Variable range hopping conduction in neutron-transmutation-doped Ge-70 : Ga

Variable range hopping conduction in neutron-transmutation-doped, isotopica lly enriched Ge-70:Ga samples for the temperature range T = 20 to 250 mK ha s been studied in the critical regime for the metal-insulator transition. E ighteen samples investigated had Ga concentrations in the range N = 0.942N( c) to 0.998N(c), where N-c is the critical Ga concentration for the metal-i nsulator transition. The low temperature resistivities rho of all samples o bey the variable range hopping conduction theory of Efros-Shklovskii type w ith an appropriate temperature dependence in the pre-factor; rho = rho(0)T( -1/3) exp(T-0/T)(1/2). The critical exponent alpha of T-0 as a function of N has been determined using the form T-0 proportional to (1 - N/N-c)(alpha) . A clear crossover of alpha from alpha approximate to 1 to 3.5 has been ob served at N approximate to 0.99N(c). The excitation of holes from the lower - to upper-Hubbard bands is the dominant conduction mechanism in the alpha approximate to 1 region, while the standard variable range hopping of holes from occupied to empty Ga sites takes place in the very vicinity of the tr ansition where alpha approximate to 3.5. The important role of the doping c ompensation is proposed in connection with the critical behavior of the ele ctrical conductivity.