Highly anisotropic energy gap in superconducting Ba(Fe0.9Co0.1)2As2 from optical conductivity measurements

We have measured the complex dynamical conductivity, sigma=sigma(1)+i sigma(2), of superconducting Ba(Fe0.9Co0.1)(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 +/- gap symmetry.