Resistivity and magnetoresistivity near the metal-insulator and superconductor-insulator transition in granular Al-Ge

Granular Al-Ge films, just on the metallic side of the critical volume fraction φ_c (50.7% Al), are found to have critical fields in excess of 1.5T at 0.47 K due to the fine granular nature of the Al. In barely insulating films where φ_c - φ < 2.2% (φ is the volume fraction of aluminum), Josephson junction coupling dominates the behaviour below T_c. For 3.4% ≤ (φ - φ) ≤ 6.2%, the resistivity diverges strongly below T_c and a large negative magnetoresistivity (MR) is observed. This behaviour is explained in terms of quasiparticle tunnelling or alternatively by the Adkins charging model. The high negative MR is explained using a magnetic field-dependent superconducting energy gap. The normal resistivity in the insulating films obeys the hopping relation ρ = ρ₀ exp(T₀/T)^x above T_c and also below T_c in a field of 3.45 T, which quenches all superconductivity in the Al below T_c = 1.61 K.