Effect of bond length and radius on superconducting transition temperature for FeAs-based superconductors

By comparing the data of lattice parameters of more than 50 new FeAs-based high temperature superconductors in two syetems, the effect of bondlength on superconducting transition temperature (T(C)) was found that, for both FeAs superconductor systems with similar ionic radii of cation A, the bond length L(As-A) between Arsenic atom and its nearest neighbor cation at the A site is in an inverse proportion to T(C), i.e. the larger the bond length L(As-A), the lower the T(C). In addition, we also found a noticeable effect of ionic radius on T(C), in which deviation from the tendency line of bond length vs. T(C) decreases with the increasing ionic radius. Both bond length-T(C) and ionic radii-T(C) relations indicate that the interaction of cation A and As ion have significant effect on superconductivity. The analysis on the electronic structure indicates that there exists the proximity of the secondary-outer p-orbit of the cation A and the 4s orbit of the As ion, both in energy space and in real space. Some high frequency individual vibrating modes would be established through exchange coupling on their inner orbits. It is worth of mention that the superconducting condensation is influenced by the interaction between the cation A and the As atoms.