本研究针对高压下高转变温度的La₃Ni₂O₇块材与常压超导但Tc减半的薄膜之间的差异，发展了一种基于对称性的唯象方法，结合DFT+U计算和实验确定的Tc及结构对称性，分析超导能隙结构。研究发现，两者均呈现s±波配对对称性和两带超导，但主导的微观配对构型不同：加压块材中超导主要由Ni-dz²轨道的面外配对主导，而薄膜中则由Ni-dx²-y²轨道的面内配对主导。Tc的降低归因于薄膜中层间与层内跳跃比率减小，导致主导配对类型从面外转向面内。该结果揭示了对称性在非常规超导中的关键作用，所发展的方法有望推广至其他非常规超导体。
The family of nickelate superconductors have long been explored as analogs of the high temperature cuprates. Nonetheless, the recent discovery that certain stoichiometric nickelates superconduct up to high $T_c$ under pressure came as a surprise. The mechanisms underlying the superconducting state remain experimentally unclear. In addition to the practical challenges posed by working in a high pressure environment, typical samples exhibit anomalously weak diamagnetic responses, which have been conjectured to reflect inhomogeneous `filamentary’ superconducting states. We perform wide-field, high-pressure, optically detected magnetic resonance spectroscopy to image the local diamagnetic responses of as grown La$_3$Ni$_2$O$_7$ samples \emph{in situ}, using nitrogen vacancy quantum sensors embedded in the diamond anvil cell. These maps confirm significant inhomogeneity of the functional superconducting responses at the few micron scale. By spatially correlating the diamagnetic Meissner response with both the local tensorial stress environment, also imaged \emph{in situ}, and stoichiometric composition, we unravel the dominant mechanisms suppressing and enhancing superconductivity. Our wide-field technique simultaneously provides a broad view of sample behavior and excellent local sensitivity, enabling the rapid construction of multi-parameter phase diagrams from the local structure-function correlations observed at the sub-micron pixel scale.
Unconventional charge and spin density-wave states are commonly observed in bilayer nickelates, drawing considerable attention due to their proximity to high-$T_c$ superconductivity in various phase diagrams. However, the nature and mechanisms of charge and spin density-waves (DWs) in nickelates remain poorly understood. Numerous experiments have reported that the charge-density-wave (CDW) transition temperature $T_{ cdw}$ and the spin-density-wave (SDW) transition temperature $T_{sdw}$ are closely related but distinct. However, in contrast to these experiments, previous mean-field-type analyses have yielded only a simple SDW phase. To resolve this key problem, this paper demonstrates that sizable CDW instabilities emerge in proportion to the SDW instability in La$3$Ni$2$O$7$.This behavior is driven by the paramagnon-interference (PMI) mechanism, which captures important electron correlations beyond mean-field theory. Therefore, (i) experimental CDW + SDW coexisting state is naturally explained. In addition, (ii) the CDW + SDW fluctuations cooperatively drive high-$T_c$ superconductivity. Notably, the predicted $s$-wave SC state is robust against the inner apical O vacancies. Furthermore, (iii) the CDW instability is highly sensitive to the size of the $d_{z^2}$-orbital hole pocket, allowing for the realization of CDW quantum criticality through carrier-doping and pressure application. We find that the coexistence of charge and spin fluctuations is essential in bilayer nickelates, with both playing a cooperative role in mediating high-$T_c$ superconductivity.
Unconventional charge- and spin-density-wave states are commonly observed in bilayer nickelates, drawing considerable attention due to their proximity to high-transition temperature ( $${T}_{{\rm{c}}}$$) superconductivity. However, the nature and origin of these density waves remain poorly understood. Experiments show that the charge-density-wave and spin-density-wave transition temperatures are closely related but distinct, while mean-field-type analyses typically have yielded only a simple spin-density-wave phase. To resolve this key problem, this paper demonstrates that sizeable charge-density-wave instabilities emerge in proportion to spin-density-wave instabilities in La3Ni2O7 due to the paramagnon-interference mechanism, which captures electron correlations beyond mean-field theories. Therefore, (i) the experimental charge- and spin-density-wave coexisting state is naturally explained, and (ii) charge- and spin-density-wave fluctuations cooperatively drive high- $${T}_{{\rm{c}}}$$superconductivity. Furthermore, the predicted s-wave superconducting state is robust against the inner-apical oxygen vacancies. We find that the coexistence of charge- and spin-fluctuations is essential in bilayer nickelates, with both playing a cooperative role in mediating high- $${T}_{{\rm{c}}}$$superconductivity.
