Stable Al-Cu-Ru icosahedral crystals: A new class of electronic alloys

Stable icosahedral ${\mathrm{Al}}_{85\mathrm{-}\mathit{x}}$ ${\mathrm{Cu}}_{\mathit{x}}$ ${\mathrm{Ru}}_{15}$ (15<x<20) alloys of good metals are characterized by conductivities below the ‘‘minimum metallic value,’’ and resistivity ratios ρ(0.5 K)/ρ(295 K) as large as 4. Strong temperature dependences which lead to changes of sign in the Hall coefficient and thermopower are also observed. Electronic densities of states are found to vary rapidly with alloy composition. The unusual electronic properties of this new class of barely metallic systems are discussed in light of current theories on electronic structure and transport of icosahedral crystals.