Knowledge

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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。

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.

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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.