Using experimental data to constrain theories of hopping conduction in NTD germanium

The mechanism for low temperature electrical conduction in neutron transmutation doped (NTD) germanium is believed to be variable range hopping (VRH). The resistance, R, at temperature T should then follow R(T ) = R0 exp (T0/T ) p, for constant (or nearly constant) values of T0, R0 and p. NTD Ge is thought to have a “Coulomb gap” in the density of states; theories then generally predict p = 0.5. However, some theories suggest larger values, such as p = 0.55. So far, experimental results have failed to distinguish between these values of p. We show that it is nevertheless practical to make sufficiently accurate measurements to do so. We present measurements for several NTD Ge samples with different doping levels, and discuss the various possible sources of error.