Optical Spectroscopy of Superconducting Ba_0.55K_0.45Fe₂As₂: Evidence for Strong Coupling to Low-Energy Bosons

Normal state optical spectroscopy on single crystals of the new iron arsenide superconductor Ba_0.55K_0.45Fe₂As₂ shows that the infrared spectrum consists of two major components: a strong metallic Drude band and a well-separated midinfrared absorption centered at 0.7 eV. It is difficult to separate the two components unambiguously but several fits using Lorentzian peaks suggest a model with a Drude peak having a plasma frequency of 1.6 to 2.1 eV and a midinfrared peak with a plasma frequency of 2.5 eV. Detailed analysis of the frequency dependent scattering rate shows that the charge carriers interact with a broad bosonic spectrum extending beyond 100 meV with a very large coupling constant λ=3.4 at low temperature. As the temperature increases this coupling weakens to λ=0.78 at ambient temperature. This suggests a bosonic spectrum that is similar to what is seen in the lower T_c cuprates.