化物具有更低的速度和更强烈的阻尼，且在更小的动量下即被过度阻尼而消失。随机相位近似（RPA）计算表明，这种差异源于镍酸盐中减少的电子跳跃和增强的长程库仑相互作用屏蔽效应，面内跳跃积分和库仑相互作用强度均显著小于铜氧化物。此外，镍酸盐的面外等离激元未显示可分辨的色散，可能与其三层耦合结构有关。温度依赖研究发现，Pr4Ni3O8的等离激元随温度升高而软化，而铜氧化物中能量几乎不变但阻尼增大，暗示镍酸盐中可能存在条纹涨落等额外关联。这些结果揭示了镍酸盐中独特的电荷屏蔽景观，电子跳跃减弱和库仑屏蔽增强是区别于铜氧化物的关键特征，可能解释了镍酸盐超导转变温度低于铜氧化物的原因，为两类材料间的类比提供了定量实验约束。

Observation of flat-bottom U-shaped energy gap in high-Tc nickelate (La,Pr)₃Ni₂O₇ thin films

Mon, 01 Jan 0001 00:00:00 +0000

利用极低温扫描隧道显微镜/谱和电输运测量，本研究首次在（La,Pr）₃Ni₂O₇薄膜中观测到能量对称、平底U形超导能隙，其零残余态密度且能隙大小超过40 meV。输运测量显示该薄膜在常压下具有超过40 K的起始超导转变温度和超过20 K的零电阻温度。隧穿谱呈现非常规的温度演化：随温度升高，U形能隙迅速填充并转变为V形能隙；同时，在14特斯拉的c轴磁场下能隙减小。这些温度与磁场依赖特性符合超导能隙行为，表明在极低温下存在无节点能隙函数。该发现揭示了双层镍氧化物高温超导的本质，并为在常压或零压下实现液氮沸点以上超导温度提供了重要启示。

On estimating superconducting shielding volume fraction from susceptibility in pressurized Ruddlesden-Popper nickelates: Response to arXiv:2602.19282

Mon, 01 Jan 0001 00:00:00 +0000

这篇回应性论文澄清了在加压Ruddlesden-Popper镍酸盐中超导屏蔽体积分数的评估方法。作者指出，他们的方法直接源于有限样品的标准静磁自洽关系（公式2-4），通过退磁因子N修正测量磁化率以得到内在磁化率，并由此估计屏蔽体积分数（f ≈ -χ）。该方法已在超导体文献中广泛应用数十年。以单晶样品S6为例，在50 GPa和5 K下，应用自洽公式计算得到超导屏蔽体积分数约为86%，而在40 GPa下约为82%。作者论证，arXiv:2602.19282中质疑的根本缺陷在于对方假设测量到的抗磁矩与超导屏蔽体积分数成线性比例，并通过计算全屏蔽麦斯纳磁矩进行简单归一化。这一假设对于强退磁的薄圆盘状样品不成立，因为内场与磁化通过退磁场自洽耦合，使得测量磁矩与屏蔽分数呈非线性关系，从而导致对方方法给出约60%的偏低估计。论文还讨论了单退磁因子框架的适用范围，指出其样品结构均匀、质量高，支持该宏观描述，而对方构造的人为相分离模型不适用于此。作者结论认为，基于静磁自洽的方法是正确且广泛采用的标准方法。

On the estimating the superconducting volume fraction from the internal magnetic susceptibility

Mon, 01 Jan 0001 00:00:00 +0000

Zhang等人[1]报道了高压下Pr₄Ni₃O₁₀单晶的零场冷却(ZFC)和场冷却(FC)数据，证实了Ruddlesden-Popper镍酸盐中的体超导电性，并利用内部磁化率χ_internal的幅度估算超导体积分数f=0.85。本文对该方法的关键假设f=|χ_internal|提出质疑，并通过反例说明：即使样品计算出的|χ_internal|达0.82，实际超导体积分数f可能低于0.10。作者指出，退磁因子N的计算依赖于非超导区域在样品中的分布，而均匀性假设并不成立——若90.2%的体积为非超导相，只要其几何形状改变退磁因子，磁化率测量值仍可匹配原始结果。因此，仅凭χ_internal无法唯一确定f，该方法在整个超导研究领域需要重新审视。

Orbital dimerization-induced first-order structural phase transition: a case study in La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

该研究以新发现的镍基超导体La₃Ni₂O₇为例，揭示了其一级结构相变背后的“轨道二聚化”机制。作者指出，标准密度泛函理论（DFT）及考虑原子内关联的LDA+DMFT方法均无法重现该相变的突然性，因为这些方法忽略了关键的原子间关联。通过构建包含低能活性轨道的多体有效哈密顿量并进行精确对角化，他们发现当Ni-O-Ni键角趋于平直时，层间Ni轨道之间会产生强烈的反铁磁超交换作用，形成自旋单态键合，从而引起总能量急剧下降。这种能量降低在特定构型处产生新的局域能量极小值，与倾斜构型共存，完美解释了实验中观测到的一级相变特征，以及高低压结构的共存现象。该机制不仅改变了晶格键合性质，还导致低能电子结构定性变化（如超导电性的出现）。该工作建立的DFT加多体修正计算框架具有普适性，适用于含开壳层d/f电子的离子材料，为理解此类体系的结构与电子性质关联提供了关键微观机理。

Orbital Signatures of Density Wave Transition in La₃Ni₂O₇-delta and La₂PrNi₂O₇-delta RP-Nickelates Probed via in-situ X-ray Absorption Near-edge Spectroscopy

Mon, 01 Jan 0001 00:00:00 +0000

The report of superconductivity (SC) with Tc~80 K in bilayer Ruddlesden-Popper (RP) nickelate La3Ni2O7-delta have sparked considerable investigations on its normal state properties and SC mechanism under pressure and at low temperature. It is believed that the density wave (DW) at ~150 K plays an important role in SC emergence, but its nature remains largely underexplored. Here, we utilized temperature-dependent in-situ Ni K-edge X-ray Absorption Near-edge Spectroscopy (XANES) to probe the Ni-3d/4p electronic states of La3Ni2O7-delta and La2PrNi2O7-delta samples down to 4.8 K, enabling us to witness the evolution of both in-plane d_(x^2-y^2)/p_x (p_y) and out-of-plane d_(3z^2-r^2)/p_z orbitals of NiO6 octahedron across the DW transition. Main edge energy associated with Ni 4p orbital shows an anomalous decline near DW transition, signifying the occurrence of lattice distortions as a hallmark of charge density wave. Below DW transition, the enlarged crystal field splitting (CFS) indicates an enhanced NiO6 octahedral distortion. Intriguingly, magnetic Pr substituents could activate the mutual interplay of d_(x^2-y^2) and d_(3z^2-r^2) orbitals. We discussed its relevance to the favored bulk SC in the pressurized polycrystalline La2PrNi2O7-delta than pristine.

Orbital-Selective d-wave Superconductivity in the Two-Band t-J Model: Possible Applications to La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

本研究采用变分蒙特卡罗方法，对包含一个巡游轨道（轨道0）和一个准局域轨道（轨道1）的双带t-J模型中的超导电性进行了探索。核心发现是，系统呈现出稳健的轨道选择性d波超导态，该超导态完全起源于巡游的轨道0。通过对超交换能级层次的分析表明，准局域化的轨道1通过倾向于形成局域轨道间束缚态来与超导电性竞争，这些束缚态作为能量缺陷破坏了相位相干性。相应地，超导序参量随轨道1占据数的增加而单调下降。受镍酸盐La₃Ni₂O₇中超导电性的启发，这些结果强调了超越单带t-J框架的多轨道物理的关键作用，并为提高超导转变温度指明了具体途径：抑制源于局域d_{z²}轨道的参与。

Origin of Spin Stripes in Bilayer Nickelate La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

针对双层镍酸盐La₃Ni₂O₇在常压下的非常规磁序，本文提出了一个忠实反映其晶体对称性的微观哈密顿量。通过大规模密度矩阵重正化群计算表明，在较大的Hund耦合JH作用下，由于隐藏的准一维性，模型中出现了(π/2,π/2)自旋条纹序，并在一定电子浓度范围内持续存在。在更对称的高压区域，当层间反铁磁耦合J⊥足够强时，模型表现出增强的层间配对倾向。该研究揭示了对角自旋条纹的微观起源，并确认Hund耦合JH和层间耦合J⊥是控制La₃Ni₂O₇磁序和配对倾向的关键要素。

Oxygen deficiency mechanism of La₃Ni₂O₇−δ under pressure

Mon, 01 Jan 0001 00:00:00 +0000

The recent discovery of superconductivity in pressurized bilayer nickelate La3Ni2O7 has triggered tremendous research interest. However, the experimentally observed oxygen deficiency implies that obtaining perfect stoichiometric single crystals is still challenging. The influence of oxygen deficiency on physical properties remains unexplained. Here, we construct a chemical potential phase diagram to characterize the stability of La3Ni2O7. The narrow stable region explains the difficulty of synthesizing pure samples. First, oxygen defect studies reveal that the interlayer apical oxygen vacancy has the highest defect concentrations and is responsible for oxygen deficiency. Second, unfolding band structures show as the oxygen-deficient variant increases, Ni

$$3d_{\;z^{2}}$$

bands shift toward a lower energy position under the Fermi level at Γ point, which is adverse to the metallization of Ni

$$3d_{\;z^{2}}$$

bands. Third, high-pressure calculations indicate that oxygen vacancy would destroy the hybridization of interlayer Ni

$$3d_{\;z^{2}}$$

orbitals, and the larger the oxygen deficiency, the higher the pressure needed to metalize the Ni

$$3d_{\;z^{2}}$$

bands. Thus, the oxygen deficiency would suppress the emergence of superconductivity in La3Ni2O7−δ. Our results elucidate the mechanism of oxygen deficiency for superconductivity in La3Ni2O7−δ and provide instructive guidance to the experimental research.

Oxygen-isotope effect on density wave transitions in La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

本研究通过氧同位素替换（¹⁶O→¹⁸O），利用电阻率和μ子自旋旋转（μSR）实验，系统探索了双层Ruddlesden-Popper镍酸盐La₃Ni₂O₇中电荷密度波（CDW）和自旋密度波（SDW）转变的同位素效应。电阻率测量显示，CDW转变温度在¹⁸O取代后显著升高约6 K，而μSR结果则表明SDW转变温度在实验误差范围内不受影响。拉曼光谱证实了同位素替换的有效性及晶格声子模的软化。这一对比鲜明的同位素响应表明，晶格振动（即电子-声子耦合）在CDW序的形成中扮演重要角色，而SDW序主要源于电子相互作用。研究结果揭示了两种密度波序的不同微观起源，并提示电子-声子耦合可能对Ruddlesden-Popper镍酸盐的超导配对机制具有潜在关联，为理论模型提供了关键约束。

Pairing mechanism and superconductivity in 1313 phase La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

通过密度泛函理论结合动力学平均场理论（DFT+DMFT）和随机相位近似（RPA）系统研究了1313相La₃Ni₂O₇的电子结构和超导机制。DMFT计算表明，单层子系统呈近绝缘态，其中d_{z²}轨道表现出莫特物理，而三层子系统保持金属性，超导主要源于后者，且其Ni-e_g轨道相对于块体La₄Ni₃O₁₀处于空穴掺杂状态。基于DMFT导出的低能有效哈密顿量，RPA分析给出三层子系统内为s^{±}波配对对称性。与块体La₄Ni₃O₁₀相比，1313相超导转变温度显著降低的原因有两个：一是空穴掺杂减弱了配对强度；二是单层子系统作为弱连接层，将相邻的三层超导层构成S-N-S约瑟夫森结，抑制了层间相位相干，从而进一步降低全局转变温度。综合结果表明，RP相La₃Ni₂O₇家族中的高温超导应归属于2222相而非1313相。

Pairing mechanism and superconductivity in pressurized La₅Ni₃O₁₁

Mon, 01 Jan 0001 00:00:00 +0000

通过密度泛函理论（DFT）和随机相位近似（RPA）计算，该研究系统分析了高压下La₅Ni₃O₁₁的电子性质与超导机制。DFT能带结构显示，这种交替堆叠双层和单层NiO₂面的材料存在两组近乎解耦的子能带，分别来自双层子系统和单层子系统。RPA分析表明，超导配对主要发生在双层子系统内，并呈现与加压La₃Ni₂O₇类似的s±波配对对称性；而单层子系统主要通过极弱的层间约瑟夫森耦合（IJC）作为桥梁，连接相邻双层以实现沿c轴的相位相干。在低压下，压力增强使IJC显著提升，从而抬升体态超导转变温度（Tc）；当压力足够高时，γ-pocket的态密度降低导致Tc逐渐下降。这一机制自然解释了实验中观测到的穹顶状Tc-压力依赖关系，并揭示了混合相与纯相镍酸盐超导体不同的压力响应。

Pairing mechanism and superconductivity in pressurized La₅Ni₃O₁₁

Mon, 01 Jan 0001 00:00:00 +0000

The discovery of superconductivity (SC) with critical temperature Tc above the boiling point of liquid nitrogen in pressurized La3Ni2O7 has sparked a surge of exploration of high-Tc superconductors in the Ruddlesden-Popper (RP) phase nickelates. More recently, the RP phase nickelate La5Ni3O11, which hosts a layered structure with alternating bilayer and single-layer NiO2 planes, has been reported to accommodate SC under pressure, exhibiting a dome-shaped pressure dependence with the highest Tc ≈ 64 K, capturing a lot of interest. Here, using density functional theory (DFT) and random phase approximation (RPA) calculations, we systematically study the electronic properties and superconducting mechanism of this material. Our DFT calculations yield a band structure including two nearly decoupled sets of sub-band structures, with one set originating from the bilayer subsystem and the other from the single-layer one. RPA-based analysis demonstrates that SC in this material occurs primarily within the bilayer subsystem exhibiting an s± wave pairing symmetry similar to that observed in pressurized La3Ni2O7, while the single-layer subsystem mainly serves as a bridge facilitating the inter-bilayer phase coherence through the interlayer Josephson coupling (IJC). Since the IJC thus attained is extremely weak, it experiences a prominent enhancement under pressure, leading to the increase of the bulk Tc with pressure initially. When the pressure is high enough, the Tc gradually decreases due to the reduced density of states on the γ-pocket. In this way, the dome-shaped pressure dependence of Tc observed experimentally is naturally understood.

