Optical conductivity of the icosahedral quasicrystal Al _75.5Mn_20.5Si₄ and its 1/1 crystalline approximant alpha -Al_72.5Mn_17.4Si_10.1

The reflectance of the metastable icosahedral quasicrystal Al _75.5Mn_20.5Si₄ and its 1/1 crystalline approximant alpha -Al_72.5Mn_17.4Si_10.1 has been measured from 50 cm^-1 to 46000 cm^-1 at temperatures from approximately=10 K to 295 K. The optical conductivity of the icosahedral sample is similar to that reported for the stable quasicrystal Al_63.5Cu _24.5Fe₁₂; starting out from a low DC value it rises linearly with frequency up to approximately=1 eV reaching a maximum at approximately=2.2 eV with a magnitude that is comparable with transition metals, above which it decreases slowly. The conductivity of the approximant phase is similar in shape, but the overall magnitude is lower. There is a broad structure at 200 cm^-1 in the quasicrystal which is resolved into at least nine sharp phonon lines in the approximant. Band structure calculations of the crystalline approximant predict a pseudogap in the electronic density of states at the Fermi surface suggesting that the unusual optical properties of the icosahedral material may also be explained by band structure arguments.