Origin of the DC and AC conductivity anisotropy in iron-based superconductors: scattering rate versus spectral weight effects

To shed light on the mechanism responsible for the in-plane resistivity anisotropy in the nematic phase of the iron-based superconductors, we investigate the impact of spin fluctuations on the anisotropic AC conductivity. On the one hand, the scattering of electrons off magnetic fluctuations causes an anisotropic scattering rate. On the other hand, the accompanying Fermi velocity renormalization contributes to both the plasma frequency and the scattering rate in antagonistic ways, giving rise to anisotropies in these quantities that exactly cancel each other in the DC limit. As a result, the effective scattering rate changes its sign as function of temperature, but the DC conductivity retains the same sign, in agreement with recent experiments.