Critical behavior of Mott variable-range hopping in Si:As near the metal-insulator transition

dc conductivity measurements of barely insulating uncompensated Si:As samples yield results showing Mott variable-range-hopping behavior [ln{σ}{∝}(T₀/T)(1/4)] in the temperature range 0.15<T<10 K. The exponent (1/4) has been confirmed using the logarithmic-derivative technique. The characteristic temperature T₀ shows scaling behavior of the form T₀{∝}(1-N/Nc{)}ᵖ$ with 2.3<p<2.9 for 1-N/${N}c0.07 and p8.4 for 0.11-N/{N}c0.16. Both the exponent (1/4) in the Mott law and the localization length exponent ν=p/3 derived from the scaling of{T}₀$ suggest a filling-in of the Coulomb gap and a reduced role of electron-electron interactions in the temperature range of these measurements.