The variable-range-hopping (VRH) resistance of n- and p-type neutron-transmutation-doped (NTD) Ge was measured at temperatures down to 30 mK and in magnetic fields up to 7 T. It is shown that the temperature dependence of the VRH resistance can be reduced to a universal curve for all samples and fields, from which one can determine the density of states (DOS) near the Fermi level. Temperature-dependent measurements show that VRH conductivity at low energies is in agreement with the existence of a soft Coulomb gap at the Fermi level. The tendency to a constant DOS at higher energies is shown: the so-called crossover phenomenon in VRH conductivity which reduces the effect of the Coulomb gap at higher magnetic fields and temperatures is discussed.

• Received 25 May 1993

DOI:https://doi.org/10.1103/PhysRevB.48.11796