ON THE SIGN AND TEMPERATURE-DEPENDENCE OF MAGNETORESISTANCE IN VARIABLE-RANGE-HOPPING IN HIGHLY COMPENSATED N-GAAS - THE ROLE OF SPIN-EFFECTS

The magnetoresistance of neutron-transmutation-doped (NTD) n-type GaAs has been measured in the variable range hopping regime at temperature s, T = 0.03 to 1.6 K. The negative hopping magnetoresistance (NHM) in small magnetic fields is strongly influenced by at least two positive hopping magnetoresistance (PHM) contributions including the effects of spin alignment. Three new effects are reported: (i) The log R versus T-1/2 plots in fixed magnetic field show two well-pronounced linear pa rts with different slopes forming a kink at a certain temperature T,wh ich shifts to higher temperatures as the applied magnetic field is inc reased, occurring at B-c/T-c congruent to 9 T/K. (ii) At the lowest te mperatures, we observed with decreasing temperatures a steep decrease and disappearance of the negative hopping magnetoresistance, followed by only (iii) a positive hopping magnetoresistance with two different slopes with log rho(B)/rho(0) proportional to B-2. We interpret this b ehavior as due to the interplay between the (coherent) quantum interfe rence with a negative sign and different incoherent mechanisms, includ ing spin-effects, which dominate the magnetoresistance at different te mperatures.