Paper

Recent experimental observations have showed some signatures of superconductivity close to 80 K in La3Ni2O7 under pressure and have raised the hope of achieving high-temperature superconductivity in bulk nickelates. However, a zero-resistance state—a key characteristic of a superconductor—was not observed. Here we show that the zero-resistance state does exist in single crystals of La3Ni2O7−δ using a liquid pressure medium at up to 30 GPa. We also find that the system remains metallic under applied pressures, suggesting the absence of a metal–insulator transition proximate to the superconductivity. Moreover, analysis of the normal state T-linear resistance reveals a link between this strange-metal behaviour and superconductivity. The association between strange-metal behaviour and high-temperature superconductivity is very much in line with other classes of unconventional superconductors, including the cuprates and Fe-based superconductors. Further investigations exploring the interplay of strange-metal behaviour and superconductivity, as well as possible competing electronic or structural phases, are essential to understand the mechanism of superconductivity in this system.

该论文将密度泛函理论加哈伯德U修正（DFT+U）与有限位移法相结合，实现了对强关联材料中声子谱和电子-声子矩阵的全哈伯德修正计算。作者将该方法应用于两种典型体系：无限层镍酸盐LaNiO₂和二氧化钌RuO₂。结果表明：在20%空穴掺杂的LaNiO₂中，哈伯德U修正微弱增强其电子-声子相互作用，但总耦合强度仍很小，不足以解释实验中约10-30 K的超导转变温度；这与近期GW修正预言的结果相反，差异源于DFT+U与GW方法所得费米面拓扑不同。在RuO₂中，哈伯德U修正消除了TiO₂衬底应变下的虚声子模，并大幅降低了电子-声子耦合，缓解了理论上过大的电子-声子耦合与实验观测到低超导转变温度之间的矛盾。该工作提供了完整纳入哈伯德U修正的电子-声子性质计算方案，并凸显了费米面形状及关联效应对声子谱和电子-声子矩阵的关键影响。

Identification of superconductivity in the Ruddlesden-Popper phases of nickelates under high pressure remains challenging. Here, we report a comprehensive study of the crystal structure, electrical resistance, and Meissner effect in single crystals of bilayer nickelate La3Ni2O7 under hydrostatic pressures up to 104 GPa. Using high-pressure X-ray diffraction, we observe a structural transition from an orthorhombic to a tetragonal phase above 40 GPa. Superconductivity emerges with a maximum onset transition temperature Tconset of 83 K at 18.0 GPa, accompanied by zero resistance. The superconducting phase is gradually suppressed and vanishes above 80 GPa, forming a right-triangle-like superconducting region. Direct-current magnetic susceptibility measurements demonstrate the Meissner effect and reveal a superconducting volume fraction of ∼41% at 22.0 GPa and 20 K, confirming the bulk nature of superconductivity in La3Ni2O7. Our results highlight the intricate relationship between superconductivity, oxygen content, and structural transitions in this material.

该工作利用高压变温拉曼光谱和偏振分析，在低至3 K、高达32.7 GPa的条件下系统追踪了La₃Ni₂O₇单晶的结构演化。基于严格的对称性选择定则，光谱中多个声子模式的消失与重整化在约14.5 GPa处指示了一个从正交Amam相到正交Fmmm相的一级结构相变，且体超导电性恰好在这一压力点出现。偏振拉曼测量进一步表明，在1.92 GPa以上，样品因退孪晶而恢复本征D₂h对称性；在超导态（3 K, 19.45 GPa）下，极化通道中仍可观测到声子模式，直接排除了四方I4/mmm相。研究结果确认，低于19.45 GPa的加压超导态本征晶体结构为正交Fmmm，而非此前争议的四方相，并揭示沿c轴呈180°的Ni–O–Ni键角是实现高超导转变温度的关键结构前提，为理解双层镍氧化物超导机制奠定了重要的结构基础。

该研究利用自旋极化扫描隧道显微镜对三层镍酸盐La₄Ni₃O₁₀中的条纹序进行了实空间成像，揭示了其局域磁性和电荷分布。实验发现，该条纹序具有四倍单胞的周期性，与铜氧化物高温超导体中的条纹序高度相似，并在费米能级附近打开了一个约66 meV的近完全能隙。更重要的是，当隧穿电子能量超过约20 meV的阈值时，可以触发离散的相位滑移，从而实现了原子尺度上的条纹动力学成像。这些结果强调了关联物理在驱动镧镍酸盐中类似条纹序中的关键作用，并显示出其与铜氧化物超导体惊人的相似性，为理解镍基超导体的配对机制提供了重要线索。