Pairing Mechanism in Bilayer Nickelate La₃Ni₂O₇ Superconductors

Mon, 01 Jan 0001 00:00:00 +0000

双层镍酸盐La₃Ni₂O₇高温超导体的配对机制研究表明，基于“基因原理”和“协同费米面规则”的统一框架可自然延伸至该双层多轨道体系。通过强关联分析，识别出两个主导反铁磁超交换通道：由内顶角氧介导的层内同轨道（d_z²）最近邻交换J⊥，以及由面内氧介导的层间不同轨道（d_z²与d_x²-y²）最近邻交换J_xz。由于双层成键-反键劈裂和d_x²-y²轨道的B₁g对称性，两通道协同作用产生稳定的s±超导态，其特征是动量空间中镜像偶与镜像奇费米面口袋之间存在内部符号反转。两个配对通道均在β口袋上最大化超导能隙，形式因子为(cosk_x-cosk_y)²。该结果将La₃Ni₂O₇纳入非常规超导的统一框架，同时揭示了其独特的高温超导配对电子环境。

Pairing properties of correlated three-leg ladders with strong interchain couplings near 1/3 filling

Mon, 01 Jan 0001 00:00:00 +0000

本文采用密度矩阵重正化群方法，研究了强链间耦合的三腿t-J梯子近1/3填充时的基态性质。在空穴掺杂进入1/3填充的自旋能隙态时，对关联函数呈幂律衰减，而自旋关联函数呈指数衰减；相反，电子掺杂并未显著增强对关联。进一步对比了三腿Hubbard模型，发现空穴掺杂态的对关联特性与t-J模型类似，但需足够大的自旋能隙。研究表明，空穴掺杂近1/3填充态有利于超导配对，这一非对称配对性质与弱耦合理论预测的相图不同，为理解三层镍酸盐超导体的电子特性提供了数值依据。

Pauli-limited upper critical field and anisotropic depairing effect of La₂.82Sr₀.18Ni₂O₇ superconducting thin film

Mon, 01 Jan 0001 00:00:00 +0000

本研究采用外延生长的La2.82Sr0.18Ni2O7薄膜（超导转变温度约31.6 K），通过高场输运测量（最高58 T）系统表征了上临界场及其各向异性。近转变温度附近，超导呈现厚度限制的二维特征；冷却后，面外相干长度减小至小于薄膜厚度（6 nm），表明体系向本征三维体超导转变。基于Ginzburg-Landau模型分析，测得零温下面内和面外上临界场分别为82 T和45 T，各向异性比γ≈1.34，与块材Ruddlesden-Popper镍酸盐相当。低温下面内方向上临界场因自旋顺磁对破裂效应被强烈压制，接近Pauli极限（58 T），而面外方向几乎不受影响。这种各向异性Pauli限制解释了上临界场各向异性降低的现象，并支持薄膜中超导本质上是三维体超导的结论。结果凸显了自旋顺磁效应在决定该类镍酸盐高场超导相图中的关键作用。

Perpendicular electric field induced s^±-wave to d-wave superconducting transition in thin film La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

受垂直电场可调控Ruddlesden–Popper双层镍酸盐La₃Ni₂O₇超导性质的启发，本研究采用动力学簇量子蒙特卡洛方法求解不平衡双轨道双层Hubbard模型。通过分析未掺杂、空穴掺杂及电子掺杂情形下电场诱导的配对对称性及其演化，发现源自d_{z²}轨道的s±波配对被抑制，同时层间d_{z²}轨道失配及电子向d_{x²-y²}轨道转移驱动了从s±波到d波的配对对称性转变。有趣的是，由d_{x²-y²}轨道产生的d波配对随电场强度呈现穹顶状行为。大规模多体计算与弱耦合方法的预期一致，为理解RP镍酸盐的超导机制提供了新见解。

Persistent structural distortions and absent superconductivity in trilayer nickelate thin films

Mon, 01 Jan 0001 00:00:00 +0000

该研究通过原子级精确合成、电输运测量、皮米级电子显微镜和同步辐射X射线衍射，系统探究了三层镍酸盐La₄Ni₃O₁₀薄膜的应变效应。尽管压缩外延应变能有效抑制母体密度波序并提升晶体对称性（如消除面外八面体旋转），但即使在最大压缩应变（-2.8%）下也未观测到超导电性。关键结构表征发现，压缩应变无法完全消除薄膜中特有的面内八面体旋转，这种旋转在每一三层单元的内层与外层中呈现层间不等效性，且持续存在。同步辐射X射线衍射显示面内旋转幅度随压缩应变单调减小，但未完全消失。相比之下，双层体系La₃Ni₂O₇中压缩应变能完全抑制所有八面体旋转从而诱导超导。该结果揭示了三层与双层体系的关键差异，表明单纯依靠外延应变工程无法实现三层镍酸盐的环境压力超导，需探索其他调控手段。

Polar, checkerboard charge order in bilayer nickelate La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

本研究利用高亮度同步辐射X射线衍射对双层镍酸盐La₃Ni₂O₇的高质量单晶进行精确结构分析，通过大动态范围探测器成功解析了之前被忽略的弱衍射信号，这些信号的强度比主Bragg反射弱近四个数量级。这些观测证据表明，晶体中存在滑移镜面对称性破缺，导致其呈现极性结构（空间群Ima2），而非先前普遍采用的中心对称模型（Fmmm）。进一步的结构精修揭示了两个不等价镍位点，其Ni–O键长差异显著，结合键价和计算，表明镍位点电荷呈现棋盘状有序分布，并与氧八面体倾斜协同作用赋予晶体极性。该电荷有序相的结构与双层锰氧化物中的极化态类似。本研究确立了常压下La₃Ni₂O₇的极化电荷有序态，指出其与压力诱导的超导电性存在竞争关系，为理解双层镍酸盐中相竞争机制和压力超导起源提供了关键结构基础。

Polymorphism in the Ruddlesden–Popper Nickelate La₃Ni₂O₇: Discovery of a Hidden Phase with Distinctive Layer Stacking

Mon, 01 Jan 0001 00:00:00 +0000

We report the discovery of a novel form of Ruddlesden–Popper (RP) nickelate that stands as the first example of long-range, coherent polymorphism in this class of inorganic solids. Rather than the well-known, uniform stacking of perovskite blocks ubiquitously found in RP phases, this newly discovered polymorph of the bilayer RP phase La3Ni2O7 adopts a novel stacking sequence in which single-layer and trilayer blocks of NiO6 octahedra alternate in a “1313” sequence. Crystals of this new polymorph are described in space group Cmmm, although we note evidence for a competing Imam variant. Transport measurements at ambient pressure reveal metallic character with evidence of a charge density wave transition with an onset at T ≈ 134 K. The discovery of such polymorphism could reverberate to the expansive range of science and applications that rely on RP materials, particularly the recently reported signatures of superconductivity in bilayer La3Ni2O7 with Tc as high as 80 K above 14 GPa.

Possible Enhancement of Superconductivity in Ambient-Pressure La₃Ni₂O₇ Thin Film

Mon, 01 Jan 0001 00:00:00 +0000

该研究采用涨落交换（FLEX）近似方法，系统分析了常压下La₃Ni₂O₇薄膜超导体在弱关联体系中的两站点两轨道模型，重点研究了空穴掺杂对超导特性的影响。通过对费米面拓扑结构的详细分析，发现当由d_{z²}反键轨道构成的δ口袋在Γ点附近形成时，δ口袋与γ口袋的嵌套以及α与β口袋之间的嵌套，在相应波矢处共同增强了s±波配对。研究进一步提出，这种由费米面嵌套驱动的自旋涨落诱导配对的增强机制，可能为提升超导转变温度提供可行途径。该工作为理解常压镍基薄膜超导体的配对机制及探索更高性能超导材料提供了理论指导。

Possible Liquid-Nitrogen-Temperature Superconductivity Driven by Perpendicular Electric Field in the Single-Bilayer Film of La₃Ni₂O₇ at Ambient Pressure

Mon, 01 Jan 0001 00:00:00 +0000

针对La₃Ni₂O₇单双层薄膜在常压下的超导转变温度（Tc）亟待提升的需求，本研究提出通过施加垂直电场驱动电荷转移来增强其超导电性。垂直电场使电子从高电势层流向低电势层，由于Ni的3d_{z²}轨道已接近半满无法容纳更多电子，流入电子主要填充低电势层的3d_{x²-y²}轨道，从而增加该层的填充率。利用简化的单轨道模型和全面的双轨道模型，结合奴隶玻色子平均场理论与密度矩阵重正化群方法，数值计算发现：填充率的增加会抑制层间s波配对，但强烈增强层内d波配对，使得底层主导的d波超导迅速上升。当层间电压达到约0.1~0.2伏时，Tc可超越液氮温度（约77 K），在常压下实现液氮温区的高温超导。该方案无需高压且避免化学掺杂无序，为在La₃Ni₂O₇超薄膜中实现高Tc超导提供了可行途径，需要实验进一步验证。

Prediction of several Co-based La₃Ni₂O₇-like superconducting materials

Mon, 01 Jan 0001 00:00:00 +0000

通过电子掺杂高压双层钴酸盐La₃Co₂O₇，预测了LaTh₂Co₂O₇、La₃Ni₂O₅Cl₂和La₃Ni₂O₅Br₂等钴基La₃Ni₂O₇类似物，这些材料具有相似的晶体结构和强关联电子态。基于密度泛函理论加动力学平均场理论（DFT+DMFT）和随机相位近似（RPA）的计算表明，钴的3d轨道呈现半满或接近半满的占据，与La₃Ni₂O₇中镍的轨道类似，并表现出强Hund耦合和质量增强。这些钴基化合物的局域磁矩（约0.64 μ_B）恰好落在镍酸盐超导的最佳窗口内（0.63–0.68 μ_B），强烈暗示其可能通过类似的自旋涨落机制实现高温超导。RPA计算进一步揭示，在电子掺杂的钴酸盐中，领先的配对对称性为s波，属于A₁ᵍ不可约表示。该工作为在钴基体系中实现高温超导电性提供了理论依据，并鼓励进一步的实验合成和表征。

Preparation and optimization of high-temperature superconducting Ruddlesden-Popper nickelate thin films

Mon, 01 Jan 0001 00:00:00 +0000

本研究采用巨大氧化原子层外延（GAE）方法在LaAlO₃和SrLaAlO₄衬底上成功制备了相纯、高质量的Ruddlesden-Popper镍酸盐Ln₃Ni₂O₇薄膜。在超强氧化性臭氧气氛下生长的薄膜无需后续退火即可超导，其中优化的Ln₃Ni₂O₇/SrLaAlO₄薄膜起始转变温度（Tc,onset）可达50 K。系统研究发现四个关键因素决定薄膜结晶质量和超导性能：精确控制阳离子化学计量可抑制二次相生成；完整的原子层逐层覆盖结合优化的界面重构能减少堆垛层错；准确调控氧含量是实现单一超导转变和高Tc,onset的必要条件。研究还发现，阳离子化学计量偏离会导致Ni-rich或Ni-deficient二次相形成，分别引发金属-绝缘体转变或高度绝缘行为，而原子层覆盖率偏差（如101.5%）虽仍可维持超导但产生残余电阻。界面重构（如预沉积半晶胞Ln₂NiO₄或对SrLaAlO₄衬底进行退火处理）可显著改善薄膜结晶性。这些结果为高质量氧化物高温超导薄膜的逐层外延生长提供了重要指导。

Pressure and strain tuning of the alternating bilayer-trilayer Ruddlesden-Popper nickelate: crystal and electronic structure

