Impurity effect on optical response in MgB2

We report on the optical response in normal state of Mg(B1−xCx)2 using single crystals and an optical microscope combined with a FTIR spectrometer. The plasma frequency estimated from a sum rule of optical conductivity is consistent with the value of band calculation. Our results demonstrate that carbon-substitution has effects of strong scattering and electron doping. To understand the whole spectral profile, we need a two-Drude picture in which the σ-band scattering rate is much smaller than the π-band one. It has been revealed that with increasing carbon substitution the σ-band scattering rate rapidly increases, while the increase of π-band scattering rate is moderate.