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 nominally uncompensated neutron-transmutation-doped isotopically enriched Ge-70:Ga samples with the Ga concentration [Ga] near N-c for the metal-insulator transition. sigma of all insulating samples obeys ln sigma proportional to -(T-0/T)(1/2) with T-0 proportional to (N-c-[Ga])/N-c while the zero temperature conductivity sigma(0) of the metallic samples is sigma(0) sigma {([Ga]-N-c)/N-c}(nu) with the critical exponent nu approximate 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.