Paper

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.

本研究采用高压高温法生长四层Ruddlesden–Popper镍酸盐La5Ni4O13块体单晶，并复现五层La6Ni5O16的合成；随后以CaH2对La5Ni4O13单晶进行拓扑还原，得到方平面相La5Ni4O10。该还原相在常压下表现为块体超导体，起始转变温度Tc^onset约18 K，零场冷屏蔽分数超过60%，比热跃变证实发生块体配对转变；同时所有晶体中超导与密度波或金属—绝缘转变共存。上临界场各向异性γ≈2.4且在T→Tc时仍保持有限，表明超导具有三维特征且四层块之间存在显著耦合。作者据此提出，方平面镍酸盐R_{n+1}Ni_nO_{2n+2}可仅通过层数调控镍价态进入超导区，无需化学替代、外加压力或外延约束。

本文利用极化分辨电子拉曼散射测量了高质量La₃Ni₂O₇单晶，在150 K以下的密度波转变中观察到显著的、对称性依赖的谱权重重新分布：B₁g通道呈现不对称峰，B₂g通道呈现对称宽峰，分别对应布里渊区X/Y点附近和对角线方向的电子激发。定量分析提取出两组SDW能隙值，B₁g通道能隙约为37.5–40.4 meV（2Δ/kBTc≈5.5–5.9），B₂g通道能隙约为23.0 meV（2Δ/kBTc≈3.4），表明前者属中等至强耦合，后者属弱耦合。这种动量选择的各向异性耦合强度无法用简单的弱耦合嵌套理论解释，揭示了非常规SDW源于各向异性电子关联。能隙的温度依赖性明显弱于平均场预期，且B₂g通道各向同性与B₁g通道弱各向异性也支持两种不同耦合机制的共存。该工作确立了La₃Ni₂O₇中SDW的电子特性，为理解压力下镍酸盐高温超导的涌现提供了微观基础。

本研究利用第一性原理密度泛函理论（GGA及GGA+U）探讨了具有双层NiO2无限层结构的La3Ni2O5F的电子与磁性。La(O/F)La阻挡层实现了NiO2双层的严格隔离，形成纯粹二维电子与磁系统。计算发现一个由间隙区域电子密度构成的E单带，该带不与任何原子轨道关联，沿M-A方向下穿费米能级，提供0.09个穴/Ni的自掺杂，使Ni实际价态变为+1.09。该E带费米面呈柱状，占布里渊区9%面积。dpσ带近乎半满但受自掺杂偏移至范霍夫奇点附近。磁响应显示出不同于以往镍酸盐的反常特性，磁化率在较大磁场下趋于消失。无磁相变的现象可归因于强烈的二维自旋涨落及偏离半填充的自掺杂效应。这些结果揭示了Ni^{1+}离子在该体系中独特的行为，以及界面阻挡层对间隙带形态的关键影响。

本研究利用原子分辨率扫描隧道显微镜和谱学技术，对生长在SrLaAlO4上的1.5个单胞(La,Pr)3Ni2O7超薄膜进行了表征。通过低温超高真空样品转移，保持了有序的√2×√2表面重构，并在隧道谱中观测到具有两个能隙尺度（约14和20 meV）且零偏压电导平坦的U形谱，表明超导能隙无节点。相比之下，若转移过程中样品在超高真空中暴露时间较长且未冷却，虽仍保持表面重构和40 K以上的输运超导转变起始温度，但隧道谱呈现V形，且宽能谱显示氧缺失会使密度波相关的谱权重混合。通过对比不同转移时间的样品，确定了获得本征超导能隙需控制氧含量，提供了双层镍酸盐超薄膜本征无节点超导能隙的原子尺度观测证据。

该研究通过将无限层钐镍氧化物超导薄膜制备成空间周期性网络结构，调控库珀对的相位相干性，实现了超导-绝缘体相变。实验中观测到周期为h/2e的磁阻振荡，直接证明了镍氧化物中2e库珀对的存在。相变主要受增强的超导涨落驱动，且库珀对在整个相变过程中参与电荷输运。研究还发现了两种异常金属态：一种出现在有限磁场下，另一种在零磁场下即可出现；这两种态均可由玻色子激发表征，暗示涡旋在基态中的动态角色。该工作确立了镍氧化物作为研究库珀对相位相干性调控下丰富玻色子相的关键平台。

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

该研究系统研究了压应变La₂LnNi₂O₇薄膜（Ln为镧系元素）在常压与高压下的超导性，以揭示常压与高压超导机制之间的关系。通过59 T磁场抑制超导态，常压正常态电阻率趋于T²行为；在立方砧腔中施加高压后，Tc从常压的41–42 K提升至16 GPa时的67–73 K。然而，通过Ln替代引起的晶格压缩虽可能模拟压力效应，却使Tc降低。在上述两种情况下，Tc均与正常态输运在T²和T线性行为之间的演化相关，表明双层镍酸盐中晶格结构变化对超导性具有重要影响，为理解常压应变薄膜与高压体材料超导态的内在联系提供了线索。

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.

本研究通过高温高压合成五层Ruddlesden-Popper相La6Ni5O16，并经拓扑还原反应制备出五层方平面（SP）镍酸盐La6Ni5O12，实现了常压下的块体超导。该材料在约18 K处表现出超导转变，经零电阻、抗磁屏蔽及比热测量证实，其中比热数据显示约25%的超导体积分数，有力支持了块体超导的存在。磁化率和扭矩磁强计测量揭示了明显的磁滞回线，证实了涡旋钉扎效应及II型超导特性。该成果突破了此前镍酸盐超导仅限于薄膜或高压环境的限制，为在常压块体样品上深入研究镍基高温超导机制提供了关键平台。