The scaling behaviour of the metal-insulator transition of isotopically engineered neutron-transmutation doped germanium

We measured on the dielectric side, with \N/N-c - 1\ less than or equal to 0.64 the critical indices of the metal-insulator transition (MIT) in n-Ge with low (1.4 and 12%) and medium (38 and 54%) compensation, prepared by neutron-transmutation doping (NTD) of isotopic mixtures of Ge-74 and Ge-70. We analyzed from the temperature dependence of hopping resistance in the variable-range hopping (VRH) regime with Coulomb gap the scaling of the localization length a, and of the dielectric constant x. At low compensation we find that the critical indices are nu approximate to 1/2, zeta approximate to 1, which increase up to nu approximate to 1, zeta approximate to 2, for medium compensations.