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After several decades of studies of high-temperature superconductivity, there is no compelling theory for the mechanism yet; however, the spin fluctua…

The discovery of superconductivity with an onset temperature of ∼80 K in pressurized bilayer Ruddlesden-Popper La3Ni2O7-δ has attracted much attention. Despite intense research, determination of the exact oxygen content and understanding of the relationship between superconductivity and oxygen content remain a big challenge. Here, we report a systematical study on the structure and physical properties of La3Ni2O7-δ polycrystalline powders which were prepared using the sol-gel method at ambient pressure and then annealed under high oxygen pressure (pO2) or in ozone. The superconducting transition of La3Ni2O7-δ at ∼80 K under high pressure is suppressed for high pO2 and ozone annealed samples. We attribute this to the combination of the following two reasons: (i) damage of the bilayer structure, as revealed by powder X-ray diffraction, scanning transmission electron microscopy and pair distribution function measurements, and (ii) hole overdoping due to the increasing of oxygen content. Our results reveal that the bilayer structure in La3Ni2O7-δ is fragile and post-annealing under mild oxidization is suitable for maintaining the integrity of the bilayer structure and increasing oxygen content.

Ruddlesden-Popper (RP) nickelates are a promising class of high-temperature superconductors, with superconducting transition temperatures exceeding the boiling point of liquid nitrogen. However, oxygen nonstoichiometry remains a persistent challenge that commonly presents in all RP nickelates. Understanding the formation of oxygen vacancies, their ordering patterns, and their impact on superconductivity is crucial, especially in the newly discovered L⁢a4⁢N⁢i3⁢O10. In this study, the first-principles structural calculations reveal the formation of in-plane oxygen vacancy chains in L⁢a4⁢N⁢i3⁢O10, a key structural feature observed under both ambient and pressurized conditions. These vacancies induce significant lattice distortion and generate residual electrons that hybridize with the Ni 𝑑𝑧2 orbital, altering the sign of the hopping integral between the 𝑑𝑧2 orbitals. Furthermore, the vacancies alter the N⁢i2+/N⁢i3+ ratio, lower the 𝑑𝑧2 orbital energy, and decrease the 𝑑𝑧2 orbital density of states at the Fermi level. Interestingly, at higher vacancy concentrations, the vacancy chains tend to align diagonally along the out-of-plane direction. The phase diagram of L⁢a4⁢N⁢i3⁢O10 exhibits a narrow stability range at ambient pressure, which expands under applied pressure, aligning with the high-oxygen-pressure conditions required for its synthesis. Importantly, these vacancy chains broaden the optical conductivity peak, which could serve as a marker for their detection. Our findings offer valuable insights into the distribution of oxygen vacancies, their role in modifying the electronic structure, and their influence on optical conductivity in L⁢a4⁢N⁢i3⁢O10.

The discovery of high-temperature superconductivity in bulk La3Ni2O7 under high hydrostatic pressure1−4 and biaxial compression in epitaxial thin films5−8 has ignited significant interest in understanding the interplay between atomic and electronic structure in these compounds. Subtle changes in the nickel-oxygen bonding environment are thought to be key drivers for stabilizing superconductivity, but specific details of which bonds and which modifications are most relevant remains so far unresolved. While direct, atomic-scale structural characterization under hydrostatic pressure is beyond current experimental capabilities, static stabilization of strained La3Ni2O7 films provides a platform well-suited to investigation with new picometer-resolution electron microscopy methods. Here, we use multislice electron ptychography (MEP)9,10 to directly measure the atomic-scale structural evolution of La3Ni2O7 thin films across a wide range of biaxial strains tuned via substrate choice. By resolving both the cation and oxygen sublattices, we study the strain-dependent evolution of atomic bonds, providing the opportunity to isolate and disentangle the effects of specific structural motifs for stabilizing superconductivity. We identify the lifting of crystalline symmetry through modification of the nickel-oxygen octahedral distortions under compressive strain as a key structural ingredient for superconductivity and identify in-plane lattice compression as a common attribute between bulk and thin film superconductivity. Building upon the detailed structures obtained by MEP, we introduce a theoretical framework to disentangle coupled structural distortions in corner-sharing octahedra11, which suggest that both known superconducting geometries of La3Ni2O7 (hydrostatic pressure and compressive strain) suppress local t2g orbital mixing in the low-energy Ni bands by raising the octahedral symmetry.

