Electrical properties of isotopically enriched   neutron-transmutation-doped ^{70} Ge:Ga near the metal-insulator transition
  We report the low temperature carrier transport properties of a series of nominally uncompensated neutron-transmutation doped (NTD) ^{70} Ge:Ga samples very close to the critical concentration N_c for the metal-insulator transition. The concentration of the sample closest to N_c is 1.0004N_c and it is unambiguously shown that the critical conductivity exponent is 0.5. Properties of insulating samples are discussed in the context of Efros and Shklovskii's variable range hopping conduction. 
Michio Watanabe, Youiti Ootuka, Kohei M. Itoh, and Eugene E. Haller