We have measured the complex dynamical conductivity, $\sigma ={\sigma }_1+i{\sigma }_2$, of superconducting $\text{Ba}{\left({\text{Fe}}_{0.9}{\text{Co}}_{0.1}\right)}_2{\text{As}}_2$ $\left(T_c=22\text{K}\right)$ 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 $15{\text{cm}}^{-1}$ (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 ${\Delta }_A=3\text{meV}$, while the larger gap is highly anisotropic with possible nodes and its rms amplitude is ${\Delta }_0=8\text{meV}$. Overall, our results are consistent with a two-band superconductor with an $s_{\pm }$ gap symmetry.

• Received 5 May 2010

DOI:https://doi.org/10.1103/PhysRevB.82.224507