Non-Ohmic hopping conduction in doped germanium atT<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 σ(E,T) =σ(0,T) 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 lnσ∼E only for field strengths greater than ∼1 V/cm, below which the conductivity is in an ‘‘Ohmic regime.’’ L, the length parameter describing the field dependence, is found to increase with decreasing temperature as L∼${\mathit{T}}^{\mathrm{-}1}$ in the variable-range-hopping regime.