  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 Gamma 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.
