Coherence peak and pair-breaking effects in the ac conductivity of BaFe$_{2-2x}$Co$_{2x}$As$_2$ epitaxial thin films

We report a study of high quality pnictide superconductor BaFe$_{1.84}$Co$_{0.16}$As$_2$ epitaxial thin films using time-domain THz spectroscopy. Near T$_c$ we find evidence for a coherence peak and qualitative agreement with the weak-coupling Mattis-Bardeen form of the conductivity. At low temperature, we find that the real part of the THz conductivity is not fully suppressed and $\sigma_2$ is significantly smaller than the Matthis-Bardeen expectation. The temperature dependence of the penetration depth $\lambda$ follows a power law with an unusually high exponent of 3.1. We interpret these results as consistent with impurity scattering induced pair-breaking. Taken together our results are strong support for an extended s$\pm$ symmetry order parameter.