NONOHMIC HOPPING CONDUCTION IN DOPED GERMANIUM AT T LESS-THAN 1-K

Measurements of the non-Ohmic behavior of the hopping conductivity of doped germanium between 300 and 700 mK show that the conductivity obeys an electric-field dependence sigma(E, T) = sigma(0, T) x exp(eEL/kT) at field strengths as low as 0.04 V/cm. This result is in disagreement with theoretical models which predict that the conductivity should obey the relation 1n-sigma-approximately E only for field strengths greater than approximately 1 V/cm, below which the conductivity is in an. ''Ohmic regime.'' L, the length paremeter describing the field dependence, is found to increase with decreasing temperature as L approximately T-1 in the variable-range-hopping regime.