Anisotropic Drude Response and the Effect of Anisotropic C Substitution inMg(B1−xCx)2

The in-plane and the out-of-plane optical spectra of small single crystals of $\mathrm{Mg}({\mathrm{B}}_{1-x}{\mathrm{C}}_x{)}_2$ were directly determined by a microscope spectroscopy technique. Contrary to previous reports, the estimated plasma frequencies for both directions are quite consistent with the band calculation. A multi-Drude picture corresponding to a multiband system is necessary to explain the whole spectral profile. The spectral changes with the carbon substitution indicate a decrease of carrier concentration as well as an increase of scattering rate, particularly, in the $\sigma$ bands.