Paper

High-pressure studies have revealed superconductivity in La4⁢Ni3⁢O10, sparking interest in its ambient-pressure properties and the underlying electronic correlations. Motivated by experimental observations of an incommensurate spin-density wave (SDW) at ambient pressure, we investigate the SDW characteristics and possible superconductivity in La4⁢Ni3⁢O10 using a multiorbital random-phase approximation (RPA). Starting with a 12-orbital tight-binding model derived from density functional theory (DFT) calculations, we include Hubbard interactions to explore the interplay between electronic correlations and magnetic instabilities. Our analysis reveals a stripe-like SDW with a wave vector 𝐐≈(±0.7⁢𝜋,0), suggesting a possible density wave instability in agreement with experiments. This configuration is driven by nesting between the 𝛼1 pocket, primarily contributed by the outer-layer Ni 𝑑𝑧2 orbitals, and the 𝛽1 pocket, contributed by both the 𝑑𝑧2 and 𝑑𝑥2−𝑦2 orbitals of the outer layer. It exhibits interlayer antiferromagnetic ordering between the top and bottom NiO layers, with the magnetic moment of the middle layer being nearly zero. We demonstrate that the Hund coupling 𝐽𝐻 is the primary driver of the observed SDW and determine the specific criterion: 𝐽𝐻>0.16⁢𝑈. Building upon our findings on the SDW mechanism, we further demonstrate that hole doping (𝛿=−0.4) enhances Fermi surface nesting, leading to the emergence of a superconducting state with a gap structure similar to that of the high-pressure phase.

本研究成功合成了高质量的长程有序杂化1313型La₃Ni₂O₇单晶（交替单层-三层结构），并对其物理性质进行了系统表征。在常压下，该材料呈现典型的半导体行为，并在170 K处出现明显的电阻率、磁化率和比热异常。¹³⁹La核磁共振谱明确证实该异常源于自旋密度波（SDW）相变。高压电输运测量表明，施加压力可以诱导金属化，但在高达65 GPa的压力下未观察到超导电性。这些发现将杂化1313型La₃Ni₂O₇确立为Ruddlesden-Popper镍酸盐家族中具有独特SDW转变的新成员，为研究杂化镍酸盐中晶体结构、电子序和超导电性之间的相互作用提供了新的平台。

采用密度泛函理论结合团簇动力学平均场方法，本文研究了双层Ruddlesden-Popper镍酸盐，发现其物理性质主要受由3z²−r²轨道单电子形成的层间“动力学单态”支配，这些单态与巡游的x²−y²平面轨道发生杂化。该杂化对静水压与面内压应变的响应截然不同：应变增强层间关联，导致轨道选择性单态配对莫特机制，而静水压主要提升面内巡游性。这一差异解释了块材与应变薄膜在角分辨光电子能谱和输运测量中的实验分歧。理论框架将此低能态视为动力学单态与巡游轨道的杂化体系，为后续理解超导电性提供了新思路。

Stabilizing and tuning superconductivity in La₃Ni₂O₇₋δ films: Oxygen recycling protocol reveals hole-doping analogue

该工作利用扫描SQUID显微镜研究了分子束外延生长在LSAT衬底上的最优掺杂无限层镍酸盐Nd1-xEuxNiO2（x=0.25）薄膜的局域超流密度与磁结构。同时进行的直流磁强计和交流磁化率成像显示，超导与弱磁织构共存，且超导态在空间上高度不均匀：接近超导转变时出现稳健的条纹状超导增强，并伴有光晕状缺陷响应及条纹间顺磁区域。这些结果表明，竞争磁相是导致该最优掺杂NENO样品超导转变异常展宽的主要因素；理解这种超导增强机制可能为提升此类材料的超导温度提供途径。

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.

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。