We have measured the complex dynamical conductivity, $\sigma = \sigma_{ 1} + i\sigma_{ 2}$, of superconducting Ba(Fe$_{ 0.9}$Co$_{ 0.1}$)$_{ 2}$As$_{ 2}$ ($T_{ c} = 22$ K) at terahertz frequencies and temperatures 2 - 30 K. In the frequency dependence of $\sigma_{ 1}$ below $T_{ c}$, we observe clear signatures of the superconducting energy gap opening. The temperature dependence of $\sigma_{ 1}$ demonstrates a pronounced coherence peak at frequencies below 15 cm$^{ -1}$ (1.8 meV). The temperature dependence of the penetration depth, calculated from $\sigma_{ 2}$, shows power-law behavior at the lowest temperatures. Analysis of the conductivity data with a two-gap model, gives the smaller isotropic s-wave gap of $\Delta_{ A} = 3$ meV, while the larger gap is highly anisotropic with possible nodes and its rms amplitude is $\Delta_{ 0} = 8$ meV. Overall, our results are consistent with a two-band superconductor with an $s_{ \pm}$ gap symmetry.
Categories: cond-mat.supr-con