新型铁基硫族超导体配对对称性及机理研究

Understanding the gap symmetry and structure should provide key clues to the microscopic pairing mechanism of the unconventional superconductors. Until now, the gap symmetry of iron chalcogenides (FeCh) is still widely debated and becomes the central issue of current superconductivity research. The newly discovered FeCh such as (Li1-xFex)OHFeSe, FeS and BaFe2S3 provide good platform for exploiting the microscopic mechanisms of FeCh-derived systems. Until now, theoretical works on these superconductors are still lacking, especially the investigation of the gap symmetry and structure. Thus, we are about to investigate the pairing symmetry and mechanism of the newly discovered FeCh using first principles and functional renormalization group studies. The main plan includes: 1. Using first principles calculations, we are about to obtain the electronic structure, magnetic sates and electron-phonon coupling. Meanwhile, the interplay between the nesting of Fermi surfaces and spin fluctuation are also discussed. Based on the bands near the Fermi energy, we can fitted them by MLWF and obtain effective tight-binding models with five Fe 3d orbitals. 2. Based on the effective models obtained in former step, we perform functional renormalized group (FRG) to study the competition/dependence between different ordered states. Moreover, the leading pairing instability and the gap function are also obtained. The aim of this project is to study the pairing symmetry and mechanism of newly discovered FeCh in theoretical perspective. Furthermore, we expect to provide critical clues to the microscopic pairing mechanism in other iron chalcogenides.

超导配对对称性及能隙结构是揭示超导微观机理的关键，但当前铁基硫族超导体（FeCh）的配对对称性仍存有广泛争议，是超导领域的热点问题。最近发现的(Li1-xFex)OHFeSe、FeS及BaFe2S3等新型FeCh拥有奇特结构或物性，为研究FeCh超导配对对称性及机理提供了良好平台。但目前这些新型超导体的理论研究还很欠缺，其配对对称性及能隙结构这一基本问题还亟待解决。本项目拟使用第一性原理和泛函重整化群等理论方法研究这些新型FeCh的配对对称性及机理。主要研究内容为：1.利用第一性原理研究其电子结构、磁性基态和电声耦合等，探讨费米面嵌套和磁性涨落对超导配对的影响，构建有效模型；2.在有效模型基础上考虑电子关联效应，利用泛函重整化群研究有序态竞争/依赖关系，揭示超导配对具体形式。本项目旨在理论研究新型FeCh的超导配对对称性和机理，也将为其它FeCh超导配对对称性和微观机理的判别提供关键信息。

铁基超导体的相关研究是近年来凝聚态理论研究的热点问题之一。在本项目中，通过第一性原理结合泛函重整化群方法，我们研究了FeS和FeSe0.5Te0.5这两种铁基硫族超导体，比较了它们的电子结构和超导配对的不同，认为Fe-3dX2-Y2（对应含两个Fe原子的元胞）轨道在费米面权重不同导致费米面面嵌套结构发生改变，从而造成不同的超导配对对称性，通过配对函数的具体形式可以明显看出轨道权重对超导配对的影响。另外，我们还研究了几种与铁基超导体结构相同的超导材料如ThNiAsN，SrIr2Ge2，SrPt2Ge2和单层CoSe等，分析了它们的能带结构，自旋磁化率还有电声耦合等性质，主要结论有ThNiAsN可能是电声耦合机制的超导体，但经空穴掺杂后有和铁基超导体相似的费米面嵌套结构；我们发现SrIr2Ge2电声耦合比较弱，而SrPt2Ge2中存在较强的电声耦合效应，并且其能带在费米能级附近具有狄拉克锥行为；单层CoSe基态为铁磁态，掺杂少量空穴后可变为共线性反铁磁基态。我们研究了不同空穴掺杂下的CoSe的电子结构和自旋磁化率，发现当大的空穴掺杂时，CoSe的费米面以及对应的自旋磁化率都与铁基超导非常相似。我们研究了单层NbF4，发现它的电子结构与铜氧化物相似，dxy轨道穿过费米能级形成四方形的费米面，泛函重整化群计算显示其在G类型反铁磁附近具有d波超导配对，从而我们认为单层NbF4可能是一种二维高温超导体。相信本项目的研究，能对理解铁基超导配对机理有更深层次的理解，还为寻找新的非常规超导体提供重要的理论指导。