Mon, 01 Jan 0001 00:00:00 +0000

通过第一性原理计算，本工作研究了混合双层-三层Ruddlesden-Popper镍酸盐La₇Ni₅O₁₇在静水压和双轴压缩应变下的晶体与电子结构。通过分析高对称P4/mmm结构中动力学不稳定声子模的不可约表示，作者确定了具有八面体倾斜的动力学稳定低对称C2/c结构。施加压力和压缩应变均会抑制八面体倾斜，使结构四方化，这一行为与传统Ruddlesden-Popper相类似。在电子结构方面，静水压和应变下的总体特征相似，但区别在于三层块中d_z²成键带的位置：在30 GPa压力下该带穿过费米能级，而任何强度的压缩应变均使其维持在费米能级以下。这种应变诱导的电子结构变化与传统双层镍酸盐中观察到的行为一致，为理解这类材料中压力与应变对超导电性的不同影响提供了关键线索。

Pressure induced redistribution of oxygen hole states in La₄Ni₃O₁₀

Mon, 01 Jan 0001 00:00:00 +0000

本文采用密度泛函计算与包含局域交换和库仑相互作用的多轨道、多原子团簇精确对角化方法，通过最小Ni₃O₁₄团簇研究了三层镍氧化物La₄Ni₃O₁₀的局域低能电子态。研究发现，在常压下，三个Ni名义上均为+2价，其中两个额外空穴中的一个局域于中心NiO₂层，与d_{x²-y²}轨道形成张-赖斯单态；另一个空穴主要占据两层间O p_z轨道的反键组合，并与三个NiO₂层的d_{z²}轨道形成的面外三自旋极化子杂化。由此，面内自旋取向交替由外层d_{x²-y²}轨道承载，层间呈反铁磁关联，同时中心层绝缘且磁矩可忽略。在高压下，两个额外空穴分别集中于一个外层和内层，各自形成d_{x²-y²}轨道的张-赖斯单态或面内三自旋极化子。团簇结果提示的可能电荷与自旋有序态突出了双层La₃Ni₂O₇与三层La₄Ni₃O₁₀之间的相似性。

Pressure-Driven Structural Transitions without a Displacive Charge-Density Wave in La₂SmNi₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

本研究利用同步辐射X射线衍射，在低温和高压下系统探究了双层镍酸盐La₂SmNi₂O₇的结构演变。常压条件下，单晶衍射揭示出一种新的单斜超结构（空间群P2₁/a），其晶胞沿c轴加倍主要源于氧原子的反铁畸变位移，且未检测到任何与电荷密度波相关的卫星反射，表明即使存在位移型电荷有序，其振幅也小于千分之几埃。施加压力后，室温粉末及单晶衍射观测到系列结构转变：约15 GPa时由单斜转变为正交对称性，约21 GPa进一步转变为四方对称性，并且中间的正交相在一定压力区间内稳定存在。在超导出现的压力—温度区域内，高质量的单晶数据实现了结构精修，提供了精确的晶格参数及键角信息，为理解超导电性的涌现奠定了结构基础。结果表明，La₂SmNi₂O₇的压力驱动结构转变无需位移型电荷密度波的参与，连续的对称性改变为理论模型深入研究电子、晶格与磁性的相互作用提供了关键约束。

Pressure-enhanced spin-density-wave transition in double-layer nickelate La₃Ni₂O₇−δ

Mon, 01 Jan 0001 00:00:00 +0000

Recently, a signature of high-temperature superconductivity above the liquid nitrogen temperature (77 K) was reported for La3Ni2O7−δ under pressure. This finding immediately stimulated intense interest in the possible mechanism of high-Tc superconductivity in double-layer nickelates. Notably, the pressure-dependent phase diagram inferred from transport measurements indicates that the superconductivity under high pressure emerges from the suppression of density-wave-like order at ambient pressure, which is similar to high-temperature superconductors. Therefore, clarifying the exact nature of the density-wave-like transition is important for determining the superconducting mechanism in double-layer nickelates. Here, nuclear magnetic resonance (NMR) spectroscopy of 139La nuclei was performed to study the density-wave-like transition in a single crystal of La3Ni2O7−δ. At high temperatures, two sets of sharp 139La NMR peaks are clearly distinguishable from a broad background signals, which are ascribed to La(1) sites from two bilayer Ruddlesden-Popper phases with different oxygen vacancy δ. As the temperature decreases, the temperature-dependent 139La NMR spectra and nuclear spin-lattice relaxation rate (1/T1) for both La(1) sites provide evidence of spin-density-wave (SDW) ordering below the transition temperature (TSDW), which is approximately 150 K. The anisotropic splitting in the NMR spectra suggests the formation of a possible double spin stripe with magnetic moments aligned along the c-axis. Furthermore, we studied the pressure-dependent SDW transition up to ∼ 2.7 GPa. Surprisingly, the TSDW inferred from NMR measurements of both La(1) sites increases with increasing pressure, which is opposite to the results from previous transport measurements under pressure and suggests an intriguing phase diagram between superconductivity and SDW. In contrast, the present 139La NMR is insensitive to the possible charge-density-wave (CDW) order in the Ni-O planes. All these results will be helpful for building a connection between superconductivity and magnetic interactions in double-layer nickelates.

Pressure-induced superconductivity in epitaxially-stabilized Pr₃Ni₂O₇ films

Mon, 01 Jan 0001 00:00:00 +0000

本研究通过外延稳定技术在LaAlO3衬底上成功制备了Pr3Ni2O7薄膜，突破了该化合物在块材中难以合成的热力学稳定性限制。常压下，无论是否经过臭氧退火处理，Pr3Ni2O7薄膜均表现出绝缘行为；但在22 GPa的高压下，薄膜呈现出T线性的金属输运特性，并出现超导电性，起始超导转变温度（Tc onset）达66 K，零电阻温度约40 K。进一步研究发现，采用更小稀土离子Nd的Nd3Ni2O7薄膜虽可被外延稳定，但在所测压力范围内未观察到超导。通过对La、Pr、Nd三种Ln3Ni2O7薄膜的比较，发现超导所需的临界压力Pc随Ln离子半径减小而增大，这一趋势与块材中Ln替代的研究结果一致。该工作表明，外延稳定技术是拓展双层镍酸盐超导家族的有效手段，为探索新型超导材料提供了重要途径。

Pressure-Invariant Isotope Effect as Evidence for Electronically Driven Intertwined Order in Pr₄Ni₃O₁₀

Mon, 01 Jan 0001 00:00:00 +0000

本研究利用μ子自旋旋转技术测量了三层Ruddlesden-Popper型镍氧化物Pr4Ni3O10中氧同位素（16O/18O）替代对自旋密度波（SDW）转变的影响。在常压下，16O和18O样品的SDW转变温度分别为158.04 K和159.81 K，呈现有限同位素位移。在液压压力下，两种同位素的转变温度均以几乎相同的速率（约-4.9 K/GPa）线性下降，使得同位素位移随压力基本不变。这一压力无关的同位素效应表明SDW转变主要源于电子关联，而非晶格动力学。结合近期非弹性X射线散射结果未发现声子软化现象，该研究支持三层Ruddlesden-Popper镍氧化物中存在由强自旋相互作用稳定的交织电荷密度波与自旋密度波序的新机制。该发现与铜氧化物中掺杂增强同位素效应的行为形成对比，为理解镍氧化物中密度波序的电子起源及其与超导电性的关系提供了重要约束。

Probing La-based nickelates with Ni 1s core-level photoelectron spectroscopy

Mon, 01 Jan 0001 00:00:00 +0000

本文通过Ni 2p与Ni 1s核心层光电子能谱对比研究了La₃Ni₂O₇、Nd₃Ni₂O₇和LaNiO₃的电子结构。由于La 3d与Ni 2p能级严重重叠且存在La高能伴峰，传统Ni 2p谱难以可靠提取La基镍酸盐的本征信号。利用硬X射线光电子能谱测量深的Ni 1s核心能级，其无自旋-轨道耦合且多极相互作用可忽略，提供了纯净的电荷转移激发视角。结果表明，Ni 1s谱能清晰区分钙钛矿LaNiO₃与双层Ruddlesden-Popper相的材料差异，并揭示La₃Ni₂O₇相比Nd₃Ni₂O₇主峰展宽、强度降低而伴峰增强。结合DFT+DMFT计算，这些谱学变化可归因于电荷转移能与杂化强度的改变，其中La₃Ni₂O₇因拉伸应变导致Ni与配体杂化减弱。该方法展示了Ni 1s核心层能谱对电子结构细微变化的敏感性，为系统表征不同应变、掺杂或层数的镍酸盐提供了有效途径。

Progress of ambient-pressure superconductivity in bilayer nickelate thin films

Mon, 01 Jan 0001 00:00:00 +0000

这篇综述总结了双层镍酸盐La₃Ni₂O₇薄膜在常压超导方面的最新进展。通过外延应变工程，利用SrLaAlO₄等衬底提供的压应变，成功在常压条件下稳定了超导相，这与块材高压超导的发现形成重要突破。实验表征方面，角分辨光电子能谱(ARPES)测量揭示了存在争议的费米面拓扑结构，不同研究组的观测结果有所差异，这可能源于薄膜生长条件的差异。在提高超导转变温度(Tc)方面，通过增大压应变和优化生长技术（如巨型氧化原子层外延），可使Tc达到约60K。理论研究聚焦于电子结构和配对对称性，弱耦合方法（如随机相位近似和泛函重整化群）预测了s±波或d波配对，而重整化平均场理论则指出节点d波配对的可能性。然而，费米面上电子袋在超导中的具体作用、晶格比与Tc的关系等关键问题尚未完全阐明。这些进展表明双层镍酸盐薄膜是一个高度可调且极具前景的高温超导研究平台。

Pseudogap and Non-Fermi-liquid criticality in double Kondo model for bilayer nickelates

Mon, 01 Jan 0001 00:00:00 +0000

该研究利用单站点动力学平均场理论（DMFT）对双层近藤晶格模型进行了系统研究，旨在探索双层镍酸盐正常态的相图。当层间隧穿不存在时，研究发现了由层间自旋耦合或空穴掺杂调谐的非费米液体临界点，该临界点将过掺杂区的标准费米液体与欠掺杂区的赝能隙金属分隔开来。这一赝能隙相被称为“第二费米液体”，其特征为具有小空穴口袋且违反微扰Luttinger定理，但不存在对称性破缺或分数化现象，其行为类似于重费米液体，准粒子残留小、有效质量大。研究还基于ancilla费米子框架提供了赝能隙和基态波函数的直观解析描述，将ancilla费米子解释为自旋极化子，并在DMFT计算中直接展示了该复合费米子的Kondo共振峰。将该分析扩展到有限层间隧穿情形后，研究应用于双层镍酸盐La₃Ni₂O₇，提出当前实验样品（x≈0.5）处于过掺杂费米液体区域，而通过电子掺杂可能进入赝能隙相和非费米液体临界区，这为理解该类材料中的异常金属行为提供了理论预测。

Raman response in superconducting multiorbital systems with application to nickelates

Mon, 01 Jan 0001 00:00:00 +0000

本研究利用电子拉曼散射方法，系统分析了超导多轨道系统的拉曼响应，并以镍酸盐为应用对象。针对三种模型——包含d_{x^2-y^2}和d_{z^2}轨道的单层与双层两轨道模型，以及仅含d_{x^2-y^2}轨道的双层单轨道模型——分别考虑了d波、s±波和s波等多种配对对称性，计算了不同拉曼对称性（A1g、B1g、B2g）下的响应特征。在两轨道模型中，采用了全多轨道方法，同时计入轨道内和轨道间散射，并与将各能带拉曼响应简单相加的加性近似进行了对比。研究发现，不同配对对称性和模型结构在拉曼谱中产生独特的指纹特征，全多轨道计算揭示了加性近似可能忽略的轨道间混合效应。这些结果有助于厘清镍酸盐超导的最小模型，判断其超导能隙的大小与对称性，并为其他多轨道超导体（如铁基超导体）的拉曼实验分析提供了通用的理论框架。

Recent progress in nickelate superconductors

Mon, 01 Jan 0001 00:00:00 +0000

This review summarizes recent advances in nickelate superconductors, covering infinite-layer, bilayer, and trilayer systems, their superconducting properti

Regulating oxygen content and superconductivity in La₃Ni₂O₇+δ

Mon, 01 Jan 0001 00:00:00 +0000

本研究通过系统调控La₃Ni₂O₇₊δ系列样品的氧含量，合成了不同相组成的材料，包括纯双层相、双层与单层-双层杂化相混合相以及含三层交生的主双层相。高压输运测量表明，这些相分别对应不同的超导转变温度（T_c），其中双层相呈现约80 K的超导电性，杂化相和三层交生相的T_c较低。氧含量不仅影响相纯度，还直接调控双层超导电性的上临界场（H_c2），纯双层相具有更高的H_c2。通过建立T_c和H_c2随氧含量变化的相图，本研究实现了对Ruddlesden-Popper镍酸盐氧含量的精确控制，为理解其高压超导机制提供了关键实验依据。

Reply to "Threefold error in the reported zero-field cooled magnetic moment of single crystal La₂SmNi₂O₇ (arXiv: 2602.23240)"

