Resistivity and magnetoresistivity measurements near the metal-insulator and superconductor-insulator transition in granular AlGe

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