Evolution of the optical spectrum with doping in Ba(Fe1-xCox)2As2

We investigated the optical spectrum of Ba(Fe1-xCox)(2)As-2 single crystals with various doping levels. It is found that the low-energy optical conductivity spectrum of this system can be decomposed into two components: a sharp Drude term and a broad "incoherent" term. For the compounds showing magnetic order, a gap appears predominantly in the incoherent component while an s-wave-like superconducting gap opens in both components for highly doped compounds. The Drude weight steadily increases as doping proceeds, consistent with electron doping in this system. On the other hand, the incoherent spectral weight is almost doping independent but its spectral feature is intimately connected with the magnetism. We demonstrate that the presence of two distinct components in the optical spectrum well explains the doping and temperature dependences of the dc resistivity.