Metal-insulator transition and localization in quasicrystalline Al70.5Pd21Re8.5-xMnx alloys

Metal-insulator transition was observed in icosahedral quasicrystals Al70.5Pd21Re85-xMnx as x changed from 5 to 0. It was found that conductivity data of the insulating samples (x less than or equal to 3.5) could be analyzed in terms of variable range hopping, i.e., sigma=sigma(0) exp[-(T-0/T)(p)], with p=1/2 for x=2, 2.5, 3, and 3.5 and p=1/4 for x=0, over the temperature range from 0.45 to 10 K. The localization length was found to be of the order of intercluster distance in the quasicrystalline structure. At decreasing x, magnetoresistance (MR) was found to change smoothly from weak-localization behavior in the metallic regime to anomalous features in the insulating regime which are distinctly different from those seen in other well-studied insulators. The persistence of positive MR in insulating Al70.5Pd21Re8.5-xMnx alloys suggests that the backscattering effect plays an important role for localized states in quasicrystalline materials. Specific heat measurement on the insulating x=2 sample yielded gamma=0.38 mJ/g atom K-2, and carrier density was found to be similar to 10(20) cm(-3) at 4.2 K from Hall effect measurement, which are at least as large as those of metallic Al-Cu-Fe quasicrystals. The finding of spatially localized states in quasiperiodic alloys cannot be explained satisfactorily by existing theories.