BaFe2As2三维紧束缚模型中的自旋涨落和超导电性
Spin fluctuations and superconductivity in a 3D tight-binding model for BaFe2As2
摘要：
尽管丰富的实验数据的Fe-磷属元素化合物的KFe 2As 2型，K = Ba，Ca，或Sr，主要的理论工作的基础上的多轨道紧束缚模型已被限制到目前为止的研究相关的1111化合物。这可以归因于122种材料的从头计算发现的更三维的电子结构，使得该系统不太适合模型开发。此外，体心四轴对称的更复杂的布里渊区（BZ）不允许电子能带结构直接展开成有效的1Fe/晶胞BZ。在这里，我们提出了一个有效的5轨道紧束缚适合的完整的DFT能带结构的BaFeAs，包括kz色散。我们比较了5-轨道自旋涨落模型，以前研究的LaOFeAs和计算RPA增强的磁化率。使用波动交换近似，以确定领先的配对不稳定性，然后，我们研究严格的二维模型计算超过一个单一的KZ削减的BZ和一个完全三维的方法之间的差异。我们发现配对态与1111材料非常相似，在空穴层上具有一般的准各向同性配对，在kz = 0处的电子层上具有节点态，但由于系统是空穴掺杂的，这些状态是带隙的。另一方面，序参量对kz的依赖性仍然很大，大部分的配对强度来自kz = pi附近的过程。这些状态表现出增强的各向异性上的空穴片和减少的各向异性上的电子片附近的BZ的顶部的趋势。
英文摘要：
Despite the wealth of experimental data on the Fe-pnictide compounds of the KFe2As2-type, K = Ba, Ca, or Sr, the main theoretical work based on multiorbital tight-binding models has been restricted so far to the study of the related 1111 compounds. This can be ascribed to the more three dimensional electronic structure found by ab initio calculations for the 122 materials, making this system less amenable to model development. In addition, the more complicated Brillouin zone (BZ) of the body-centered tetragonal symmetry does not allow a straightforward unfolding of the electronic band structure into an effective 1Fe/unit cell BZ. Here we present an effective 5-orbital tight-binding fit of the full DFT band structure for BaFeAs including the kz dispersions. We compare the 5-orbital spin fluctuation model to one previously studied for LaOFeAs and calculate the RPA enhanced susceptibility. Using the fluctuation exchange approximation to determine the leading pairing instability, we then examine the differences between a strictly two dimensional model calculation over a single kz cut of the BZ and a completely three dimensional approach. We find pairing states quite similar to the 1111 materials, with generic quasi-isotropic pairing on the hole sheets and nodal states on the electron sheets at kz = 0 which however are gapped as the system is hole doped. On the other hand, a substantial kz dependence of the order parameter remains, with most of the pairing strength deriving from processes near kz = pi. These states exhibit a tendency for an enhanced anisotropy on the hole sheets and a reduced anisotropy on the electron sheets near the top of the BZ.
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