Pnictogen height as a switch between high-T_c nodeless and low-T_c nodal pairings in the iron-based superconductors

Effects of the lattice structure on the spin-fluctuation mediated superconductivity is studied theoretically for various iron pnictides. We get a general picture that the gap function and the strength of the superconducting instability are determined by multiple spin fluctuation modes arising from several nestings among disconnected pieces of the Fermi surface, which work constructively or competitively as dictated by the lattice structure. Specifically, the appearance of the Fermi surface around (π,π) in the unfolded Brillouin zone is sensitive to the pnictogen height hPn above the Fe plane, where hPn is shown to act as a switch between high-Tc nodeless and low-Tc nodal pairings.