| A study of the electronic transport properties and low temperature specific heats of well-ordered icosahedral (i-) phases in the Al-Cu-Fe, Al-Cu-Ru-Si, and Al-Pd-Re systems is presented. Several unusual features were seen, including low dc conductivities, low electronic contributions to the specific heat, and large and strongly temperature dependent Hall coefficients, and thermoelectric powers. Band structure effects, enhanced by the unique structure factor due to icosahedral symmetry are responsible for these features. It was also noted that the electronic properties were sensitive to small compositional changes of {dollar}<{dollar}2 at.%. These features suggest a rapidly varying conductivity spectrum {dollar}sigma({lcub}cal E{rcub}){dollar} in these materials. Electronic properties of the rhombohedral phase of Al-Cu-Fe show similar behavior to those of the i-phase at the same composition.; While semimetallic behavior is observed in i-phases in the Al-Cu-Fe and Al-Cu-Ru-Si systems, i-phase in the Al-Pd-Re system showed insulating-like electrical resistivity. The low-temperature resistivity values of {dollar}{lcub}sim1{rcub}Omega{dollar}-cm are comparable to those of doped semiconductors on the insulating side of the metal-insulator transition. It is unusual that the effective carrier concentration ({dollar}rm{lcub}sim1{rcub}0sp{lcub}20{rcub} cmsp{lcub}-3{rcub}),{dollar} obtained from Hall effect, and the electronic contribution to the specific heat {dollar}gammasim0.1{dollar} mJ/g atom K{dollar}sp2{dollar} are quite similar to those for the semimetallic i-phases despite the low temperature resistivity being higher by a factor of 100.; Low temperature resistivity is enhanced by a factor of {dollar}sim{dollar}10 in the Al-Pd-Re samples following additional heat treatment at 600{dollar}spcirc{dollar}C-700{dollar}spcirc{dollar}C but the electronic structure appears to be unaffected. This behavior has not been seen before in quasicrystals. It is thought to be due to a subtle structural transition to a more ordered i-phase upon annealing. The power-law dependence of the low temperature conductivity, {dollar}(sigmaproptosqrt{lcub}T{rcub}{dollar} prior to low temperature anneal, {dollar}sigmapropto T{dollar} after annealing) may be an effect of the quasiperiodicity leading to critical states. |
