磁受挫诱导的非常规超导材料探索和物性研究

The achievements in understanding and application of superconductors relies strongly on the discovery of new superconducting materials. As a significant characteristic of the iron base superconductors and a majority of the unconventional superconductors, superconductivity is usually found as the neighbor of the antiferromagnetic (AFM) state. The key to induce superconductivity is whether the AFM ordering in the parent material can be suppressed effectively. Compared with the traditional carrier doping and chemical pressure methods, magnetic frustration is induced by the crystal structure and hence not limited by the solid solution limit and the chemical stability of the parent material, and meanwhile it can efficiently suppressed the AFM ordering. The latest results on FeSe superconductors shows that magnetic frustration is responsible for the absence of AFM ordering and even superconductivity in the parent FeSe system, which could be expected to be an important method for the exploration of new high-TC materials. This project, based on our former research on superconductors, aims at exploring new superconducting materials with crystal structures that inherent with magnetic frustration effects, including triangular lattice, Kagome lattice, pyrochlore lattice etc. We further introducing carrier doping and chemical pressure to the already unstable AFM ground state found in the majority of geometric magnetic frustration materials, and trying to explore and build novel magnetic frustrated superconducting materials with controllable carrier density and chemical stress. Based on our extensive experiences in the new superconductor investigation and crystal growth, our project intent to provide a more in-depth understanding on the electromagnetic properties of the magnetic frustrated materials under carrier doping or chemical stress etc., and pave the way for understanding their superconducting mechanism.

新超导体的发现对超导领域发展具有重要推动作用。作为铁基以及多数非常规超导体的显著特征，超导电性常出现在反铁磁序不稳定的区域，诱导超导电性的关键在于抑制母体材料的长程磁有序。相对于载流子注入和化学压力等常规手段，由晶体结构诱导的磁受挫机制可避免材料固溶极限和结构稳定性的影响，且可有效抑制母体的反铁磁序。最新工作表明，磁受挫机制对于诱导FeSe等体系的高温超导电性有重要作用，该机制有望成为探索新型高温超导材料的重要方法。基于前期在新超导材料领域的工作基础，本项目提出在三角型晶格、Kagome晶格、烧绿石结构等几种典型磁受挫体系中，利用其反铁磁序不稳定的基态，通过掺杂取代、化学插层等物性调控技术，额外引入载流子和化学应力，尝试探索和构筑新型磁受挫超导材料。进而生长高质量晶体，系统研究其结构物性，深入理解载流子及化学压力等对磁受挫材料电磁性质的调控机制，揭示磁受挫超导材料的超导机理。

作为宏观量子现象的超导电性具有重要的基础科学价值和巨大的应用前景。前期工作表明，磁受挫机制对于诱导FeSe等体系的高温超导电性有重要作用，该机制有望成为探索新型高温超导材料的重要方法。在本项目的支持下，项目组针对FeSe，FeAl2Se4，Fe4Nb2O9等一批三角晶格、四方晶格及Kagome晶格的磁性及磁阻挫体系开展了系统研究，表征了相关材料的本征磁性、磁阻措及非传统超导电性。其中，针对FeSe，FeAl2Se4等磁性被充分抑制的材料，开发了包括化学插层、水热脱嵌、等价取代等调控手段，系统研究了载流子注入以及化学应力对高温超导电性的影响。进一步在FeSe基高温超导体的连续电子掺杂、空穴掺杂和化学应力调控等方向上做出了部分原创性成果。发表包括JACS (1篇)、Adv. Func. Matt. (1篇)、 Phy. Rev. B (4篇)和Phy. Rev. M (2篇)在内的SCI学术论文12篇。.主要代表性成果包括：.1.生长了三角晶格反铁磁体FeAl2Se4的单晶，因磁阻措，该材料在低至0.4 K时未检测到长程有序。其比热在低温下呈现罕见的双峰结构和T2幂律，这分别归因于材料内潜在的四极自旋关联和Halperin-Saslow模式。我们的结果与具有海森堡和双二次自旋相互作用的自旋-1系统的早期理论一致，该发现提供了对于高自旋受挫量子磁体磁性的新认识。.2.针对磁阻措母体FeSe等材料，创新性的发展了一种低温离子置换的方法，克服了范德瓦尔斯材料中阴离子插层的困难。成功向 FeSe、NiSe、FeS 等材料中引入了大量 S/Se 离子，并制备出了首个空穴掺杂的 FeSe 基超导体 (S/Se)x(NH3)yFe2Se2，揭开FeSe空穴掺杂研究帷幕。.3.通过低温中子衍射数据，确定了三方晶格Fe4Nb2O9在奈尔温度以下的磁结构，并揭示了其T-str的结构相变。通过对称相关张量分析，证明了T-N和T-str之间磁电效应的微观起源是自旋电流和d-p杂化机制。.4. 利用化学压力效应，在FeSe1-xSx系统中实现了高达37.5K的超导转变，为下一步铁硫族高温超导体的探索，打下了新的材料基础。.上述项目成果推动了铁基超导材料及磁阻挫材料领域的研究和发展。