DENSITY-OF-STATES NEAR THE FERMI-LEVEL FROM MEASUREMENTS OF VARIABLE-RANGE-HOPPING MAGNETORESISTANCE IN GERMANIUM

The variable-range-hopping (VRH) resistance of n- and p-type neutron-t ransmutation-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 de pendence of the VRH resistance can be reduced to a universal curve for all samples and fields, from which one can determine the density of s tates (DOS) near the Fermi level. Temperature-dependent measurements s how that VRH conductivity at low energies is in agreement with the exi stence of a soft Coulomb gap at the Fermi level. The tendency to a con stant DOS at higher energies is shown: the so-called crossover phenome non in VRH conductivity which reduces the effect of the Coulomb gap at higher magnetic fields and temperatures is discussed.