常压下重锶掺杂的La$_{2}$SrNi$_{2}$O$_{7-δ}$作为四方Ruddlesden-Popper相
Heavily Sr-Doped La$_{2}$SrNi$_{2}$O$_{7-δ}$ as a Tetragonal Ruddlesden-Popper Phase at Ambient Pressure
- Nanjing University（南京大学）
- Sun Yat-sen University（中山大学）
- Anhui University（安徽大学）
机构由 AI 辅助整理，请以论文原文为准。
AI总结：
该研究合成了重锶掺杂的La$_{2}$SrNi$_{2}$O$_{7-δ}$，证实其为常压下的四方RP相，发现该材料无超导电性，归因于Ni-3d$_{z^2}$带偏离半填充状态，为双层镍酸盐超导研究提供了关键线索。
AI中文摘要：
高压下的块体双层Ruddlesden-Popper（RP）镍酸盐或通过压缩应变制备的薄膜中已发现高温超导电性。超导态下，通常观测到沿c轴具有直Ni-O-Ni键的四方结构，同时密度波序被抑制或消失。因此，这些因素是否足以在常压下实现超导电性仍是未解决的问题。本研究首次通过熔剂法在高温高压（HPHT）条件下成功合成了重锶掺杂的La$_{2}$SrNi$_{2}$O$_{7-δ}$。X射线衍射和扫描透射电子显微镜（STEM）证实该材料采用四方（I4/mmm）结构，沿c轴的Ni-O-Ni键角为180°。电阻测量显示其具有金属行为，低温下出现回升，且未观测到密度波特征。然而，压力或氧含量变化均未诱导出超导电性。密度泛函理论计算表明，锶掺杂引入的空穴主要掺杂到Ni-3d$_{z^2}$轨道，导致常压下费米面上出现极大的γ口袋，并显著降低Ni-3d$_{z^2}$轨道的占据。结合实验观测与理论计算，将超导电性的缺失归因于Ni-3d$_{z^2}$带严重偏离半填充状态，而该状态对于层间反铁磁相互作用及配对至关重要。本研究揭示了双层镍酸盐体系实现超导电性的重要问题。
英文摘要：
High-temperature superconductivity has been found in bilayer Ruddlesden-Popper (RP) nickelates in bulk samples under high pressure, or in thin films via compressive strain. In the superconducting state, a tetragonal structure with a straight Ni-O-Ni bond along c-axis has been commonly observed, together with the suppression or diminishing of the density-wave orders. Therefore, it remains an open question whether these factors are sufficient for achieving superconductivity at ambient pressure. Here we report the first successful synthesis of heavily Sr-doped La$_{2}$SrNi$_{2}$O$_{7-δ}$ under high-pressure and high-temperature (HPHT) conditions with a flux method. X-ray diffraction and scanning transmission electron microscopy (STEM) confirm that the material adopts a tetragonal (I4/mmm) structure with an 180$^{\circ}$ Ni-O-Ni bond angle along c-axis. Resistance measurements reveal metallic behavior with a low-temperature upturn and no density-wave features are observed. However, neither pressure nor oxygen variation induces superconductivity. Density functional theory calculations indicate that the holes introduced by Sr doping are predominantly doped into the Ni-3d$_{z^2}$ orbital, leading to the appearance of a very large $γ$ pocket on the Fermi surface at ambient pressure and significantly reducing the occupation of the Ni-3d$_{z^2 }$ orbital. Combining the experimental observations with theoretical calculations, we attribute the absence of superconductivity to the serious deviation from the half-filling state of the Ni-3d$_{z^2 }$ band, which is crucial for the interlayer antiferromagnetic interaction and thus for pairing. Our work unravels important issues for achieving superconductivity in bilayer nickelate system.