本研究采用多轨道紧束缚模型分析薄层双层镍酸盐La₃Ni₂O₇中的非费米液体输运现象，重点考察多带效应对霍尔系数的影响。通过格林函数方法推导了包含准量子度量（qQM）项的严格霍尔系数公式，发现该qQM项的温度依赖性在强关联多带体系中至关重要。计算表明，Ni d₂²轨道自旋涨落导致更强的准粒子阻尼，而Ni dₓ²₋ᵧ²轨道形成冷点。La₃Ni₂O₇中霍尔系数的显著温度依赖源于空穴带正贡献与电子带负贡献的竞争，qQM项使得低温下正霍尔系数增强，并解释了实验中观察到的T线性电阻率和霍尔系数随温度降低而上升的行为。此外，该qQM项对描述Nernst系数及其他涉及速度二阶导数的输运现象也起关键作用。研究揭示了自旋涨落诱导的轨道选择性重整化在非费米液体输运中的核心机制，为理解该体系的奇异输运性质提供了理论框架。

The bilayer nickelate La3Ni2O7 under pressure has recently emerged as a promising system for high-Tc superconductivity. In this work, we investigate the fate of the superconducting properties in La3Ni2O7 under pressure, focusing on the effects of structural deformation and apical oxygen vacancies. Employing a low-energy effective t-J∥-J⊥ model for the $$3{d}_{{x}^{2}-{y}^{2}}$$orbitals within the slave-boson mean-field approach, we demonstrate that the pairing strength is significantly enhanced in the high-pressure tetragonal I4/mmm phase compared to the ambient pressure orthorhombic Amam phase. Furthermore, by simulating random configurations of apical oxygen vacancies, we show that oxygen vacancies suppress both pairing strength and superfluid density. These results underscore the critical role of pressure and oxygen stoichiometry in tuning the SC of La3Ni2O7, providing key insights into optimizing its high-Tc behavior.

The recent discovery of superconductivity in the bilayer Ruddlesden-Popper nickelate La 3 Ni 2 O 7 under high pressure has generated much interest in the superconducting pairing mechanism of nickelates. Despite extensive work, the superconducting pairing symmetry in La 3 Ni 2 O 7 remains unresolved, with conflicting results even for identical methods. We argue that different superconducting states in La 3 Ni 2 O 7 are in close competition and highly sensitive to the choice of interaction parameters as well as pressure-induced changes in the electronic structure. Our study uses a multiorbital Hubbard model, incorporating all Ni 3 d and O 2 p states. We analyze the superconducting pairing mechanism of La 3 Ni 2 O 7 within the random phase approximation and find a transition between d -wave and sign-changing s -wave pairing states as a function of pressure and interaction parameters, which is driven by spin fluctuations with different wave vectors. These spin fluctuations with incommensurate wave vectors cooperatively stabilize a superconducting order parameter with d x 2 − y 2 symmetry for realistic model parameters. Simultaneously, their competition may be responsible for the absence of magnetic order in La 3 Ni 2 O 7 , demonstrating that magnetic frustration and superconducting pairing can arise from the same set of incommensurate spin fluctuations.

The effects of Co doping in a series of La3Ni2−xCoxO7−δ samples before and after high oxygen pressure annealing have been investigated. The structural refinemen

本研究采用六端法对Ruddlesden-Popper镍酸盐双层与三层单晶进行高精度输运测量，自洽提取了面内与面外电阻率。结果发现，样品呈现强本征电子各向异性，面外电阻率表现出非单调温度依赖，揭示了普遍的层间相干至非相干渡越。在压力下，最大超导转变温度与环境压强下的电阻率各向异性成反比，表明更强的层间电子相干性有利于超导。此外，面外电阻率是磁性与密度波序的灵敏探针，而面内电阻率响应较弱。这些结果突出层间相干性作为关键调控参数，既追踪磁性关联，又与超导电性紧密相关，为镍基高温超导的微观理论提供了严格约束。