Paper

This review summarizes recent advances in nickelate superconductors, covering infinite-layer, bilayer, and trilayer systems, their superconducting properti

本研究通过系统调控La₃Ni₂O₇₊δ系列样品的氧含量，合成了不同相组成的材料，包括纯双层相、双层与单层-双层杂化相混合相以及含三层交生的主双层相。高压输运测量表明，这些相分别对应不同的超导转变温度（T_c），其中双层相呈现约80 K的超导电性，杂化相和三层交生相的T_c较低。氧含量不仅影响相纯度，还直接调控双层超导电性的上临界场（H_c2），纯双层相具有更高的H_c2。通过建立T_c和H_c2随氧含量变化的相图，本研究实现了对Ruddlesden-Popper镍酸盐氧含量的精确控制，为理解其高压超导机制提供了关键实验依据。

针对Korolev和Talantsev对Li等人《Nature》论文中超导相分数计算的批评，本文作者逐一回应：首先，经实验确认低温尾巴的弱上升源于背景，且未观察到顺磁迈斯纳效应，因此场冷数据可用于计算超导相分数；其次，退磁效应必须基于实际测量磁矩随超导相分数f的变化，而Korolev等错误地将退磁场视为常数，导致其公式低估f约三分之二（因子接近1/3），解释了其计算结果仅为报道值（约62.1%）约三分之一的原因；最后，样品的多种表征（能谱、X射线衍射、核四极共振、扫描透射电镜等）证实为均匀高质量块体单晶，不存在多个离散超导区域。因此，Li等人《Nature》论文中超导相分数的计算方法并未被Korolev等的分析推翻。

The recent discovery of a superconductivity signature in La3Ni2O7-δ under a pressure of 14 GPa, with a superconducting transition temperature of around 80 K, has attracted considerable attention. An important aspect of investigating electronic structures is discerning the extent to which the electronic ground state of La3Ni2O7-δ resembles the parent state of the cuprate superconductor, a charge transfer insulator with long-range antiferromagnetism. Through X-ray absorption spectroscopy, we reveal the influence of oxygen ligands on the electronic ground states of the Ni ions, displaying a charge transfer nature akin to cuprate but with distinct orbital configurations. Additionally, in La3Ni2O7-δ films, we detect a superlattice reflection (1/4, 1/4, L) at the Ni L absorption edge using resonant X-ray scattering measurements. Further examination of the resonance profile indicates that the reflection originates from the Ni d orbitals. By evaluating the reflection’s azimuthal angle dependence, we confirm the presence of collinear antiferromagnetic spin ordering and charge-like anisotropy ordered with the same periodicity. Our findings reveal a microscopic relationship between these two components in the temperature dependence of the scattering intensity of the reflection. This investigation enriches our understanding of high-temperature superconductivity in La3Ni2O7-δ under high pressure.

本研究采用行列式量子蒙特卡罗方法，在三轨道Emery模型基础上加入具有三维色散的间隙$s$轨道，模拟无限层镍酸盐的低能电子结构。大规模计算发现：强关联效应显著缩小了间隙$s$轨道产生的电子口袋，但在20%空穴掺杂下口袋仍存续，大小与ARPES实验观测相当；$d_{x^2-y^2}$轨道色散受到强烈重正化，沿$k_z$方向的弱色散与实验一致。此外，与常规三轨道模型相比，引入$s$轨道后系统的短程反铁磁关联明显增强。这些结果揭示了强关联和多轨道效应在决定无限层镍酸盐低能电子态和自旋关联中的关键作用，表明必须在实际多轨道框架中处理相互作用驱动的多体物理。

Ruddlesden-Popper镍氧化物在块材高压和薄膜外延约束条件下均表现出超导电性，但高度依赖于样品质量、氧含量、缺陷和应力状态。本文提出，亚稳态RP晶格仅在Ni-O框架的局部约束变形处于一个有界的剪切应变窗口内时才进入超导态；该变形控制着八面体旋转、层间Ni-O-Ni键角以及Ni dz²与dx²-y²轨道间的耦合。这一剪切应力约束超导电性（SSCS）框架统一解释了此前观察到的压力阈值、可逆性、空间不均匀性、压力介质依赖性、薄膜-衬底敏感性以及可重复性难题。SSCS场景并不替代键角、键长、轨道占据、氧化学计量或载流子密度等传统因素的作用，而是指明这些因素协同稳定超导态所需的力学与对称性条件。镍氧化物超导体表现出的脆性与异质性并非外在复杂性，而是超导态本身的核心诊断特征。该视角为改善可重复性提供了具体实验路径，并将压缩块材、外延薄膜、化学替代样品及杂化RP结构中的物理机制统一在同一概念框架内。

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

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。