HOPPING CONDUCTION AND METAL-INSULATOR-TRANSITION IN ISOTOPICALLY ENRICHED NEUTRON-TRANSMUTATION-DOPED GE-70-GA

We report on the electrical conductivity sigma of a series of nominall y uncompensated neutron-transmutation-doped isotopically enriched Ge-7 0:Ga samples with the Ga concentration [Ga] near N-c for the metal-ins ulator transition. sigma of all insulating samples obeys ln sigma prop ortional to -(T-0/T)(1/2) with T-0 proportional to (N-c-[Ga])/N-c whil e the zero temperature conductivity sigma(0) of the metallic samples i s sigma(0) sigma {([Ga]-N-c)/N-c}(nu) with the critical exponent nu ap proximate to 0.5. The values of N-c obtained from the two independent scalings of T-0 and sigma(0) are identical, i.e., nu approximate to 0. 5 is established unambiguously for uncompensated Ge:Ga.