Highly anisotropic energy gap in superconducting Ba(Fe_0.9Co_0.1)₂As₂ from optical conductivity measurements

We have measured the complex dynamical conductivity, sigma = sigma₁+i sigma₂, of superconducting Ba(Fe_0.9Co_0.1)₂As₂ (T_c = 22 K) at terahertz frequencies and temperatures 2–30 K. In the frequency dependence of sigma₁ below T_c, we observe clear signatures of the superconducting energy gap opening. The temperature dependence of sigma₁ demonstrates a pronounced coherence peak at frequencies below 15 cm⁻¹ (1.8 meV). The temperature dependence of the penetration depth, calculated from sigma₂, 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 Delta₀ = 8 meV. Overall, our results are consistent with a two-band superconductor with an s± gap symmetry.