Conduction mechanisms in some icosahedral and amorphous phases

Quasicrystalline icosahedral and amorphous phases prepared as thin films or bulk samples in the systems Al-Pd-Re, Al-Cu-Fe and Ti-Zr-Ni, have been investigated with respect to their temperature dependence of conductance and thermopower, in the range between 10 and 300 K. Various conduction mechanisms yield different exponent values in the power-law temperature dependence of conductance, T ζ . A semi-metal behaviour (ζ ≅ 1.5) is found in the amorphous phase and in the disordered icosahedral Al-Pd-Re film and Ti-Zr-Ni ribbons, as well as in icosahedral Al-Cu-Fe films. Lower ζ values found in the quasi-perfect icosahedral Al-Pd-Re bulk sample and in two of the icosahedral Al-Cu-Fe films suggest that electron localization in a hierarchy of clusters is the main conduction mechanism in these samples. The thermopower data support the existence of a pseudo-gap at the Fermi level and suggest a similar short-range order in both amorphous and icosahedral phases. The special sensitivity of the thermopower to the DOS details at E F is responsible for the strong variation of its magnitude with the degree of icosahedral order.