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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.

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。

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。

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

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。

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。

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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。

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