Optical Conductivity of Insulating Al-Based Alloys: Comparison of Quasiperiodic and Periodic Systems

We report on the optical conductivity of the structurally ordered icosahedral quasicrystal ${\mathrm{Al}}_{70}$ ${\mathrm{Pd}}_{20}$ ${\mathrm{Re}}_{10}$ and the crystal ${\mathrm{Al}}_2$Ru obtained from the reflectance measured over a wide frequency range. Our results show that both the crystal and the quasicrystal are semiconductors and have quite similar band structure. The major difference between the two materials arises from the localization of the electronic states deep inside the band gap of the quasicrystal in spite of the metallic carrier density. We speculate that the quasiperiodic symmetry favors carrier localization.