Paper

Layered perovskites─including the Dion–Jacobson, Ruddlesden–Popper, and Aurivillius families─exhibit a wide range of correlated electron phenomena, from high-temperature superconductivity to multiferroicity. Here, we report a new family of layered perovskites realized through topochemical oxidation of Lan+1NinO3n+1+δ (n = 1–4) Ruddlesden–Popper nickelate thin films. Postgrowth ozone annealing induces a substantial c-axis expansion─17.8% for La2NiO4+δ (n = 1)─that monotonically decreases with increasing n. Surface synchrotron X-ray diffraction and coherent Bragg rod analysis (COBRA) reveal that this structural expansion arises from the intercalation of approximately δ ≈ 0.7–1.0 oxygen atoms into interstitial sites within the rock salt spacer layers, far exceeding the previous record of δ ≈ 0.3 for any Ruddlesden–Popper oxide. These oxygen-intercalated phases form a new class of layered perovskites with a spacer layer composition intermediate between the Ruddlesden–Popper and Aurivillius phases. Furthermore, oxygen intercalation induces metallicity, enhances nickel–oxygen hybridization, and suppresses oxygen octahedral rotations, a feature associated with high-temperature superconductivity in Ruddlesden–Popper nickelates. Our work establishes topochemical oxidation as a powerful approach to accessing highly oxidized, metastable phases across a broad range of layered oxide systems, offering new platforms to engineer electronic properties via intercalation chemistry.

Topotactical Hydrogen Induced Single-Band d -Wave Superconductivity in La₂ NiO 4

本研究利用压力-温度单晶X射线衍射与从头算密度泛函理论计算，揭示助熔剂生长的三层镍酸盐La₄Ni₃O₁₀中四方相（I4/mmm）到单斜相（P2₁/c）的转变不存在中间斜方Bmab相，而是一个直接的结构相变，并伴随两倍超结构的形成，体现为公度超晶格反射的出现。该转变温度可在14 GPa压力下从约1030 K连续抑制至20 K，说明压力有效稳定了四方相。此外，在助熔剂生长晶体中首次通过X射线衍射探测到与非公度密度波有序相关的微弱卫星反射，补充了此前仅见于浮区法晶体的结果，拉曼光谱在130 K以下观察到额外声子模式也进一步印证了该有序态。从头算计算与实验观察吻合。这项工作澄清了La₄Ni₃O₁₀结构对称性的长期争议，并支持超导电性在高压恢复四方对称性后出现，为后续电子结构及超导机理研究提供了关键的晶体学基础。

The metal-insulator transition of La 3 Ni 2 O 7-δ with 2-dimensional electrons has been studied. Various physical properties can be understood by introducing a model of charge orderings in the NiO …

该研究构建了双层镍氧化物超导能隙结构的微观理论模型，模型中dx²-y²对称性的传导带与局域化的d3z²-r²自旋共存。强层间耦合使局域磁矩形成单重态基态，其虚拟的单重态-三重态激发（即“triplon”）介导了传导电子间的配对相互作用，由此产生带间s±波配对，两个能带（α和β）上的序参量符号相反。理论结果自然解释了实验中观测到的关键特征：尽管α带态密度较小，但其超导能隙更大，且能隙呈现出由非局域Kondo耦合导致的显著动量空间各向异性。这些发现有力支持了triplon介导的配对机制作为双层镍氧化物超导的微观起源。

通过第一性原理计算与奇异模式泛函重整化群方法，系统研究了La3Ni2O7/LaAlO3薄膜中层间镍-镍距离对基态的影响。结果显示，较小的层间距导致C型自旋密度波（层间铁磁耦合），较大的层间距则产生G型自旋密度波（层间反铁磁耦合），而在这两种相之间，出现了以镍3d₃z²⁻ᵣ²轨道配对为主导的s±波超导态。该结果解释了实验中薄膜在常压下超导的成因，并预测施加压力会降低超导转变温度，直至系统进入C型自旋密度波。这一预言若经实验验证，将为理解该体系中电子关联的本质提供深刻见解，因为C型自旋密度波在巡游电子图像中自然实现，而在局域磁矩图像中则难以形成（后者层间自旋始终反铁磁耦合）。

