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Abstract
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.
Highlights
Indicates that the superconductivity is locally optimized at a certain bond angle larger than 109 deg., i.e., between 113 deg. and 120 deg., in contradiction with Lee’s plot[9] as well as the previous theoretical expectations
It is known that the Fermi surface of the iron based superconductors is mainly governed by the Fe-Pn-Fe bond angle, or the pnictogen height measured from the Fe plane[10,11]
Motivated by recent experiments on LnFeAsxP1-xO1-yFy which observe non-monotonic evolution of the superconducting Tc against the arsenic/phosphorus ratio, we have studied the correlation between the local lattice structure, the orbital character of the Fermi surface, and superconductivity
Summary
Indicates that the superconductivity is locally optimized at a certain bond angle larger than 109 deg., i.e., between 113 deg. (arsenic end) and 120 deg. (phosphorus end), in contradiction with Lee’s plot[9] as well as the previous theoretical expectations. The number of the Fermi surfaces does not change in the intermediate regime of the arsenic content, so that the previous study[10,11] cannot explain this local optimization of Tc. an unrevealed feature of the iron-based materials regarding this local optimization of superconductivity should be present. An unrevealed feature of the iron-based materials regarding this local optimization of superconductivity should be present Given this background, we study the correlation between the local lattice structure, the orbital character of the Fermi surface, and Tc in the 1111 system with isovalent doping. 120 deg., which corresponds to the intermediate regime of the arsenic (or phosphorus) content The origin of this local optimization is traced back to the variation of the orbital character and the density of states of the hole Fermi surfaces around the Γ point, which is controlled by the bond angle. The consistency with the experiment strongly indicates the importance of the spin fluctuation played in this series of superconductors
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