Origin of gap anisotropy in spin fluctuation models of the iron pnictides

Abstract

We discuss the large gap anisotropy found for the ${A}_{1g}$ ($s$ wave) state in random-phase approximation spin fluctuation and functional renormalization-group calculations and show how the simple arguments leading to isotropic sign-switched $s$-wave states in these systems need to be supplemented by a consideration of pair scattering within Fermi-surface sheets and between the individual electron sheets as well. In addition, accounting for the orbital makeup of the states on the Fermi surface is found to be crucial.