Origin of the lattice structure sensitivity of the superconductivity in the iron pnictides

For the iron pnictide superconductors, we construct a five orbital model from first principles calculations [1], to which we apply random phase approximation to investigate spin fluctuation mediated pairing. In both 1111 and 122 materials, we show that the appearance/disappearance of the hole Fermi surface arising from the $d_{X^2-Y^2}$ orbital is sensitive to the pnictogen height measured from the iron planes, which in turn affects superconductivity [2]. In particular, the superconducting gap has a fully gapped sign reversing s-wave symmetry when the pnictogen height is high, while they tend to have nodes when the height is low. We also discuss some differences between 1111 and 122 materials.\\[4pt] [1] K. Kuroki et al., Phys. Rev. Lett. 101 (2008) 087004, erratum: Phys. Rev. Lett. 102 (2009) 109902(E).\\[0pt] [2] K. Kuroki et al., Phys. Rev. B 79 (2009) 224511.