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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.

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

本研究利用氦气作传压介质，通过同步辐射单晶X射线衍射构建了化学计量比双层镍酸盐La₃Ni₂O₇在静水压下的本征压力–温度结构相图。常压下该材料呈极性正交Am2m结构，伴随不等价Ni位间的电荷有序和NiO₆八面体倾斜。随压力升高，正交晶格畸变逐步减小，同时代表电荷有序的超结构反射强度线性下降。在约10 GPa时，体系发生从电荷有序Am2m相到四方I4/mmm相的直接转变，无中间Amam相出现，正交孪晶分裂消失，晶胞参数趋于四方对称。该四方相特征为八面体倾斜完全消除，形成直线型层间Ni–O–Ni键，并在超导出现的压力–温度区域稳定存在。这一结构相变恰好与电阻测量中起始温度约68 K的体超导开始出现吻合，表明超导态位于I4/mmm四方结构框架内。上述结果澄清了超导相结构归属的争议，确立了La₃Ni₂O₇的本征结构演化路径，为理解镍酸盐高温超导的微观起源提供了关键结构基础。

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2 条关联记录

该研究利用高压光学浮区法生长的La3Ni2O7单晶，结合中子衍射与非弹性中子散射，在常压母相中确定q=(0,0.5,0.5)的单一条纹型磁序，并测得约5 meV的自旋能隙、面内各向异性激发以及具有双层周期的显著调制，后者表明层间存在反铁磁耦合。作者以包含强层间交换和相互竞争面内交换的双层Heisenberg型模型描述色散；弹性散射难以区分单条纹与双条纹，而非弹性谱成为判别依据。绝对归一化显示，其自旋波带宽仅约为铜氧化物的25%，但相应能量处的局域动态磁化率增强，总涨落矩与铜氧化物相当。该结果建立了不同于铜氧化物的磁框架，并被视为与镍酸盐超导相关；文中并未确立配对通道，磁性对配对的角色仍是待检验前提。

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。

该研究利用常压拉曼散射系统测量了双层镍酸盐 La₃Ni₂O₇ 的电子、磁性和晶格激发。进入 153 K 以下的自旋密度波态后，A₁g 通道出现突变的电子谱隙及约 50 meV 的等吸收点，而 B₁g 和 B₂g 通道由显著的双磁振子激发主导，表明沿 Ni–O 键及其对角方向存在两种面内自旋交换相互作用。基于自旋波理论的计算进一步指出，B₂g 双磁振子模式源于由镍 d_{x²−y²} 轨道介导的两个键对角反铁磁相互作用之间的竞争；低于 10 meV 的低能双磁振子激发则来自较弱的自旋矩，支持自旋歧化。研究还发现 B₁g 声子从 280 K 到 4.5 K 的异常软化，提示存在导致棋盘状呼吸调制的初生晶格不稳定性。这些结果将竞争性键对角相互作用、自旋歧化和晶格不稳定性联系起来，为理解镍酸盐超导机理提供了关键要素。

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。