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 phi(c) (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 (phi(c) - phi) < 2.2% (where phi is the volume percent of aluminum), superconducting fluctuations and Josephson-junction coupling dominate the behavior below T(c) (T(c) congruent-to 1.6 K). For the more insulating films where 3.4% less-than-or-equal-to (phi(c) - phi) less-than-or-equal-to 6.2%, the resistivity diverges strongly below T(c) 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 (phi < phi(c)) obeys the hopping relation rho = rho0exp(T0/T)x above T(c) and also below T(c) in a field of 3.48 T, which quenches all superconductivity in the films.