通过结合密度泛函理论和随机相位近似，系统研究了压缩和拉伸应变对1313-La3Ni2O7薄膜超导电性的影响。研究发现，无论压应变还是拉应变，单层与三层块体间均存在自掺杂效应，且在拉伸应变下最为显著。在LSAO衬底引入的压缩应变下，即使考虑衬底锶离子迁移带来的空穴掺杂，超导电性也难以出现，这与实验一致。然而，在KTO衬底施加的拉伸应变下，三层子系统中原本未穿过费米能级的能带下移，在M点出现一个小型空穴型γ口袋，与Γ点的小型电子型σ口袋通过近(π,π)波矢相连。随机相位近似计算揭示，此时三层子系统可形成稳定的s±波配对态，其序参量在这两个口袋间发生符号反转。进一步分析表明，γ口袋大小对配对至关重要，过大的γ口袋会抑制超导。该工作预测了应变驱动的电子结构重构，并提出通过拉伸应变工程可在环境压力下实现1313-La3Ni2O7超导电性的设计原则。

本研究开发了可在高达40 GPa、7 T磁场和5 K低温下同时工作的超快磁压力光谱平台，并将其应用于三层镍酸盐Pr₄Ni₃O₁₀中准粒子动力学的磁压力演化探测。实验观察到电荷密度波（CDW）转变附近准粒子弛豫出现显著的临界减慢现象，该现象在施加压力后消失。在更高压力下，低温弛豫时间反而变长，与初始超导关联特征一致。然而，高达7 T的磁场几乎不改变弛豫行为，且未观察到涡旋诱导的预瓶颈动力学——后者在对照体超导样品中是稳健特征——表明当前压力条件下可能的超导态并非体态，而是丝状或强不均匀的。该磁压力超快能力为解决关联量子材料中压力诱导超导电性与交织有序的悬而未决问题开辟了新途径。

Ultrafast optical evidence of coexisting density waves in bilayer nickelate La₃Ni₂O₇

The discovery of high-temperature superconductivity in La3⁢Ni2⁢O7 at pressures above 14 GPa has spurred extensive research efforts. Yet, fundamental aspects of the superconducting phase, including the possibility of a filamentary character, are currently subjects of controversial debates. Conversely, a crystal structure with NiO6 octahedral bilayers stacked along the 𝑐-axis direction was consistently posited in initial studies on La3⁢Ni2⁢O7. Here, we reassess this structure in optical floating zone-grown La3⁢Ni2⁢O7 single crystals that show signs of filamentary superconductivity. Employing scanning transmission electron microscopy and single-crystal x-ray diffraction under high pressures, we observe multiple crystallographic phases in these crystals, with the majority phase exhibiting alternating monolayers and trilayers of NiO6 octahedra, signifying a profound deviation from the previously suggested bilayer structure. Using density functional theory, we disentangle the individual contributions of the monolayer and trilayer structural units to the electronic band structure of La3⁢Ni2⁢O7, providing a firm basis for advanced theoretical modeling and future evaluations of the potential of the monolayer-trilayer structure for hosting superconductivity.在高于 14 GPa 的压力下， La3⁢Ni2⁢O7 中高温超导性的发现激发了广泛的研究。然而，超导相的基本性质，包括其丝状特征的可能性，目前仍存在争议。相反，在对 La3⁢Ni2⁢O7 的早期研究中，人们一直假设其晶体结构为沿 𝑐 轴方向堆叠的 NiO6 八面体双层。本文中，我们重新评估了光学浮区法生长的 La3⁢Ni2⁢O7 单晶中的这种结构，这些单晶表现出丝状超导的迹象。利用扫描透射电子显微镜和高压下的单晶 X 射线衍射技术，我们观察到这些晶体中存在多种晶相，其中主要相由 NiO6 八面体的单层和三层交替构成，这表明其与先前提出的双层结构存在显著偏差。利用密度泛函理论，我们分离出单层和三层结构单元对 La3⁢Ni2⁢O7 电子能带结构的各个贡献，为高级理论建模和未来评估单层-三层结构承载超导性的潜力提供了坚实的基础。