The discovery of pressure-induced superconductivity in two- and three-layer Ruddlesden-Popper nickelates has generated significant interest in these materials as a platform for unconventional superconductivity. While their ground state exhibits magnetism, a direct determination of their magnetic structure remains elusive. Understanding this aspect is crucial, as magnetism may play a role in the pairing mechanism of superconductivity in these compounds. We resolve the magnetic structures of the bilayer (2222) polymorphs of La3Ni2O7 and La2PrNi2O7 using neutron powder diffraction (NPD) and muon-spin rotation/relaxation (muSR). Magnetic neutron scattering appears below approximately 150 K in both compounds and is observed at the (qx, 1/2, 0) position, with qx = 0 and 1/2 for La3Ni2O7 and qx = 0 for La2PrNi2O7. Within a single layer, alternating low (0.05 - 0.075 muB) and high (0.66 muB) magnetic moment stripes form. These layers stack antiferromagnetically along the c-direction to form bilayers. The presence of two propagation vectors (qx = 0 and 1/2) in undoped La3Ni2O7 suggests the coexistence of two magnetic stacking polymorphs within a single crystallographic phase. The muSR spectra further confirm these magnetic structures. Our findings provide a detailed understanding of the magnetic ground state in bilayer nickelates, offering insights into possible precursor states that may influence the emergence of superconductivity in these materials.
The recent discovery of superconductivity in La3Ni2O7−δ under high pressure with a transition temperature around 80 K (ref. 1) has sparked extensive experimental2–6 and theoretical efforts7–12. Several key questions regarding the pairing mechanism remain to be answered, such as the most relevant atomic orbitals and the role of atomic deficiencies. Here we develop a new, energy-filtered, multislice electron ptychography technique, assisted by electron energy-loss spectroscopy, to address these critical issues. Oxygen vacancies are directly visualized and are found to primarily occupy the inner apical sites, which have been proposed to be crucial to superconductivity13,14. We precisely determine the nanoscale stoichiometry and its correlation to the oxygen K-edge spectra, which reveals a significant inhomogeneity in the oxygen content and electronic structure within the sample. The spectroscopic results also reveal that stoichiometric La3Ni2O7 has strong charge-transfer characteristics, with holes that are self-doped from Ni sites into O sites. The ligand holes mainly reside on the inner apical O and the planar O, whereas the density on the outer apical O is negligible. As the concentration of O vacancies increases, ligand holes on both sites are simultaneously annihilated. These observations will assist in further development and understanding of superconducting nickelate materials. Our imaging technique for quantifying atomic deficiencies can also be widely applied in materials science and condensed-matter physics.
本研究通过定制金刚石对顶砧旋转器，对Pr₄Ni₃O₁₀单晶进行原位高压角度依赖电输运测量，证实了其超导各向异性。在50.2 GPa压力下，样品出现超导转变，临界温度约31 K。通过测量垂直和平行于ab面的上临界场，得到各向异性参数γ约为1.6，该值随温度升高而减小，在接近超导临界温度时趋近于1。根据Ginzburg‑Landau模型拟合，零温上临界场平行和垂直于ab面分别为89.9 T和57.3 T，并计算出ab面内和c轴方向的相干长度分别为2.4 nm和1.5 nm。与铜氧化物及铁基超导体对比发现，Pr₄Ni₃O₁₀的各向异性行为符合双带模型，其中层内量子限域诱导了层间相干性，从而呈现出三维超导特性。该研究不仅证实了块体Ruddlesden‑Popper镍酸盐中存在各向异性超导，而且为理解维度对高温超导机制的影响提供了关键见解。
针对双层镍酸盐La₃Ni₂O₇中配对机制的争议，本文利用角分辨光电子能谱（ARPES）和扫描隧道显微镜（STM）揭示的低各向异性无节点全间隙作为约束，提出以布里渊区对角线上的配对能隙为关键探针。对称性分析表明，沿该对角线上dx2-y2与dz2轨道间的杂化消失，使得γ口袋和α/β口袋的能隙分别反映两个轨道的本征配对强度。基于dz2轨道主导的杂化驱动配对机制会导致α/β口袋在对角线方向出现能隙节点，与实验观测的U形dI/dV谱相矛盾；而dx2-y2轨道主导的洪特规则驱动配对机制则产生整个费米面上的均匀全间隙，与ARPES和STM结果一致。弱耦合随机相近似计算也因dz2轨道的态密度优势在对角线附近给出节点或近节点行为，与实验冲突。因此，该工作澄清了dx2-y2轨道在配对中的主导地位，并确立洪特规则驱动的配对机制为La₃Ni₂O₇中最相关的超导配对图像。