Mon, 01 Jan 0001 00:00:00 +0000

针对Korolev和Talantsev对Li等人《Nature》论文中超导相分数计算的批评，本文作者逐一回应：首先，经实验确认低温尾巴的弱上升源于背景，且未观察到顺磁迈斯纳效应，因此场冷数据可用于计算超导相分数；其次，退磁效应必须基于实际测量磁矩随超导相分数f的变化，而Korolev等错误地将退磁场视为常数，导致其公式低估f约三分之二（因子接近1/3），解释了其计算结果仅为报道值（约62.1%）约三分之一的原因；最后，样品的多种表征（能谱、X射线衍射、核四极共振、扫描透射电镜等）证实为均匀高质量块体单晶，不存在多个离散超导区域。因此，Li等人《Nature》论文中超导相分数的计算方法并未被Korolev等的分析推翻。

Resolving the electronic ground state of La₃Ni₂O₇-δ films

Mon, 01 Jan 0001 00:00:00 +0000

The recent discovery of a superconductivity signature in La3Ni2O7-δ under a pressure of 14 GPa, with a superconducting transition temperature of around 80 K, has attracted considerable attention. An important aspect of investigating electronic structures is discerning the extent to which the electronic ground state of La3Ni2O7-δ resembles the parent state of the cuprate superconductor, a charge transfer insulator with long-range antiferromagnetism. Through X-ray absorption spectroscopy, we reveal the influence of oxygen ligands on the electronic ground states of the Ni ions, displaying a charge transfer nature akin to cuprate but with distinct orbital configurations. Additionally, in La3Ni2O7-δ films, we detect a superlattice reflection (1/4, 1/4, L) at the Ni L absorption edge using resonant X-ray scattering measurements. Further examination of the resonance profile indicates that the reflection originates from the Ni d orbitals. By evaluating the reflection’s azimuthal angle dependence, we confirm the presence of collinear antiferromagnetic spin ordering and charge-like anisotropy ordered with the same periodicity. Our findings reveal a microscopic relationship between these two components in the temperature dependence of the scattering intensity of the reflection. This investigation enriches our understanding of high-temperature superconductivity in La3Ni2O7-δ under high pressure.

Role of interstitial s orbital in a model of infinite-layer nickelates

Mon, 01 Jan 0001 00:00:00 +0000

本研究采用行列式量子蒙特卡罗方法，在三轨道Emery模型基础上加入具有三维色散的间隙$s$轨道，模拟无限层镍酸盐的低能电子结构。大规模计算发现：强关联效应显著缩小了间隙$s$轨道产生的电子口袋，但在20%空穴掺杂下口袋仍存续，大小与ARPES实验观测相当；$d_{x^2-y^2}$轨道色散受到强烈重正化，沿$k_z$方向的弱色散与实验一致。此外，与常规三轨道模型相比，引入$s$轨道后系统的短程反铁磁关联明显增强。这些结果揭示了强关联和多轨道效应在决定无限层镍酸盐低能电子态和自旋关联中的关键作用，表明必须在实际多轨道框架中处理相互作用驱动的多体物理。

Shear-stress-constrained superconductivity in Ruddlesden-Popper nickelates

Mon, 01 Jan 0001 00:00:00 +0000

Ruddlesden-Popper镍氧化物在块材高压和薄膜外延约束条件下均表现出超导电性，但高度依赖于样品质量、氧含量、缺陷和应力状态。本文提出，亚稳态RP晶格仅在Ni-O框架的局部约束变形处于一个有界的剪切应变窗口内时才进入超导态；该变形控制着八面体旋转、层间Ni-O-Ni键角以及Ni dz²与dx²-y²轨道间的耦合。这一剪切应力约束超导电性（SSCS）框架统一解释了此前观察到的压力阈值、可逆性、空间不均匀性、压力介质依赖性、薄膜-衬底敏感性以及可重复性难题。SSCS场景并不替代键角、键长、轨道占据、氧化学计量或载流子密度等传统因素的作用，而是指明这些因素协同稳定超导态所需的力学与对称性条件。镍氧化物超导体表现出的脆性与异质性并非外在复杂性，而是超导态本身的核心诊断特征。该视角为改善可重复性提供了具体实验路径，并将压缩块材、外延薄膜、化学替代样品及杂化RP结构中的物理机制统一在同一概念框架内。

Signature of Superconductivity in Pressurized La₄Ni₃O₁₀

Mon, 01 Jan 0001 00:00:00 +0000

The discovery of high-temperature superconductivity near 80 K in bilayer nickelate La3Ni2O7 under high pressures has renewed the exploration of superconducting nickelate in bulk materials. The extension of superconductivity in other nickelates in a broader family is also essential. Here, we report the experimental observation of superconducting signature in trilayer nickelate La4Ni3O10 under high pressures. By using a modified sol-gel method and post-annealing treatment under high oxygen pressure, we successfully obtained polycrystalline La4Ni3O10 samples with different transport behaviors at ambient pressure. Then we performed high-pressure electrical resistance measurements on these samples in a diamond-anvil-cell apparatus. Surprisingly, the signature of possible superconducting transition with a maximum transition temperature (T c) of about 20 K under high pressures is observed, as evidenced by a clear drop of resistance and the suppression of resistance drops under magnetic fields. Although the resistance drop is sample-dependent and relatively small, it appears in all of our measured samples. We argue that the observed superconducting signal is most likely to originate from the main phase of La4Ni3O10. Our findings will motivate the exploration of superconductivity in a broader family of nickelates and shed light on the understanding of the underlying mechanisms of high-T c superconductivity in nickelates.

Signatures of ambient pressure superconductivity in thin film La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

Recently, the bilayer nickelate La3Ni2O7 has been discovered as a new superconductor with transition temperature Tc near 80 K under high pressure1–3. Despite extensive theoretical and experimental work to understand the nature of its superconductivity4–29, the requirement of extreme pressure restricts the use of many experimental probes and limits its application potential. Here we present signatures of superconductivity in La3Ni2O7 thin films at ambient pressure, facilitated by the application of epitaxial compressive strain. The onset Tc varies roughly from 26 to 42 K, with higher Tc values correlating with smaller in-plane lattice constants. We observed the co-existence of other Ruddlesden–Popper phases within the films and dependence of transport behaviour with ozone annealing, suggesting that the observed low zero resistance Tc of around 2 K can be attributed to stacking defects, grain boundaries and oxygen stoichiometry. This finding initiates numerous opportunities to stabilize and study superconductivity in bilayer nickelates at ambient pressure, and to facilitate the broad understanding of the ever-growing number of high temperature and unconventional superconductors in the transition metal oxides.

Signatures of superconductivity near 80 K in a nickelate under high pressure

Mon, 01 Jan 0001 00:00:00 +0000

Signatures of superconductivity near 80 K in a nickelate under high pressure

Soft point-contact Andreev reflection spectroscopy in a palm-type cubic anvil-pressure cell

Mon, 01 Jan 0001 00:00:00 +0000

研究人员将软点接触安德列夫反射谱技术集成到棕榈型立方压砧高压腔中，通过基底锚定与外部导线分线策略，在高达15 GPa的静水压下稳定构建多个点接触结。对单质超导体Nb的基准测量验证了方法的可靠性，得到零温超导能隙比2Δ(0)/k_B T_c≈3.3。进一步应用于kagome金属超导体CsCr3Sb5和双层镍酸盐超导体La2PrNi2O7，观测到与常规BCS超导体截然不同的尖锐零偏压电导峰，并系统研究了其随温度、磁场和压力的演化。分析表明，这些谱学特征与非常规超导电性及可能的d波配对对称性相符，为理解其配对机制提供了直接谱学证据。该工作成功搭建了连接宏观电输运和微观光谱探针的高压实验平台，为广泛探索压力诱导非常规超导体的配对对称性开辟了新途径。

Spin and orbital excitations in undoped infinite layers: a comparison between superconducting PrNiO₂ and insulating CaCuO₂

Mon, 01 Jan 0001 00:00:00 +0000

该研究通过动量分辨与偏振分辨共振非弹性X射线散射（RIXS）测量，系统比较了未掺杂超导无限层镍酸盐PrNiO₂与绝缘铜酸盐CaCuO₂的自旋和轨道激发特性。结果显示，PrNiO₂的面内磁交换积分（约46 meV）明显小于CaCuO₂（约82 meV），而面外交换积分相近（约6-7 meV），表明两种材料均支持三维反铁磁有序，且自旋-自旋关联的三维性类似。轨道激发（3d态内跃迁）在单离子模型下吻合良好，但Ni-dxy峰的能量显著低于Cu-dxy，且色散方向相反——镍酸盐中呈现最近邻轨道超交换耦合驱动的轨道激发传播，而铜酸盐则为次近邻耦合主导。尽管两者电荷转移能差异显著（镍酸盐更大），但自旋与轨道激发特性总体上高度相似，仅在Ni-dxy峰能量和色散上表现出关键区别，这归因于轨道超交换耦合机制的不同。该工作揭示了无限层镍酸盐与铜酸盐在磁性和轨道动力学上的核心共性，同时指出镍酸盐中自旋涨落能量更小、掺杂电荷在金属位点上的局域化更强。

Spin correlations in La₃Ni₂O₇ thin films

Mon, 01 Jan 0001 00:00:00 +0000

该工作利用共振非弹性X射线散射（RIXS）系统研究了外延应变从约-2%至+1.9%的La₃Ni₂O₇（LNO）薄膜的电子和自旋激发。在呈现常压超导（起始转变温度>40 K）的压缩应变LNO/SrLaAlO₄薄膜中，观察到与块材LNO相似的dd激发和自旋动力学，但自旋激发带宽增大约10 meV，表明层间反铁磁交换作用Jz增强；而拉伸应变的LNO/SrTiO₃薄膜则表现出自旋激发和Ni 3dz²相关dd激发的显著抑制。这一演化反映了应变对Ni 3dz²-O 2pz杂化和层间距离的调控，进而影响层间磁耦合强度。结果表明，外延应变能有效调制双层镍酸盐的层间反铁磁超交换作用，且增强的Jz与常压超导的出现密切相关，支持层间磁交换作用促进层间配对的理论图像。

Spin Fluctuations in the Rare-Earth Doped Bilayer Nickelates

Mon, 01 Jan 0001 00:00:00 +0000

通过非弹性中子散射研究了稀土Pr和Nd掺杂的双层镍酸盐La₂LnNi₂O₇₋δ（Ln=La, Pr, Nd）粉末在常压下的自旋涨落。在未掺杂的La₃Ni₂O₇₋δ中观测到的45 meV平坦自旋涨落模式，在掺杂后分裂为43和48 meV两个模式，并在约60 meV处出现另一弱模式。其中La₂NdNi₂O₇₋δ的自旋涨落强度显著高于La₃Ni₂O₇₋δ和La₂PrNi₂O₇₋δ。这些结果与基于条纹型反铁磁海森堡模型的描述一致，表明稀土掺杂增强了层间磁耦合，层间交换耦合SJ⊥从约60 meV提升至69–73 meV，层内耦合保持较弱（≤3.5 meV）。这种增强可能解释了稀土掺杂后超导转变温度从80 K升至接近100 K的现象。该工作揭示了稀土掺杂对双层镍酸盐自旋动力学和超导配对的调控作用。

Spin-charge-orbital order in nickelate superconductors

Mon, 01 Jan 0001 00:00:00 +0000

Spin-charge-orbital order in nickelate superconductors

Spin-density wave and superconductivity in La₄Ni₃O₁₀ under ambient pressure

Mon, 01 Jan 0001 00:00:00 +0000

High-pressure studies have revealed superconductivity in La4⁢Ni3⁢O10, sparking interest in its ambient-pressure properties and the underlying electronic correlations. Motivated by experimental observations of an incommensurate spin-density wave (SDW) at ambient pressure, we investigate the SDW characteristics and possible superconductivity in La4⁢Ni3⁢O10 using a multiorbital random-phase approximation (RPA). Starting with a 12-orbital tight-binding model derived from density functional theory (DFT) calculations, we include Hubbard interactions to explore the interplay between electronic correlations and magnetic instabilities. Our analysis reveals a stripe-like SDW with a wave vector 𝐐≈(±0.7⁢𝜋,0), suggesting a possible density wave instability in agreement with experiments. This configuration is driven by nesting between the 𝛼1 pocket, primarily contributed by the outer-layer Ni 𝑑𝑧2 orbitals, and the 𝛽1 pocket, contributed by both the 𝑑𝑧2 and 𝑑𝑥2−𝑦2 orbitals of the outer layer. It exhibits interlayer antiferromagnetic ordering between the top and bottom NiO layers, with the magnetic moment of the middle layer being nearly zero. We demonstrate that the Hund coupling 𝐽𝐻 is the primary driver of the observed SDW and determine the specific criterion: 𝐽𝐻>0.16⁢𝑈. Building upon our findings on the SDW mechanism, we further demonstrate that hole doping (𝛿=−0.4) enhances Fermi surface nesting, leading to the emergence of a superconducting state with a gap structure similar to that of the high-pressure phase.

