[论文解读] Structure determination and coexistence of superconductivity and antiferromagnetic order in (Li0.8Fe0.2)OHFeSe

X. F. Lu, N. Z. Wang+11 more|arXiv (Cornell University)|Aug 9, 2014

Iron-based superconductors research参考文献 15被引用 168

一句话总结

本研究通过一种新颖的水热法合成了空气稳定的 (Li0.8Fe0.2)OHFeSe，其超导转变温度约为 40 K。结合 X 射线衍射与中子粉末衍射以及核磁共振技术，作者明确确定了晶体结构，并证实了超导性与反铁磁有序共存，为研究铁基超导体机制提供了稳定平台。

ABSTRACT

FeSe-derived superconductors show some unique behaviors relative to iron-pnictide superconductors, which are very helpful to understand the mechanism of superconductivity in high-Tc iron-based superconductors. The low-energy electronic structure of the heavily electron-doped AxFe2Se2 (A=K, Rb, Cs) demonstrates that interband scattering or Fermi surface nesting is not a necessary ingredient for the unconventional superconductivity in iron-based superconductors. The superconducting transition temperature (Tc) in the one-unit-cell FeSe on SrTiO3 substrate can reach as high as ~65 K, largely transcending the bulk Tc of all known iron-based superconductors. However, in the case of AxFe2Se2, the inter-grown antiferromagnetic insulating phase makes it difficult to study the underlying physics. Superconductors of alkali metal ions and NH3 molecules or organic-molecules intercalated FeSe and single layer or thin film FeSe on SrTiO3 substrate are extremely air-sensitive, which prevents the further investigation of their physical properties. Therefore, it is urgent to find a stable and accessible FeSe-derived superconductor for physical property measurements so as to study the underlying mechanism of superconductivity. Here, we report the air-stable superconductor (Li0.8Fe0.2)OHFeSe with high temperature superconductivity at ~40 K synthesized by a novel hydrothermal method. The crystal structure is unambiguously determined by the combination of X-ray and neutron powder diffraction and nuclear magnetic resonance. It is also found that an antiferromagnetic order coexists with superconductivity in such new FeSe-derived superconductor. This novel synthetic route opens a new avenue for exploring other superconductors in the related systems. The combination of different structure characterization techniques helps to complementarily determine and understand the details of the complicated structures.

研究动机与目标

• 开发一种稳定、易获取的 FeSe 基超导体，用于物理性质测量，以克服现有体系的空气敏感性与相容性问题。
• 利用互补的衍射与光谱技术，精确确定 (Li0.8Fe0.2)OHFeSe 的晶体结构。
• 研究该新型超导体中超导性与磁有序共存的特性。
• 建立一种新的合成路线，用于探索相关 FeSe 基材料。

提出的方法

• 采用一种新颖的水热合成方法，制备出单相、空气稳定的 (Li0.8Fe0.2)OHFeSe 样品。
• 利用 X 射线粉末衍射（XRD）和中子粉末衍射（NPD）高精度测定晶体结构。
• 应用核磁共振（NMR）光谱技术探测局部电子与磁环境。
• 综合 XRD、NPD 与 NMR 数据，实现对晶体结构的全面且无歧义的确定。
• 测量电输运性质与磁化率，以确认超导转变温度（Tc）。
• 分析磁性数据，以识别反铁磁有序的存在及其性质。

实验结果

研究问题

• RQ1 能否合成一种稳定、对空气不敏感的 FeSe 基超导体，以实现可靠的物理性质测量？
• RQ2 (Li0.8Fe0.2)OHFeSe 的精确晶体结构是什么？其与其它 FeSe 基相有何不同？
• RQ3 在 (Li0.8Fe0.2)OHFeSe 中，反铁磁有序是否与超导性共存？若共存，其条件为何？
• RQ4 XRD、NPD 与 NMR 技术的联合应用如何实现对复杂、无序体系的精确结构表征？

主要发现

• 新颖的水热法成功制备出单相、空气稳定的 (Li0.8Fe0.2)OHFeSe，其超导转变温度（Tc）约为 40 K。
• X 射线与中子粉末衍射无歧义地确定了晶体结构，揭示其为具有有序 OH 与 Li/Fe 分层排列的层状四角结构。
• NMR 测量证实了局域磁有序的存在，表明反铁磁有序与超导性共存。
• 磁化率与电阻率测量在约 40 K 处显示出清晰的超导转变，未发现竞争相的迹象。
• XRD、NPD 与 NMR 的结合提供了互补的结构信息，解决了复杂、无序体系中的结构模糊性问题。
• 该体系中超导性与反铁磁有序的共存表明电子相之间存在复杂相互作用，对理解高温铁基超导体机制具有重要意义。