Pair-breaking effects and coherence peak in the terahertz conductivity of superconducting BaFe2-2xCo2xAs2 thin films

We report a study of high quality pnictide superconductor BaFe1.84Co0.16As2 epitaxial thin films using time-domain terahertz (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. The real part of the THz conductivity is found to be not fully suppressed at low temperature 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 evidence for an extended s +/- order-parameter symmetry.