Effects of $c/a$ Anisotropy and Local Crystal Structure on   Superconductivity in   $A\mathrm{Fe_{2}}(\mathrm{As}_{1-x}\mathrm{P}_{x}\mathrm{)_{2}}$   ($A$=Ba$_{1-y}$Sr$_y$, Sr$_{1-y}$Ca$_y$ and Eu)
  We investigated the effects of $c/a$ anisotropy and local crystal structure on superconductivity (SC) in As/P solid solution systems, $A\mathrm{Fe_{2}}(\mathrm{As}_{1-x}\mathrm{P}_{x}\mathrm{)_{2}}$ ($A$122P) with various $A$ ions. With decreasing $A$ site atomic size from $A$=Ba to Eu, the structural anisotropy decreases, and the rate of decreasing with $x$ also increases. The rapid narrowing of the region of antiferromagnetic composition ($x$) can be considered to be a result of this anisotropy change due mainly to the change in the Fermi surface (FS) nesting condition. By contrast, although the structural anisotropy systematically changes, the maximum $T_{\mathrm{c}}$ values are almost the same in all $A$122P systems except for Eu122P. These results indicate that the modification of the FS topology via the structural anisotropy does not affect SC. However local structural parameters, such as pnictogen height, are crucial for $T_{\mathrm{c}}$. 
Toru Adachi, Yusuke Nakamatsu, Tatsuya Kobayashi, Shigeki Miyasaka,   Setsuko Tajima, Masayoshi Ichimiya, Masaaki Ashida, Ha...
cond-mat.supr-con