Optical study on clathrates Sr₈Ga₁₆Ge₃₀ and β-Eu₈Ga₁₆Ge₃₀

The optical reflectivity spectra of clathrates Sr₈Ga₁₆Ge₃₀ and β-Eu₈Ga₁₆Ge₃₀ have been measured to investigate their electronic structure. A clear Drude shape was observed in the spectrum of Sr₈Ga₁₆Ge₃₀. On the other hand, a broad Drude shape due to magnetic scattering by Eu 4f electrons and a peak originated from Eu 4f states were observed in the spectrum of β-Eu₈Ga₁₆Ge₃₀. No clear change of optical conductivity due to the magnetic order was observed in β-Eu₈Ga₁₆Ge₃₀ and no evidence of the rattling in Sr₈Ga₁₆Ge₃₀ observed in contrast to α-Eu₈Ga₁₆Ge₃₀. These results essentially originated from the large carrier density of these materials.