ELECTRON CONDUCTION WITHIN LANDAU-LEVEL TAILS OF MEDIUM-MOBILITY GAASALGAAS HETEROSTRUCTURES/

The temperature dependence of both components, rho(xx)(T) and rho(xy)( T), of the resistivity tensor has been studied at T greater than or eq ual to 4.2 K within integer quantum Hall effect plateaux around fillin g factors nu = 2 and nu = 4 of medium-mobility GaAs/AlGaAs heterostruc tures. In the middle of the mobility gap standard activated conductivi ty has been found with activation energies Delta scaling well with HBA R omega(c)/2. At filling factors slightly below nu = 2 another contrib ution adds to the activated conductivity at T less than or equal to 12 K. This additional contribution can be further enhanced at higher mea suring direct currents. We suggest that it arises due to enhanced elec tric-field-assisted tunnelling across potential barriers separating lo calized states within the bulk of the sample. This effect contributes to the backscattering across the sample, leading to an enhanced longit udinal conductivity. The additional contribution to sigma(xx)(T) can b e reasonably well fitted to the formula for the variable range hopping in strong magnetic fields, indicating that the hopping can persist ev en at temperatures well above 4.2 K.