Resistivity and magnetoresistivity measurements 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 φc (50.7% Al) are found to have critical fields in excess of 1.5 T at 0.47 K due to the fine granular nature of the Al. In barely insulating films where (φc-φ)<2.2% (where φ is the volume percent of aluminum), superconducting fluctuations and Josephson-junction coupling dominate the behavior below Tc (Tc1.6 K). For the more insulating films where 3.4%≤(φc-φ)≤6.2%, the resistivity diverges strongly below Tc and a large negative magnetoresistivity is observed. This behavior is explained in terms of quasiparticle tunneling or alternatively by the Adkins charging model; the magnetoresistivity is explained in terms of a magnetic-field-dependent superconducting energy gap. The upper critical fields of all the samples are found to be almost independent of the bulk resistivities, supporting a picture of weakly coupled grains. The normal resistivity in the insulating films (φ<φc) obeys the hopping relation ρ=ρ0exp(T0/T)x above Tc and also below Tc in a field of 3.48 T, which quenches all superconductivity in the films.