再论LiFeAs中轨道涨落介导的超导电性：非平凡自旋轨道相互作用对能带结构和超导能隙函数的影响
Revisit of the Orbital-Fluctuation-Mediated Superconductivity in LiFeAs: Nontrivial Spin-Orbit Interaction Effects on the Bandstructure and Superconducting Gap Function
摘要：
ARPES研究给出的LiFeAs（Tc = 18 K）的精确能隙结构为我们理解铁基超导体中的配对机制提供了重要的信息。LiFeAs能隙结构最显著的特征是"最大能隙出现在Z点附近的小孔中"。这一结果已经很自然地用轨道涨落的观点来解释了（T。Saito等人，Phys.Rev.B90，035104（2014）），而通过自旋波动方案获得相反的结果。在本文中，我们研究的间隙结构的LiFeAs考虑到自旋轨道相互作用（SOI），最近的ARPES研究，揭示了显着的SOI诱导修改的费米表面拓扑结构的动机。为此，我们参考不同的ARPES群给出的能带结构，构造了两种可能的有限SOI紧束缚模型。此外，我们推广了有限SOI多轨道系统的能隙方程，并应用轨道自旋涨落理论计算了能隙函数。基于SOI诱导的能带结构，LiFeAs中带隙结构的主要特征仅在（xz/yz-xy）轨道之间存在强轨道间相互作用的情况下自然再现。因此，LiFeAs中的实验能隙结构是轨道涨落配对机制的有力证据。
英文摘要：
Precise gap structure in LiFeAs (Tc = 18 K) given by ARPES studies offers us significant information to understand the pairing mechanism in iron-based superconductors. The most remarkable characteristics in LiFeAs gap structure would be that "the largest gap emerges on the tiny hole-pockets around Z point". This result had been naturally explained in terms of the orbital-fluctuation scenario (T. Saito et al., Phys. Rev. B 90, 035104 (2014)), whereas an opposite result is obtained by the spin-fluctuation scenario. In this paper, we study the gap structure in LiFeAs by taking the spin-orbit interaction (SOI) into account, motivated by the recent ARPES studies that revealed the significant SOI-induced modification of the Fermi surface topology. For this purpose, we construct the two possible tight-binding models with finite SOI by referring the bandstructures given by different ARPES groups. In addition, we extend the gap equation for multiorbital systems with finite SOI, and calculate the gap functions by applying the orbital-spin fluctuation theory. On the basis of both SOI-induced band structures, main characteristics of the gap structure in LiFeAs are naturally reproduced only in the presence of strong inter-orbital interactions between (xz/yz - xy) orbitals. Thus, the experimental gap structure in LiFeAs is a strong evidence for the orbital-fluctuation pairing mechanism.
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