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

We investigated four series of Ge samples of isotopic mixtures of Ge-74 and Ge-70 doped by the neutron transmutation doping technique with a compensat ion degree K = 1.4, 12, 38 and 54% and n-type conductivity which show a str ong dependence of the critical impurity concentration N-c on K. For barely insulating samples we determined the critical scaling exponents of the loca lization length and of the dielectric constant by analysis of the temperatu re dependence of hopping resistance and of the positive magnetoresistance a t variable range hopping with Coulomb gap. For low disorder (K = 1.4 and 12 %) the critical exponents of the localization length and the dielectric con stant are nu = 0.66 +/- 0.27 and xi = 1.07 +/- 0.27, which nearly double fo r medium disorder (K = 38 and 54%) to nu = 1.59 +/- 0.32 and xi = 1.94 +/- 0.2, respectively.