Spin-density-wave transition in monolayer-trilayer La₃Ni₂O₇ single crystals

Mon, 01 Jan 0001 00:00:00 +0000

本研究成功合成了高质量的长程有序杂化1313型La₃Ni₂O₇单晶（交替单层-三层结构），并对其物理性质进行了系统表征。在常压下，该材料呈现典型的半导体行为，并在170 K处出现明显的电阻率、磁化率和比热异常。¹³⁹La核磁共振谱明确证实该异常源于自旋密度波（SDW）相变。高压电输运测量表明，施加压力可以诱导金属化，但在高达65 GPa的压力下未观察到超导电性。这些发现将杂化1313型La₃Ni₂O₇确立为Ruddlesden-Popper镍酸盐家族中具有独特SDW转变的新成员，为研究杂化镍酸盐中晶体结构、电子序和超导电性之间的相互作用提供了新的平台。

Squeezing dynamical singlets in bilayer nickelates

Mon, 01 Jan 0001 00:00:00 +0000

采用密度泛函理论结合团簇动力学平均场方法，本文研究了双层Ruddlesden-Popper镍酸盐，发现其物理性质主要受由3z²−r²轨道单电子形成的层间“动力学单态”支配，这些单态与巡游的x²−y²平面轨道发生杂化。该杂化对静水压与面内压应变的响应截然不同：应变增强层间关联，导致轨道选择性单态配对莫特机制，而静水压主要提升面内巡游性。这一差异解释了块材与应变薄膜在角分辨光电子能谱和输运测量中的实验分歧。理论框架将此低能态视为动力学单态与巡游轨道的杂化体系，为后续理解超导电性提供了新思路。

Stabilizing and tuning superconductivity in La₃Ni₂O₇−δ films: Oxygen recycling protocol reveals hole-doping analogue

Mon, 01 Jan 0001 00:00:00 +0000

Stabilizing and tuning superconductivity in La₃Ni₂O₇−δ films: Oxygen recycling protocol reveals hole-doping analogue

Strong interlayer magnetic exchange coupling in La₃Ni₂O₇−δ revealed by inelastic neutron scattering

Mon, 01 Jan 0001 00:00:00 +0000

After several decades of studies of high-temperature superconductivity, there is no compelling theory for the mechanism yet; however, the spin fluctua…

Strong oxidizing annealing of bilayer La₃Ni₂O₇-δ results in suppression of superconductivity under high pressure

Mon, 01 Jan 0001 00:00:00 +0000

The discovery of superconductivity with an onset temperature of ∼80 K in pressurized bilayer Ruddlesden-Popper La3Ni2O7-δ has attracted much attention. Despite intense research, determination of the exact oxygen content and understanding of the relationship between superconductivity and oxygen content remain a big challenge. Here, we report a systematical study on the structure and physical properties of La3Ni2O7-δ polycrystalline powders which were prepared using the sol-gel method at ambient pressure and then annealed under high oxygen pressure (pO2) or in ozone. The superconducting transition of La3Ni2O7-δ at ∼80 K under high pressure is suppressed for high pO2 and ozone annealed samples. We attribute this to the combination of the following two reasons: (i) damage of the bilayer structure, as revealed by powder X-ray diffraction, scanning transmission electron microscopy and pair distribution function measurements, and (ii) hole overdoping due to the increasing of oxygen content. Our results reveal that the bilayer structure in La3Ni2O7-δ is fragile and post-annealing under mild oxidization is suitable for maintaining the integrity of the bilayer structure and increasing oxygen content.

Structural features and electronic properties of La₄Ni₃O₁₀ with oxygen vacancies

Mon, 01 Jan 0001 00:00:00 +0000

Ruddlesden-Popper (RP) nickelates are a promising class of high-temperature superconductors, with superconducting transition temperatures exceeding the boiling point of liquid nitrogen. However, oxygen nonstoichiometry remains a persistent challenge that commonly presents in all RP nickelates. Understanding the formation of oxygen vacancies, their ordering patterns, and their impact on superconductivity is crucial, especially in the newly discovered L⁢a4⁢N⁢i3⁢O10. In this study, the first-principles structural calculations reveal the formation of in-plane oxygen vacancy chains in L⁢a4⁢N⁢i3⁢O10, a key structural feature observed under both ambient and pressurized conditions. These vacancies induce significant lattice distortion and generate residual electrons that hybridize with the Ni 𝑑𝑧2 orbital, altering the sign of the hopping integral between the 𝑑𝑧2 orbitals. Furthermore, the vacancies alter the N⁢i2+/N⁢i3+ ratio, lower the 𝑑𝑧2 orbital energy, and decrease the 𝑑𝑧2 orbital density of states at the Fermi level. Interestingly, at higher vacancy concentrations, the vacancy chains tend to align diagonally along the out-of-plane direction. The phase diagram of L⁢a4⁢N⁢i3⁢O10 exhibits a narrow stability range at ambient pressure, which expands under applied pressure, aligning with the high-oxygen-pressure conditions required for its synthesis. Importantly, these vacancy chains broaden the optical conductivity peak, which could serve as a marker for their detection. Our findings offer valuable insights into the distribution of oxygen vacancies, their role in modifying the electronic structure, and their influence on optical conductivity in L⁢a4⁢N⁢i3⁢O10.

Structural modifications in strain-engineered bilayer nickelate thin films

Mon, 01 Jan 0001 00:00:00 +0000

The discovery of high-temperature superconductivity in bulk La3Ni2O7 under high hydrostatic pressure1−4 and biaxial compression in epitaxial thin films5−8 has ignited significant interest in understanding the interplay between atomic and electronic structure in these compounds. Subtle changes in the nickel-oxygen bonding environment are thought to be key drivers for stabilizing superconductivity, but specific details of which bonds and which modifications are most relevant remains so far unresolved. While direct, atomic-scale structural characterization under hydrostatic pressure is beyond current experimental capabilities, static stabilization of strained La3Ni2O7 films provides a platform well-suited to investigation with new picometer-resolution electron microscopy methods. Here, we use multislice electron ptychography (MEP)9,10 to directly measure the atomic-scale structural evolution of La3Ni2O7 thin films across a wide range of biaxial strains tuned via substrate choice. By resolving both the cation and oxygen sublattices, we study the strain-dependent evolution of atomic bonds, providing the opportunity to isolate and disentangle the effects of specific structural motifs for stabilizing superconductivity. We identify the lifting of crystalline symmetry through modification of the nickel-oxygen octahedral distortions under compressive strain as a key structural ingredient for superconductivity and identify in-plane lattice compression as a common attribute between bulk and thin film superconductivity. Building upon the detailed structures obtained by MEP, we introduce a theoretical framework to disentangle coupled structural distortions in corner-sharing octahedra11, which suggest that both known superconducting geometries of La3Ni2O7 (hydrostatic pressure and compressive strain) suppress local t2g orbital mixing in the low-energy Ni bands by raising the octahedral symmetry.

Structural stability, electronic structure, and magnetic properties of the single-layer trilayer La₃Ni₂O₇ polymorph

Mon, 01 Jan 0001 00:00:00 +0000

该研究通过第一性原理计算和群论分析，系统研究了交替单层-三层（1313）堆叠的La₃Ni₂O₇多晶型的结构稳定性、电子结构与磁性质。常压下，最高对称性Cmmm结构在布里渊区高对称点存在多个不稳定声子支，其对应的畸变可导致实验上报道的另一种空间群Imma，该结构显示出NiO₆八面体倾斜。磁性分析表明，常压下该材料的电子结构主要由三层块主导，单层块处于Mott绝缘态。压力下，四方P4/mmm结构趋于稳定，与实验一致。研究揭示了八面体倾斜并非超导的必要条件，并阐明了不同空间群之间的对称关联及压力驱动的结构相变机制。

Structural symmetry effects on the competition of density waves and superconductivity in bilayer nickelates

Mon, 01 Jan 0001 00:00:00 +0000

本文利用功能重正化群方法，研究了双层镍酸盐La₃Ni₂O₇在常压与高压晶体结构下自旋密度波序与超导的竞争。通过对比两种结构的弱耦合多轨道模型，发现随着Hund耦合增大，主导不稳定性从超导转变为具有特征波矢Q₁≈(π/2,π/2)的自旋密度波，与实验一致。令人意外的是，常压与高压结构的非相互作用磁化率和fRG主导不稳定性几乎相同，表明压力下超导的出现不能仅由低能电子结构变化解释。进一步分析表明，抑制正交畸变是关键因素：当体系趋近四方极限时，对称性相关的自旋密度波涨落近乎简并，从而阻碍长程磁有序并增强配对相互作用。这些结果揭示了晶格对称性是调控双层镍酸盐中竞争有序态的核心参量，并提示通过单轴应变降低正交畸变有望在常压下实现体超导。

Structural transition, electric transport, and electronic structures in the compressed trilayer nickelate La₄Ni₃O₁₀

Mon, 01 Jan 0001 00:00:00 +0000

Atomic structure and electronic band structure are fundamental properties for understanding the mechanism of superconductivity. Motivated by the discovery of pressure-induced high-temperature superconductivity at 80 K in the bilayer Rud-dlesden-Popper nickelate La3Ni2O7, the atomic structure and electronic band structure of the trilayer nickelate La4Ni3O10 under pressure up to 44.3 GPa are investigated. A structural transition from the monoclinic P21/a space group to the tetragonal I4/mmm around 12.6–13.4 GPa is identified, accompanied by a drop of resistance below 7 K. Density functional theory calculations suggest that the bonding state of Ni

$$3{d_{{z^2}}}$$

orbital rises and crosses the Fermi level at high pressures, which may give rise to possible superconductivity observed in resistance under pressure in La4Ni3O10. The trilayer nickelate La4Ni3O10 shows some similarities with the bilayer La3Ni2O7 and has unique properties, providing a new platform to investigate the underlying mechanism of superconductivity in nickelates.

Structure Responsible for the Superconducting State in La₃Ni₂O₇ at High-Pressure and Low-Temperature Conditions

Mon, 01 Jan 0001 00:00:00 +0000

Very recently, a new superconductor with Tc = 80 K has been reported in nickelate (La3Ni2O7) at around 15–40 GPa conditions (Nature, 621, 493, 2023), which is the second type of unconventional superconductor, besides cuprates, with Tc above liquid nitrogen temperature. However, the phase diagram plotted in this report was mostly based on the transport measurement under low-temperature and high-pressure conditions, and the assumed corresponding X-ray diffraction (XRD) results were carried out at room temperature. This encouraged us to carry out in situ high-pressure and low-temperature synchrotron XRD experiments to determine which phase is responsible for the high Tc state. In addition to the phase transition from the orthorhombic Amam structure to the orthorhombic Fmmm structure, a tetragonal phase with the space group of I4/mmm was discovered when the sample was compressed to around 19 GPa at 40 K where the superconductivity takes place in La3Ni2O7. The calculations based on this tetragonal structure reveal that the electronic states that approached the Fermi energy were mainly dominated by the eg orbitals (3dz2 and 3dx2–y2) of Ni atoms, which are located in the oxygen octahedral crystal field. The correlation between Tc and this structural evolution, especially Ni–O octahedra regularity and the in-plane Ni–O–Ni bonding angles, is analyzed. This work sheds new light to identify what is the most likely phase responsible for superconductivity in double-layered nickelate.

Studies on Successive Electronic State Changes in Systems with NiO₂ Planes–139La-NMR/NQR–

Mon, 01 Jan 0001 00:00:00 +0000

139 La-NMR/NQR measurements of La 3 Ni 2 O 7-δ (δ∼0.0 and δ∼0.08), and La 4 Ni 3 O 10 have been performed. 139 La-NMR and transport and magnetic studies have also been carried out for Tl(La 2 Sr 2 )Ni 2 O 9 . Anomalous temperature ( T ) dependence of the longitudinal relaxation rates 1/ T 1 has been found at temperatures T A ∼(140–150 K) for all the systems. In Tl(La 2 Sr 2 )Ni 2 O 9 , there exists a transition to a magnetically ordered state at T ∼20 K. The large broadening of the NMR spectra observed for Tl(La 2 Sr 2 )Ni 2 O 9 below 20 K indicates that the system is in the charge ordered state in the temperature region between 20 K and T A with localized magnetic moments at Ni sites. The NQR intensity of La 4 Ni 3 O 10 begins to decrease rapidly with decreasing T at T A ∼140 K and almost disappears at T ∼120 K (wipeout). By arguing results of the present experimental studies, we propose that in all the systems with NiO 2 planes studied here exhibit similar type transitions to the charge ordered states at temperatures T A , all of which are in the narrow T region around 140 K. It has also been found that all the systems exhibit resistivity anomalies in the T region of (450–550) K, which suggests that they have a tendency of similar type changes or transitions of their electronic states at the temperatures, too.