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该研究通过第一性原理计算和群论分析，系统研究了交替单层-三层（1313）堆叠的La₃Ni₂O₇多晶型的结构稳定性、电子结构与磁性质。常压下，最高对称性Cmmm结构在布里渊区高对称点存在多个不稳定声子支，其对应的畸变可导致实验上报道的另一种空间群Imma，该结构显示出NiO₆八面体倾斜。磁性分析表明，常压下该材料的电子结构主要由三层块主导，单层块处于Mott绝缘态。压力下，四方P4/mmm结构趋于稳定，与实验一致。研究揭示了八面体倾斜并非超导的必要条件，并阐明了不同空间群之间的对称关联及压力驱动的结构相变机制。

本文通过第一性原理计算研究了新合成的双无限层氧氟化物La₃Ni₂O₅F的结构稳定性、电子结构与磁性，并探讨了化学压力与外延应变的影响。声子谱证实了该晶体结构的动力学稳定性，其电子结构呈现高度二维的类铜氧化物费米面，主要由Ni-d_(x²-y²)态主导，伴随稀土离子自掺杂导致的~d^(1.2)有效填充，电荷转移能接近铜氧化物区间。化学压力和外延应变下，这类电子特征保持显著稳健，镍-d_(x²-y²)填充仅发生适度变化。自旋极化计算揭示出多种面内与面间自旋排列的近简并磁构型，压应变进一步增强磁阻挫，却不显著改变电子结构。因此，La₃Ni₂O₅F被确立为有前景的铜氧化物类似物，晶格工程可有效精细调控其电子与磁学性质。

本文利用功能重正化群方法，研究了双层镍酸盐La₃Ni₂O₇在常压与高压晶体结构下自旋密度波序与超导的竞争。通过对比两种结构的弱耦合多轨道模型，发现随着Hund耦合增大，主导不稳定性从超导转变为具有特征波矢Q₁≈(π/2,π/2)的自旋密度波，与实验一致。令人意外的是，常压与高压结构的非相互作用磁化率和fRG主导不稳定性几乎相同，表明压力下超导的出现不能仅由低能电子结构变化解释。进一步分析表明，抑制正交畸变是关键因素：当体系趋近四方极限时，对称性相关的自旋密度波涨落近乎简并，从而阻碍长程磁有序并增强配对相互作用。这些结果揭示了晶格对称性是调控双层镍酸盐中竞争有序态的核心参量，并提示通过单轴应变降低正交畸变有望在常压下实现体超导。

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Atomic structure and electronic band structure are fundamental properties for understanding the mechanism of superconductivity. Motivated by the discovery of pressure-induced high-temperature superconductivity at 80 K in the bilayer Rud-dlesden-Popper nickelate La3Ni2O7, the atomic structure and electronic band structure of the trilayer nickelate La4Ni3O10 under pressure up to 44.3 GPa are investigated. A structural transition from the monoclinic P21/a space group to the tetragonal I4/mmm around 12.6–13.4 GPa is identified, accompanied by a drop of resistance below 7 K. Density functional theory calculations suggest that the bonding state of Ni $$3{d_{{z^2}}}$$orbital rises and crosses the Fermi level at high pressures, which may give rise to possible superconductivity observed in resistance under pressure in La4Ni3O10. The trilayer nickelate La4Ni3O10 shows some similarities with the bilayer La3Ni2O7 and has unique properties, providing a new platform to investigate the underlying mechanism of superconductivity in nickelates.