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 inv estigated with respect to their temperature dependence of conductance and t hermopower, in the range between 10 and 300 K. Various conduction mechanism s yield different exponent values in the power-law temperature dependence o f conductance, Tt. A semi-metal behaviour (zeta congruent to 1.5) is found in the amorphous phase and in the disordered icosahedral Al-Pd-Rt: film and Ti-Zr-Ni ribbons, as well as in icosahedral Al-Cu-Fe films. Lower zeta val ues 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 h ierarchy 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 p hases. 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 o f icosahedral order.