AxFe2-ySe2（A=K，Rb，Cs）高温超导体及相关化合物的中子散射研究

In 2008, the discovery of iron based superconductors has attracted great attention, because these new superconductors could help to unravel the mystery of high Tc superconductivity. Since its discovery, significant progress had been made in understanding the magnetism and electronic structure of this system. It was widely believed that the Fermi surface nesting was important for both antiferromagnetsm and electron pairing. However, the discovery of AxFe2-ySe2 (A=K, Rb, Cs) superconductors have challenged the Fermi surface nesting scenario because the antiferromagnetism and band structure of AxFe2-ySe2 seem to be completely different from those of other iron based superconductors. Moreover, as phase separation often exists in AxFe2-ySe2 sample, it is difficult to measure the intrinsic properties of the superconducting phase. To date, it remains elusive on what is real magnetic ground state of the parent compound and how magnetism is coupled with superconductivity in this system. Here, we propose to use neutron scattering techniques to study the crystal structure, magnetic structure, spin excitations and their couplings with superconductivity in AxFe2-ySe2. The success of these measurements will shed some new light on the understanding of antiferromagnestim and its relationship with superconducting pairing of iron based superconductors.

2008年，铁基超导体的发现吸引了科学家们的广泛关注，因为它可能为解决高温超导机理提供新的线索。很快人们对铁基超导体的磁性和电子能带结构都有了较为深入的认识: 许多人认为费米面的嵌套对反铁磁的形成和超导配对都有重要的作用。可是在2010年底，KxFe2-ySe2 超导体的发现又激发了科学家们新的兴趣，因为这个体系的能带结构和磁结构似乎和别的铁基超导体有很大的区别，它对费米面嵌套模型提出了挑战。由于KxFe2-ySe2样品常常存在相分离现象，这使得对超导相及其母体相的本征性质的研究存在一定的难度, 因此这个体系的母体的基态以及磁与超导配对的关系仍不清楚。在本项目中，我们将利用中子散射技术对KxFe2-ySe2超导体及其可能的半导体态的母体化合物的晶格结构，磁结构以及自旋激发与超导电性的耦合进行系统深入的研究，期望能理解反铁磁，特别是局域磁矩的反铁磁对超导配对的贡献。

2010年，KxFe2-ySe2 高温超导体的发现激发了研究人员的广泛兴趣，因为这个体系的能带结构和磁基态和别的铁基超导体(FeAs类)有很大的区别，它对人们在FeAs类超导体中建立的费米面嵌套模型提出了挑战。因此理解 KxFe2-ySe2以及相关的FeSe类超导体的磁和超导以及电子结构的关系对全面的理解高温超导机理意义重大。在本项目的支持下，研究团队在这个方向的主要研究成果如下： (1)率先在KxFe2-ySe2中发现条纹反铁磁半导体相并确定其磁哈密顿量[J. Zhao* et al., Phys. Rev. Lett. 112, 177002 (2014)]；(2)率先在硫掺杂的KxFe2-ySe2-zSz发现反相位库珀配对到同相位配对的新奇相变[Q. Wang, J. Zhao* et al., Phys. Rev. Lett. 116, 197004 (2016)]； (3)发现铁硒中磁涨落和向列相、超导的强耦合[Q. Wang , J. Zhao * et al., Nature Materials 15, 159-163 (2016)]；（4）首次在铁硒中发现奈尔和条纹自旋涨落共存并实验确立了其磁基态[Q. Wang, J. Zhao* et al., Nature Communications 7, 12182 (2016)]；（5）首次发现重电子掺杂铁硒材料中扭曲的自旋激发色散关系[ B. Pan，J. Zhao* et al., Nature Communications 8, 123 (2017)]。