Analysis of the critical behavior of the metal-insulator transition by variation of the compensation in neutron transmutation doped ⁷⁴Ge-⁷⁰Ge crystals

We investigated four series of Ge samples of isotopic mixtures of ⁷⁴Ge and ⁷⁰Ge doped by the neutron transmutation doping technique with a compensation degree K = 1.4, 12, 38 and 54% and n-type conductivity which show a strong dependence of the critical impurity concentration N_c on K. For barely insulating samples we determined the critical scaling exponents of the localization length and of the dielectric constant by analysis of the temperature dependence of hopping resistance and of the positive magnetoresistance at variable range hopping with Coulomb gap. For low disorder (K = 1.4 and 12%) the critical exponents of the localization length and the dielectric constant are v = 0.66 ± 0.27 and ζ = 1.07 ± 0.27, which nearly double for medium disorder (K = 38 and 54%) to v = 1.59 ± 0.32 and ζ = 1.94 ± 0.2, respectively.