铁基超导体中自旋轨道耦合效应的微观理论研究

Spin-orbit coupling was always thought to be unimportant in iron-based superconductors. However, recent angle-resolved photoemission spectroscopy and neutron scattering experiments directly observe its effects, including spin-splitting of the energy bands and reorientation of iron spins at lower temperature. Combined with earlier experimental observation of anisotropic spin excitation spectra and possible triplet pairing, it's reasonable to take spin-orbit coupling into consideration for a full description of iron-based superconductors. Starting with microscopic lattice models with spin-orbit coupling and applying the random phase approximation and functional renormalization group method, this project is going to study the effects of spin-orbit coupling on the magnetic and superconducting properties of iron-based superconductors. The studies include: (1)static spin-orientation through calculations of static polarized spin susceptibilities; (2)pairing symmetry and microscopic pairing form through functional renormalization group studies; (3)calculations of physical properties in the superconducting state, such as spin excitation spectra for different spin polarizations, quasi-particle scattering interference spectra, the Bogoliubov-de Gennes bands of an open-bounded system, etc. These theoretical studies may explain the many exotic and inconsistent experimental observations and promote the full theoretical description of iron-based superconductors.

一直以来，铁基超导体中的自旋轨道耦合效应都被认为可以忽略。然而，近期的光电子光谱和中子散射实验却直接观测到了这种效应的存在，包括能带的自旋劈裂和降温时铁自旋极化的重新取向。结合先前实验观测到“122”体系自旋激发谱的各向异性以及LiFeAs中可能的三态配对，有理由认为完整描述铁基超导体可能需要考虑自旋轨道耦合。本项目拟从包含自旋轨道耦合的微观晶格模型出发，采用无规相近似和泛函重整化群方法，研究自旋轨道耦合对铁基超导体磁性和超导性质的影响，包括：（1）通过计算正常态静态极化自旋磁化率得到静态的自旋极化取向；（2）通过泛函重整化群计算得到超导配对对称性和配对算符的微观表达形式；（3）超导态物性计算，包括自旋激发谱、准粒子散射相干谱、开边界体系的Bogoliubov-de Gennes能带等。这些理论研究有可能解释不同实验中观测到的奇异现象，促进更为完整的铁基超导体理论描述。

关联电子系统中电荷，自旋与超导自由度的竞争与关联是凝聚态物理中的一个热点和难点问题之一。结合自旋轨道耦合，诸如拓扑超导体等新的非平庸拓扑相将有可能出现。到目前为止，已有大量的成熟先进的方法对理论模型或是实际材料中的相关效应进行了详细研究。本项目中，我们利用奇异模泛函重整化群方案研究了几种典型体系的关联效应，包括：. 1. 对于一维扩展Hubbard模型，我们研究了其相互作用相图随掺杂的变化。在1/8掺杂时，我们在自旋密度波和键序波之间发现了一种新奇的扩展s波超导态。在1/4掺杂时，我们发现在自旋密度波和电荷密度波之间会出现新的p波超导态。. 2. 对于存在自旋轨道耦合的Kagome格子，随着掺杂，我们发现了120度旋转的元胞内反铁磁、f波超导、p+ip拓扑超导的演化。由于自旋轨道耦合的存在，超导态将是单态和三态的混合。手征性的p+ip拓扑超导态陈数为1，在边界上将会出现零能的weyl费米子。. 从理论方面讲，我们提出了自旋或者电荷涨落诱导非常规超导的统一图景。对于实际材料，我们的研究可以解释某些现象并对实验提供一些理论指导。