Superconducting dome and field-enhanced superconductivity of PLD synthesized Nd₁₋ₓEuₓNiO₂ thin films

Mon, 01 Jan 0001 00:00:00 +0000

研究人员采用脉冲激光沉积结合氢化钙拓扑还原法，成功制备了一系列无限层结构 Nd₁₋ₓEuₓNiO₂ 薄膜，掺杂范围扩展至 x=0–0.7。电输运测量揭示在 0.2≤x≤0.5 区间存在一个超导穹顶，其掺杂宽度大于分子束外延制备的样品，与化学溶液法相当。其中 x=0.3 的薄膜表现出最优超导转变温度约 31 K，显著高于其他真空外延技术所得数值，表明脉冲激光沉积是制备高质量、高转变温度该类镍氧化物超导薄膜的有效途径。磁输运实验在欠掺杂和过掺杂区域均观测到强健的磁场增强与重入超导行为，这归因于 Eu²⁺ 局域磁矩在外场下的极化产生内部交换场，部分补偿外场所致；仅靠 Jaccarino-Peter 效应不足以完全解释该现象，暗示存在其他机制。在刚高于起始超导转变温度的低温区，霍尔电阻呈现非线性特征，且未伴随明显的磁滞，这可能源于磁性杂质散射。这些结果突出了磁性稀土 Eu²⁺ 离子在赋予无限层镍氧化物奇异物理属性中的关键作用。

Superconducting Dome in La_3-xSrₓNi₂O_7-δ Thin Films

Mon, 01 Jan 0001 00:00:00 +0000

The ambient-pressure superconductivity in La3⁢Ni2⁢O7 thin films via compressive epitaxial strain provides a highly accessible platform for diverse characterization techniques, facilitating the studies of high-temperature superconductivity. Here, we systematically map the phase diagram and reveal the superconducting dome with an electron-hole crossover in compressively strained La3−𝑥⁢Sr𝑥⁢Ni2⁢O7−𝛿 thin films by simultaneously tuning Sr doping and oxygen content. The maximum transition temperature (𝑇𝑐) coincides with an anomalous sign change in the Hall coefficient (𝑅𝐻), reminiscent of electron-doped cuprates, which may signal a Fermi surface reconstruction. Beyond the superconducting dome, a ln⁡1/𝑇 insulating regime and a 𝑇-linear resistivity regime are also resolved, resembling behaviors observed in cuprates and infinite-layer nickelates. This work reveals a dome-shaped relationship between 𝑇𝑐 and 𝑅𝐻 and establishes a key framework for understanding unconventional superconductivity in nickelate systems.

Superconducting Lanthanum Nickel Oxides with Bilayered and Trilayered Crystal Structures

Mon, 01 Jan 0001 00:00:00 +0000

2023年，双层镍氧化物La₃Ni₂O₇在约14 GPa高压下被发现超导，临界温度近80 K，其结构与高温铜氧化物相似，随后又发现了三层La₄Ni₃O₁₀的超导性。这两种化合物属于Ruddlesden–Popper相，由NiO₂方晶格层与LaO岩盐层交替堆叠构成。当前研究主要沿三个方向展开：扩展化合物化学多样性、通过元素替代提升超导转变温度、以及阐明超导配对机制。然而，关键实验必须在高压下进行，这给机理研究带来困难，因此开发在更低压力甚至常压下表现超导的镍氧化物具有重要意义。本文综述了这些体系的现有知识，重点介绍已较为成熟的样品合成与表征方法，并简要概述其电子性质，旨在为后续材料探索和物理机制理解提供基础。

Superconducting phase diagram of multi-layer square-planar nickelates

Mon, 01 Jan 0001 00:00:00 +0000

本研究系统构建了多层方形平面镍酸盐Nd_{n+1}Ni_nO_{2n+2}（n=4–8）的超导相图，发现n=4至7的化合物呈现超导转变迹象，起始临界温度最高达12.9 K（n=6），n=8仅显示微弱超导关联。通过降低层数n，超导各向异性因钕位4f电子效应发生反转——电子结构趋近铜酸盐特性，且磁涨落在超导区及过掺杂非超导区持续存在。值得注意的是，该超导区与化学掺杂无限层镍酸盐重叠，揭示了不同结构实现在方形平面镍酸盐中的共性与差异。这一工作确立了通过原子精度层状设计合成新型镍基超导体的通用模板。

Superconductivity and magnetism in bilayer nickelates: itinerant perspective

Mon, 01 Jan 0001 00:00:00 +0000

该研究从巡游视角探讨了双层镍氧化物的超导与磁性。基于对压缩应变薄膜角分辨光电子能谱的紧束缚拟合，作者引入了标准在位排斥相互作用（包括轨道内U、轨道间U′、Hund耦合JH和配对跳跃JP），并通过随机相位近似（RPA）考虑粒子-空穴涨落对这些裸相互作用进行重整，从而得到有效配对相互作用。结果表明，在强Hund耦合区域，s波超导和(π/2, π/2)自旋密度波（SDW）有序是优势基态；而在弱Hund耦合下，d波配对和(π, π)自旋密度波成为主导基态。该结果与先前密度矩阵重正化群（DMRG）研究定性一致，强调了Hund耦合在决定体系超导配对对称性和磁性类型中的关键作用。

Superconductivity and normal-state transport in compressively strained La₂PrNi₂O₇ thin films

Mon, 01 Jan 0001 00:00:00 +0000

The discovery of superconductivity under high pressure in Ruddlesden–Popper phases of bulk nickelates has sparked great interest in stabilizing ambient-pressure superconductivity in the thin-film form using epitaxial strain. Recently, signs of superconductivity have been observed in compressively strained bilayer nickelate thin films with an onset temperature exceeding 40 K, although with broad, two-step-like transitions. Here we report the intrinsic superconductivity and normal-state transport properties in compressively strained La2PrNi2O7 thin films, achieved through a combination of isovalent Pr substitution, growth optimization and precision ozone annealing. The superconducting onset occurs above 48 K, with zero resistance reached above 30 K, and the critical current density at 1.4 K is 100-fold larger than previous reports. The normal-state resistivity exhibits quadratic temperature dependence indicative of Fermi liquid behaviour, and other phenomenological similarities to transport in overdoped cuprates suggest parallels in their emergent properties.

Superconductivity in an infinite-layer nickelate

Mon, 01 Jan 0001 00:00:00 +0000

The discovery of unconventional superconductivity in (La,Ba)2CuO4 (ref. 1) has motivated the study of compounds with similar crystal and electronic structure, with the aim of finding additional superconductors and understanding the origins of copper oxide superconductivity. Isostructural examples include bulk superconducting Sr2RuO4 (ref. 2) and surface-electron-doped Sr2IrO4, which exhibits spectroscopic signatures consistent with a superconducting gap3,4, although a zero-resistance state has not yet been observed. This approach has also led to the theoretical investigation of nickelates5,6, as well as thin-film heterostructures designed to host superconductivity. One such structure is the LaAlO3/LaNiO3 superlattice7–9, which has been recently proposed for the creation of an artificially layered nickelate heterostructure with a singly occupied

$${d}_{{x}^{2}-{y}^{2}}$$

band. The absence of superconductivity observed in previous related experiments has been attributed, at least in part, to incomplete polarization of the eg orbitals10. Here we report the observation of superconductivity in an infinite-layer nickelate that is isostructural to infinite-layer copper oxides11–13. Using soft-chemistry topotactic reduction14–20, NdNiO2 and Nd0.8Sr0.2NiO2 single-crystal thin films are synthesized by reducing the perovskite precursor phase. Whereas NdNiO2 exhibits a resistive upturn at low temperature, measurements of the resistivity, critical current density and magnetic-field response of Nd0.8Sr0.2NiO2 indicate a superconducting transition temperature of about 9 to 15 kelvin. Because this compound is a member of a series of reduced layered nickelate crystal structures21–23, these results suggest the possibility of a family of nickelate superconductors analogous to copper oxides24 and pnictides25.

Superconductivity in bilayer La₃Ni₂O₇: A review focusing on the strong-coupling Hund's rule assisted pairing mechanism

Mon, 01 Jan 0001 00:00:00 +0000

双层La₃Ni₂O₇中的高温超导源于其独特的双轨道双层电子结构，其中3d_z²轨道近半填充且局域化，通过内顶角氧2p_z轨道产生强层间反铁磁交换；而3d_x²-y²轨道约四分之一填充且高度巡游。在强耦合下，Hund规则耦合使同一镍位上两个轨道的自旋对齐，将层间反铁磁交换有效传递至巡游的3d_x²-y²轨道，形成有效耦合J⊥。这一机制可简化为针对3d_x²-y²带的强耦合双层t-J-J⊥模型，其中J⊥驱动电子形成层间库珀对，实现高临界温度的扩展s波配对超导。同时，强局域的3d_z²电子倾向于形成层间梯子单态，由于缺乏相位相干性，这些单态不直接参与超导凝聚，而是导致赝能隙相的出现。该综述系统阐述了这一强耦合Hund规则辅助配对理论，为理解该体系的高温超导机制提供了统一框架。

Superconductivity in doped symmetric mass generation insulator: a quantum Monte-Carlo study

Mon, 01 Jan 0001 00:00:00 +0000

本研究采用无符号问题的量子蒙特卡洛模拟，系统研究了一个具有强层间反铁磁交换耦合和局域哈伯德排斥相互作用的双层费米子模型，该模型是实现对称质量生成（SMG）绝缘体的原型。数值精确结果明确表明，在掺杂SMG绝缘体相后，体系中涌现出稳健的超导配对，且哈伯德排斥相互作用显著增强了超导序参量。鉴于该模型可能捕获高压下高温超导体La₃Ni₂O₇的关键特征，此项工作确立了从掺杂SMG母态出发实现超导电性的新范式，为未来实验探索提供了重要理论指导。

Superconductivity in monolayer-trilayer phase of La₃Ni₂O₇ under high pressure

Mon, 01 Jan 0001 00:00:00 +0000

The discovery of 80 K superconductivity in pressurized bilayer Ruddlesden-Popper (RP) nickelate La$_3$Ni$_2$O$_7$ has established a new high-temperature superconductor family. The quest to understand the governing principles of RP nickelate superconductivity has become a central focus in condensed matter physics. Here, we report a critical advance by synthesizing and investigating a distinct structural polymorph of the same compound: the monolayer-trilayer (1313) hybrid phase of La$_3$Ni$_2$O$_7$. Under high pressure, synchrotron X-ray diffraction and Raman spectroscopy reveal a structural transition from the orthorhombic $Cmmm$ to the tetragonal $P4/mmm$ space group at 13~GPa. Above 19 GPa, the phase exhibits a clear superconducting transition, confirmed by a zero-resistance state, albeit at a significantly reduced temperature of 3.6 K. The stark contrast with the 80 K transition in the bilayer phase provides a uniquely clean experimental comparison. Our results demonstrate that the superconducting transition temperature is directly governed by the nature of the interlayer coupling, and the bilayer NiO$_6$ block as the essential structural motif for achieving high-$T_\text{c}$ superconductivity in the RP nickelates.

Superconductivity in pressurized trilayer La₄Ni₃O₁₀−δ single crystals

Mon, 01 Jan 0001 00:00:00 +0000

Superconductivity in pressurized trilayer La₄Ni₃O₁₀−δ single crystals

Superconductivity in Ruddlesden-Popper nickelates: a review of recent progress, focusing on thin films

Mon, 01 Jan 0001 00:00:00 +0000

近年来，Ruddlesden-Popper（RP）镍氧化物超导研究取得重要突破。本文系统回顾了该领域的实验与理论进展，重点聚焦薄膜体系。核心发现包括：在高压下，双层La₃Ni₂O₇（Tc~80 K）和三层La₄Ni₃O₁₀相继呈现超导电性；尤为关键的是，在提供压缩应变的衬底上制备的La₃Ni₂O₇超薄膜中实现了常压超导，这突破了高压限制，使得角分辨光电子能谱（ARPES）等此前无法在超导态使用的实验技术得以应用。理论方面，该体系需同时考虑Ni的e_g和a_{1g}两个轨道，以及双层内强烈的层间耦合形成的“二聚体”图像，并表现出类似铜氧化物的奇特金属行为和强关联特征。本文通过对比不同RP镍氧化物的异同，为理解关联电子系统中的高温超导机制提供了新视角，并展望了未来研究方向。

Superconductivity onset above 60 K in ambient-pressure nickelate films

Mon, 01 Jan 0001 00:00:00 +0000

本研究采用巨大氧化原子层外延法，在极端非平衡条件下于SrLaAlO4衬底上生长(La,Pr)3Ni2O7薄膜，实现了常压下约63 K的超导起始转变温度，零电阻温度达约37 K，抗磁信号起始约23 K。该方法通过高温和原位充分氧化克服了亚稳态超导相的结构不稳定性，X射线衍射和扫描透射电镜证实薄膜具有大范围晶态纯度。输运测量显示正常态电阻温度幂律指数α从低起始转变温度样品的费米液体行为（α≈2）向高起始转变温度样品的奇异金属行为（α≈1）系统演化，直接将增强的超导性与非费米液体行为关联。互电感技术绘制的涡旋熔化相图揭示二维熔化极限被抑制至近零，层间耦合强度显著强于铋系铜氧化物。这些结果表明镍酸盐是常压下具有强层间耦合的奇异金属高温超导体。

Superconductor-insulator transitions in infinite-layer nickelates controlled via operando monitored reduction

Mon, 01 Jan 0001 00:00:00 +0000

该研究通过开发原位监测还原（OMR）方法，实现了对无限层镍酸盐超导体中Ni 3d轨道电子占据从约3d⁷到3d⁹的超宽范围连续调控，从而可控地驱动超导体-绝缘体转变（SIT）。结合同步辐射X射线吸收谱和扫描透射电镜对氧原子的分析，精确标定了电子占据态，并进一步利用离子液体门控和磁场调制SIT。Nernst效应测量显示，与铜氧化物不同，配对在电阻开始下降时即已启动，而迈斯纳效应仅在零电阻态才出现，标志着全局相位相干性的建立。角度依赖的磁输运研究表明，转变温度区间内超导呈现二维与三维特征的混合，表明所观察的SIT偏离了经典二维模型。这些结果在氧含量-磁场-温度参数空间内为理解无限层镍酸盐中结构与电子相变的相互作用提供了独特视角。

Suppressed density wave and emergent negative magnetoresistance in Tb-doped La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

The suppression of density wave in bilayer nickelate La3⁢Ni2⁢O7 under pressure has been identified as a critical factor enabling pressure-induced high-temperature superconductivity. However, this density wave state exhibits remarkable stability against most alternative tuning methods except the high-pressure technique. Herein through systematic investigations of Tb doping effects on electrical transport and magnetic properties, we observe a gradual suppression of density wave transition temperature with the increasing Tb concentration, accompanied by the emergence of negative magnetoresistance persisting up to 14 T. Magnetic susceptibility measurements further reveal the formation of a doping-induced spin-glass state, which likely accounts for the observed negative magnetoresistance phenomenon. This work establishes an effective chemical doping approach to manipulate the density wave state and correlated quantum state in La3⁢Ni2⁢O7, offering new insights into the mechanism of high-temperature superconductivity and potential pathways toward achieving ambient-pressure superconductivity in bulk nickelate crystals.

