Usui Hidetomo, Suzuki Katsuhiro, Kuroki Kazuhiko
Department of Physics, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka, 560-0043, Japan.
Sci Rep. 2015 Jun 15;5:11399. doi: 10.1038/srep11399.
Motivated by recent experimental investigations of the isovalent doping iron-based superconductors LaFe(AsxP1-x)O1-yFy and NdFe(AsxP1-x)O1-yFy, we theoretically study the correlation between the local lattice structure, the Fermi surface, the spin fluctuation-mediated superconductivity, and the composition ratio. In the phosphides, the dXZ and dYZ orbitals barely hybridize around the Γ point to give rise to two intersecting ellipse shape Fermi surfaces. As the arsenic content increases and the Fe-As-Fe bond angle is reduced, the hybridization increases, so that the two bands are mixed to result in concentric inner and outer Fermi surfaces, and the orbital character gradually changes to dxz and dyz, where x-y axes are rotated by 45 degrees from X-Y. This makes the orbital matching between the electron and hole Fermi surfaces better and enhances the spin fluctuation within the dxz/yz orbitals. On the other hand, the hybridization splits the two bands, resulting in a more dispersive inner band. Hence, there is a trade-off between the density of states and the orbital matching, thereby locally maximizing the dxz/yz spin fluctuation and superconductivity in the intermediate regime of As/P ratio. The consistency with the experiment strongly indicate the importance of the spin fluctuation played in this series of superconductors.
受近期对等价掺杂铁基超导体LaFe(AsxP1-x)O1-yFy和NdFe(AsxP1-x)O1-yFy的实验研究启发，我们从理论上研究了局域晶格结构、费米面、自旋涨落介导的超导性与成分比之间的相关性。在磷化物中，dXZ和dYZ轨道在Γ点附近几乎不发生杂化，从而产生两个相交的椭圆形费米面。随着砷含量的增加以及Fe-As-Fe键角减小，杂化增强，使得两个能带混合，形成同心的内外费米面，且轨道特征逐渐变为dxz和dyz，其中x-y轴相对于X-Y轴旋转了45度。这使得电子和空穴费米面之间的轨道匹配更好，并增强了dxz/yz轨道内的自旋涨落。另一方面，杂化使两个能带分裂，导致内层能带更具色散性。因此，在态密度和轨道匹配之间存在权衡，从而在As/P比的中间区域使dxz/yz自旋涨落和超导性在局部达到最大值。与实验结果的一致性有力地表明了自旋涨落在这一系列超导体中所起的重要作用。
