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。
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。
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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。
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.
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.