The evolution of pairing correlation with 3d_z2 electron filling in a bilayer two-orbital model for La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

通过密度矩阵重正化群方法，该工作在一维最小几何结构上系统研究了加压双层镍氧化物La₃Ni₂O₇中有效双层双轨道模型的超导配对关联随Ni 3d_{z²}轨道填充的演化。通过调节轨道化学势差异，将3d_{z²}轨道从1/12掺杂连续调控至近半填充，发现超导关联在半填充附近被显著抑制，表明该轨道的巡游性有利于配对。此外，在电荷涨落较大的区域配对关联增强，暗示电荷序与超导之间存在竞争。这些结果支持了既有的理论图像：层间反铁磁超交换提供配对胶水，而巡游的3d_{z²}轨道通过杂化将配对传递并建立长程相干性。研究明确了3d_{z²}轨道巡游性对超导的关键作用，为理解该体系中非常规配对机制提供了重要依据。

Theoretical proposal of superconductivity in hole-doped reduced bilayer nickelate La₃Ni₂O₆: a manifestation of orbital-space bilayer model with incipient bands

Mon, 01 Jan 0001 00:00:00 +0000

本研究将多轨道Hubbard模型与双层Hubbard模型建立对应，提出轨道空间双层模型（OSBM），其中轨道能级差ΔE的作用类似于真实空间双层模型中的层间跃迁，且超导在incipient-band区域得到增强。基于此，理论预测在适当空穴掺杂下，还原型双层镍酸盐La₃Ni₂O₆可作为OSBM超导体候选。通过第一性原理构建紧束缚模型，发现由于缺乏外部顶角氧，Ni的d_{x²-y²}轨道与其他d轨道之间存在大的ΔE。利用涨落交换近似计算表明，在incipient-band情形下，由轨道间相互作用驱动可产生s±波超导电性，超导能隙函数在d_{x²-y²}带与其他d轨道带之间变号。同时，研究还考察了原子替代和压力下晶体结构的能量与动力学稳定性。尽管La₃Ni₂O₇与La₃Ni₂O₆化学式相似，但该工作指出后者可能实现完全不同的配对机制。

Theoretical study of superconductivity in freestanding infinite-layer nickelate membranes under pressure: mitigation of excess correlation enhances T_c

Mon, 01 Jan 0001 00:00:00 +0000

该研究基于第一性原理计算构建了七轨道有效模型，采用涨落交换（FLEX）近似对自由站立无限层镍酸盐Nd₀.₈₅Sr₀.₁₅NiO₂膜在压力下的超导电性进行了理论分析。结果表明，随着压力增大，超导转变温度Tc单调上升，与近期实验结果一致。这种增强归因于Ni原子极低价态导致的过强电子关联得到缓解，使得有效电子相互作用参数U显著降低，从而减少了准粒子阻尼并增强了自旋涨落介导的d波配对。此外，声子计算确认晶体结构在90 GPa内保持动态稳定。研究通过对比不同U值的模型，指出仅采用较大的U值（约5.1 eV）才能复现实验趋势，而较小U值会导致Tc过早饱和甚至出现穹顶形状，从而支持了无限层镍酸盐中过强关联抑制超导、压力缓解关联进而提升Tc的机制。

Theoretical study on ambient pressure superconductivity in La₃Ni₂O₇ thin films: structural analysis, model construction, and robustness of s±-wave pairing

Mon, 01 Jan 0001 00:00:00 +0000

该研究通过理论方法分析了La₃Ni₂O₇薄膜在常压下的超导电性。研究首先基于第一性原理结构优化构建模型哈密顿量，固定面内晶格常数至实验所用衬底（LSAT、LAO和SLAO），并额外采用基于实验测定晶体结构的模型。运用考虑完全动量和频率依赖的格林函数与配对相互作用的涨落交换近似（FLEX）求解线性化Eliashberg方程。结果表明，电子结构（包括γ-费米口袋的存在与否）取决于所采用的晶体结构及能带计算中是否包含+U修正，但s±-波配对对称性始终保持鲁棒。这种鲁棒性主要源于配对由有限能量自旋涨落介导，该机制对费米面拓扑细节不敏感，并在轨道表示中产生近乎与动量无关的层间d_{3z²−r²}配对间隙函数。另一方面，薄膜的超导转变温度（约40 K）约为加压块材（约80 K）的一半，这一现象在FLEX框架内仅能通过采用基于实验测定晶体结构得到的小层间跃迁参数|t⊥|模型来理解，但也不排除其他因素的可能性。

Three-Dimensional Electronic Structures in Superconducting Ruddlesden-Popper Bilayer Nickelate Films

Mon, 01 Jan 0001 00:00:00 +0000

通过采用低温超高真空暗箱转移技术保持样品表面质量，本研究利用角分辨光电子能谱（ARPES）结合不同光子能量，系统解析了超导(La,Pr,Sm)₃Ni₂O₇/SrLaAlO₄薄膜的三维电子能带结构。实验揭示了轨道依赖的维度性：dx²-y²主导的能带呈现准二维特征，而dz²主导的γ能带则显示出明显的kz色散。在所有观测能带的高对称方向均发现有限能隙，其中γ能带的温度依赖分析表明其超导能隙约18 meV，比值2Δ/kBTc~8，远超弱耦合BCS极限。此外，费米能级附近谱权重的抑制在超导转变温度以上仍持续存在，且观测到普遍存在的瀑布状谱特征，表明电子相互作用的显著影响。这些发现强调了第三维度及dz²轨道在镍酸盐超导机制中的关键作用，对理论模型施加了重要约束。

Threefold error in the reported zero-field cooled magnetic moment of single crystal La₂SmNi₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

该论文指出，Li等人在高压镍酸盐超导体La₂SmNi₂O₇单晶的零场冷（ZFC）和场冷（FC）磁化测量中，对超导相分数的计算存在三重错误。首先，由于顺磁迈斯纳效应（Wohlleben效应），FC模式下样品的磁矩可能为正或负，因此不能用于计算超导相分数。其次，重新分析Li等人的ZFC数据后发现，按照其自身采用的计算方法，超导相分数应为22.8%，而非他们报告的62.1%，两者相差约三倍，主要源于退磁因子计算差异（Li等人取0.849，而作者使用Brandt公式算得0.81548）。第三，即使ZFC磁矩值计算正确，也无法直接用于确定超导相体积分数，因为存在无数种小于样品实际尺寸的超导区域形状和分布，均可产生相同的测量磁矩。作者强调，他们认同Li等人的实验证实了加压镍酸盐的体超导电性，但认为上述计算错误需要纠正，以免误导后续研究。

Time-reversal symmetry breaking superconductivity with electronic glass in nickelate (La, Pr, Sm)₃Ni₂O₇ films

Mon, 01 Jan 0001 00:00:00 +0000

研究团队通过对(La, Pr, Sm)3Ni2O7双层镍酸盐薄膜进行电输运测量，发现了伴随电子玻璃行为的时间反演对称性破缺超导电性。该超导态出现在接近零电阻态的低温区域，表现出三个显著特征：一是非常规的磁电阻回滞，这是时间反演对称性破缺的直接证据，且在不同磁场方向下均保持稳健，与涡旋钉扎或长程磁有序有本质区别；连续氧还原同时削弱了超导电性和磁滞，揭示了它们与特定镍3d电子轨道的关联。二是电流-电压响应在零场下表现出磁历史依赖性和非互易性，进一步证实了自发的内禀时间反演对称性破缺。三是移除磁场后电阻呈现对数缓慢弛豫，这是玻璃态动力学的标志。这些现象首次在镍基超导体中揭示了兼具自发时间反演对称性破缺和内在玻璃态特性的超导态，为理解高温超导机理提供了重要的现象学和概念性突破。

Topochemical Oxidation of Ruddlesden–Popper Nickelates Reveals Distinct Structural Family: Oxygen-Intercalated Layered Perovskites

Mon, 01 Jan 0001 00:00:00 +0000

Layered perovskites─including the Dion–Jacobson, Ruddlesden–Popper, and Aurivillius families─exhibit a wide range of correlated electron phenomena, from high-temperature superconductivity to multiferroicity. Here, we report a new family of layered perovskites realized through topochemical oxidation of Lan+1NinO3n+1+δ (n = 1–4) Ruddlesden–Popper nickelate thin films. Postgrowth ozone annealing induces a substantial c-axis expansion─17.8% for La2NiO4+δ (n = 1)─that monotonically decreases with increasing n. Surface synchrotron X-ray diffraction and coherent Bragg rod analysis (COBRA) reveal that this structural expansion arises from the intercalation of approximately δ ≈ 0.7–1.0 oxygen atoms into interstitial sites within the rock salt spacer layers, far exceeding the previous record of δ ≈ 0.3 for any Ruddlesden–Popper oxide. These oxygen-intercalated phases form a new class of layered perovskites with a spacer layer composition intermediate between the Ruddlesden–Popper and Aurivillius phases. Furthermore, oxygen intercalation induces metallicity, enhances nickel–oxygen hybridization, and suppresses oxygen octahedral rotations, a feature associated with high-temperature superconductivity in Ruddlesden–Popper nickelates. Our work establishes topochemical oxidation as a powerful approach to accessing highly oxidized, metastable phases across a broad range of layered oxide systems, offering new platforms to engineer electronic properties via intercalation chemistry.

Topotactical Hydrogen Induced Single-Band d -Wave Superconductivity in La₂ NiO 4

Mon, 01 Jan 0001 00:00:00 +0000

Topotactical Hydrogen Induced Single-Band d -Wave Superconductivity in La₂ NiO 4

Tracing the horizon of tetragonal-to-monoclinic distortion in pressurized trilayer nickelate La₄Ni₃O₁₀

Mon, 01 Jan 0001 00:00:00 +0000

本研究利用压力-温度单晶X射线衍射与从头算密度泛函理论计算，揭示助熔剂生长的三层镍酸盐La₄Ni₃O₁₀中四方相（I4/mmm）到单斜相（P2₁/c）的转变不存在中间斜方Bmab相，而是一个直接的结构相变，并伴随两倍超结构的形成，体现为公度超晶格反射的出现。该转变温度可在14 GPa压力下从约1030 K连续抑制至20 K，说明压力有效稳定了四方相。此外，在助熔剂生长晶体中首次通过X射线衍射探测到与非公度密度波有序相关的微弱卫星反射，补充了此前仅见于浮区法晶体的结果，拉曼光谱在130 K以下观察到额外声子模式也进一步印证了该有序态。从头算计算与实验观察吻合。这项工作澄清了La₄Ni₃O₁₀结构对称性的长期争议，并支持超导电性在高压恢复四方对称性后出现，为后续电子结构及超导机理研究提供了关键的晶体学基础。

Transport, Magnetic and Thermal Properties of La₃Ni₂O₇-δ

Mon, 01 Jan 0001 00:00:00 +0000

The metal-insulator transition of La 3 Ni 2 O 7-δ with 2-dimensional electrons has been studied. Various physical properties can be understood by introducing a model of charge orderings in the NiO …

Triplon-mediated pairing and the superconducting gap structure in bilayer nickelates

