摘要：Motivated by recent experimental investigations of the isovalent doping iron-based superconductors LaFe(As x P1- x )O1- y F y and NdFe(As x P1- x )O1- y F y , 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 d XZ and d YZ 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 d xz and d yz , 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 d xz/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 d xz/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.
