Hopping transport in an insulating quasicrystal bar of Al₇₀Pd_22.5Re_7.5 near the metal-insulator transition

We have observed that the conductivity σ(T) for the Al₇₀Pd_22.5Re_7.5 quasicrystal, with a resistivity ratio r = R(4.2 K)/R(300 K) = 13.2, obeys the variable-range hopping law, σ(T) = σ₀/ exp[(T₀/ T)^μ, in the temperature range between 64 mK and 1.6 K. The hopping exponent μ is extracted to be 0.23, close to the Mott exponent of 1/4, and T₀ is 3.5 K. This insulating behaviour is consistent with the prediction of a previously determined scaling law that bulk Al₇₀Pd_22.5Re_7.5 samples having r ≥ 12.8 will be insulating. Large positive magnetoresistances (MRs) were observed in this sample. The percentage change in the MR = ΔR(B, T)/R(0, T), as high as 185% at T = 0.11 K and B = 17 T, is the largest value ever reported in Al-Pd-Re QCs, to our knowledge. The difficulties using existing MR theories to explain the MR data for this insulating sample near the metal-insulator transition are discussed.