Mon, 01 Jan 0001 00:00:00 +0000

该研究构建了双层镍氧化物超导能隙结构的微观理论模型，模型中dx²-y²对称性的传导带与局域化的d3z²-r²自旋共存。强层间耦合使局域磁矩形成单重态基态，其虚拟的单重态-三重态激发（即“triplon”）介导了传导电子间的配对相互作用，由此产生带间s±波配对，两个能带（α和β）上的序参量符号相反。理论结果自然解释了实验中观测到的关键特征：尽管α带态密度较小，但其超导能隙更大，且能隙呈现出由非局域Kondo耦合导致的显著动量空间各向异性。这些发现有力支持了triplon介导的配对机制作为双层镍氧化物超导的微观起源。

Tunable superconductivity and spin density wave in La₃Ni₂O₇/LaAlO₃ thin films

Mon, 01 Jan 0001 00:00:00 +0000

通过第一性原理计算与奇异模式泛函重整化群方法，系统研究了La3Ni2O7/LaAlO3薄膜中层间镍-镍距离对基态的影响。结果显示，较小的层间距导致C型自旋密度波（层间铁磁耦合），较大的层间距则产生G型自旋密度波（层间反铁磁耦合），而在这两种相之间，出现了以镍3d₃z²⁻ᵣ²轨道配对为主导的s±波超导态。该结果解释了实验中薄膜在常压下超导的成因，并预测施加压力会降低超导转变温度，直至系统进入C型自旋密度波。这一预言若经实验验证，将为理解该体系中电子关联的本质提供深刻见解，因为C型自旋密度波在巡游电子图像中自然实现，而在局域磁矩图像中则难以形成（后者层间自旋始终反铁磁耦合）。

Tunable Superconductivity in 1313-La₃Ni₂O₇: Suppressed under Compression and Possible s± Pairing under Tension

Mon, 01 Jan 0001 00:00:00 +0000

通过结合密度泛函理论和随机相位近似，系统研究了压缩和拉伸应变对1313-La3Ni2O7薄膜超导电性的影响。研究发现，无论压应变还是拉应变，单层与三层块体间均存在自掺杂效应，且在拉伸应变下最为显著。在LSAO衬底引入的压缩应变下，即使考虑衬底锶离子迁移带来的空穴掺杂，超导电性也难以出现，这与实验一致。然而，在KTO衬底施加的拉伸应变下，三层子系统中原本未穿过费米能级的能带下移，在M点出现一个小型空穴型γ口袋，与Γ点的小型电子型σ口袋通过近(π,π)波矢相连。随机相位近似计算揭示，此时三层子系统可形成稳定的s±波配对态，其序参量在这两个口袋间发生符号反转。进一步分析表明，γ口袋大小对配对至关重要，过大的γ口袋会抑制超导。该工作预测了应变驱动的电子结构重构，并提出通过拉伸应变工程可在环境压力下实现1313-La3Ni2O7超导电性的设计原则。

Ultrafast Magneto-Pressure Spectroscopy and Control of Correlated Phases in a Trilayer Nickelate

Mon, 01 Jan 0001 00:00:00 +0000

本研究开发了可在高达40 GPa、7 T磁场和5 K低温下同时工作的超快磁压力光谱平台，并将其应用于三层镍酸盐Pr₄Ni₃O₁₀中准粒子动力学的磁压力演化探测。实验观察到电荷密度波（CDW）转变附近准粒子弛豫出现显著的临界减慢现象，该现象在施加压力后消失。在更高压力下，低温弛豫时间反而变长，与初始超导关联特征一致。然而，高达7 T的磁场几乎不改变弛豫行为，且未观察到涡旋诱导的预瓶颈动力学——后者在对照体超导样品中是稳健特征——表明当前压力条件下可能的超导态并非体态，而是丝状或强不均匀的。该磁压力超快能力为解决关联量子材料中压力诱导超导电性与交织有序的悬而未决问题开辟了新途径。

Ultrafast optical evidence of coexisting density waves in bilayer nickelate La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

Ultrafast optical evidence of coexisting density waves in bilayer nickelate La₃Ni₂O₇

Unconventional Crystal Structure of the High-Pressure Superconductor La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

The discovery of high-temperature superconductivity in La3⁢Ni2⁢O7 at pressures above 14 GPa has spurred extensive research efforts. Yet, fundamental aspects of the superconducting phase, including the possibility of a filamentary character, are currently subjects of controversial debates. Conversely, a crystal structure with NiO6 octahedral bilayers stacked along the 𝑐-axis direction was consistently posited in initial studies on La3⁢Ni2⁢O7. Here, we reassess this structure in optical floating zone-grown La3⁢Ni2⁢O7 single crystals that show signs of filamentary superconductivity. Employing scanning transmission electron microscopy and single-crystal x-ray diffraction under high pressures, we observe multiple crystallographic phases in these crystals, with the majority phase exhibiting alternating monolayers and trilayers of NiO6 octahedra, signifying a profound deviation from the previously suggested bilayer structure. Using density functional theory, we disentangle the individual contributions of the monolayer and trilayer structural units to the electronic band structure of La3⁢Ni2⁢O7, providing a firm basis for advanced theoretical modeling and future evaluations of the potential of the monolayer-trilayer structure for hosting superconductivity.在高于 14 GPa 的压力下， La3⁢Ni2⁢O7 中高温超导性的发现激发了广泛的研究。然而，超导相的基本性质，包括其丝状特征的可能性，目前仍存在争议。相反，在对 La3⁢Ni2⁢O7 的早期研究中，人们一直假设其晶体结构为沿 𝑐 轴方向堆叠的 NiO6 八面体双层。本文中，我们重新评估了光学浮区法生长的 La3⁢Ni2⁢O7 单晶中的这种结构，这些单晶表现出丝状超导的迹象。利用扫描透射电子显微镜和高压下的单晶 X 射线衍射技术，我们观察到这些晶体中存在多种晶相，其中主要相由 NiO6 八面体的单层和三层交替构成，这表明其与先前提出的双层结构存在显著偏差。利用密度泛函理论，我们分离出单层和三层结构单元对 La3⁢Ni2⁢O7 电子能带结构的各个贡献，为高级理论建模和未来评估单层-三层结构承载超导性的潜力提供了坚实的基础。

Unconventional Superconductivity in La₃Ni₂O₇ from the Perspective of Symmetry

Mon, 01 Jan 0001 00:00:00 +0000

本研究针对高压下高转变温度的La₃Ni₂O₇块材与常压超导但Tc减半的薄膜之间的差异，发展了一种基于对称性的唯象方法，结合DFT+U计算和实验确定的Tc及结构对称性，分析超导能隙结构。研究发现，两者均呈现s±波配对对称性和两带超导，但主导的微观配对构型不同：加压块材中超导主要由Ni-dz²轨道的面外配对主导，而薄膜中则由Ni-dx²-y²轨道的面内配对主导。Tc的降低归因于薄膜中层间与层内跳跃比率减小，导致主导配对类型从面外转向面内。该结果揭示了对称性在非常规超导中的关键作用，所发展的方法有望推广至其他非常规超导体。

Uncovering origins of heterogeneous superconductivity in La₃Ni₂O₇ using quantum sensors

Mon, 01 Jan 0001 00:00:00 +0000

The family of nickelate superconductors have long been explored as analogs of the high temperature cuprates. Nonetheless, the recent discovery that certain stoichiometric nickelates superconduct up to high $T_c$ under pressure came as a surprise. The mechanisms underlying the superconducting state remain experimentally unclear. In addition to the practical challenges posed by working in a high pressure environment, typical samples exhibit anomalously weak diamagnetic responses, which have been conjectured to reflect inhomogeneous `filamentary’ superconducting states. We perform wide-field, high-pressure, optically detected magnetic resonance spectroscopy to image the local diamagnetic responses of as grown La$_3$Ni$_2$O$_7$ samples \emph{in situ}, using nitrogen vacancy quantum sensors embedded in the diamond anvil cell. These maps confirm significant inhomogeneity of the functional superconducting responses at the few micron scale. By spatially correlating the diamagnetic Meissner response with both the local tensorial stress environment, also imaged \emph{in situ}, and stoichiometric composition, we unravel the dominant mechanisms suppressing and enhancing superconductivity. Our wide-field technique simultaneously provides a broad view of sample behavior and excellent local sensitivity, enabling the rapid construction of multi-parameter phase diagrams from the local structure-function correlations observed at the sub-micron pixel scale.

Unified mechanism of charge-density-wave and high-T_c superconductivity protected from oxygen vacancies in bilayer nickelates

Mon, 01 Jan 0001 00:00:00 +0000

Unconventional charge and spin density-wave states are commonly observed in bilayer nickelates, drawing considerable attention due to their proximity to high-$T_c$ superconductivity in various phase diagrams. However, the nature and mechanisms of charge and spin density-waves (DWs) in nickelates remain poorly understood. Numerous experiments have reported that the charge-density-wave (CDW) transition temperature $T_{ cdw}$ and the spin-density-wave (SDW) transition temperature $T_{sdw}$ are closely related but distinct. However, in contrast to these experiments, previous mean-field-type analyses have yielded only a simple SDW phase. To resolve this key problem, this paper demonstrates that sizable CDW instabilities emerge in proportion to the SDW instability in La$3$Ni$2$O$7$.This behavior is driven by the paramagnon-interference (PMI) mechanism, which captures important electron correlations beyond mean-field theory. Therefore, (i) experimental CDW + SDW coexisting state is naturally explained. In addition, (ii) the CDW + SDW fluctuations cooperatively drive high-$T_c$ superconductivity. Notably, the predicted $s$-wave SC state is robust against the inner apical O vacancies. Furthermore, (iii) the CDW instability is highly sensitive to the size of the $d_{z^2}$-orbital hole pocket, allowing for the realization of CDW quantum criticality through carrier-doping and pressure application. We find that the coexistence of charge and spin fluctuations is essential in bilayer nickelates, with both playing a cooperative role in mediating high-$T_c$ superconductivity.

Unified mechanism of charge-density-wave and high-Tc superconductivity protected from oxygen vacancies in bilayer nickelates

Mon, 01 Jan 0001 00:00:00 +0000

Unconventional charge- and spin-density-wave states are commonly observed in bilayer nickelates, drawing considerable attention due to their proximity to high-transition temperature (

$${T}_{{\rm{c}}}$$

) superconductivity. However, the nature and origin of these density waves remain poorly understood. Experiments show that the charge-density-wave and spin-density-wave transition temperatures are closely related but distinct, while mean-field-type analyses typically have yielded only a simple spin-density-wave phase. To resolve this key problem, this paper demonstrates that sizeable charge-density-wave instabilities emerge in proportion to spin-density-wave instabilities in La3Ni2O7 due to the paramagnon-interference mechanism, which captures electron correlations beyond mean-field theories. Therefore, (i) the experimental charge- and spin-density-wave coexisting state is naturally explained, and (ii) charge- and spin-density-wave fluctuations cooperatively drive high-

$${T}_{{\rm{c}}}$$

superconductivity. Furthermore, the predicted s-wave superconducting state is robust against the inner-apical oxygen vacancies. We find that the coexistence of charge- and spin-fluctuations is essential in bilayer nickelates, with both playing a cooperative role in mediating high-

$${T}_{{\rm{c}}}$$

superconductivity.

Unraveling Spin Density Wave Order in Layered Nickelates La₃Ni₂O₇ and La₂PrNi₂O₇ via Neutron Diffraction

Mon, 01 Jan 0001 00:00:00 +0000

The discovery of pressure-induced superconductivity in two- and three-layer Ruddlesden-Popper nickelates has generated significant interest in these materials as a platform for unconventional superconductivity. While their ground state exhibits magnetism, a direct determination of their magnetic structure remains elusive. Understanding this aspect is crucial, as magnetism may play a role in the pairing mechanism of superconductivity in these compounds. We resolve the magnetic structures of the bilayer (2222) polymorphs of La3Ni2O7 and La2PrNi2O7 using neutron powder diffraction (NPD) and muon-spin rotation/relaxation (muSR). Magnetic neutron scattering appears below approximately 150 K in both compounds and is observed at the (qx, 1/2, 0) position, with qx = 0 and 1/2 for La3Ni2O7 and qx = 0 for La2PrNi2O7. Within a single layer, alternating low (0.05 - 0.075 muB) and high (0.66 muB) magnetic moment stripes form. These layers stack antiferromagnetically along the c-direction to form bilayers. The presence of two propagation vectors (qx = 0 and 1/2) in undoped La3Ni2O7 suggests the coexistence of two magnetic stacking polymorphs within a single crystallographic phase. The muSR spectra further confirm these magnetic structures. Our findings provide a detailed understanding of the magnetic ground state in bilayer nickelates, offering insights into possible precursor states that may influence the emergence of superconductivity in these materials.

Visualization of oxygen vacancies and self-doped ligand holes in La₃Ni₂O₇−δ

Mon, 01 Jan 0001 00:00:00 +0000

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.

Weakly anisotropic superconductivity of Pr₄Ni₃O₁₀ single crystals

Mon, 01 Jan 0001 00:00:00 +0000

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。

What Does the Single-Particle Spectrum Imply on the Pairing Nature and Pairing Mechanism in La₃Ni₂O₇?

Mon, 01 Jan 0001 00:00:00 +0000

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。