Crossover from negative to positive magnetoresistance in a Si delta-doped GaAs single quantum well

We have performed magnetoresistance measurements on a Si delta-doped GaAs single quantum well. With increasing temperature TT, a crossover from negative magnetoresistance (NMR) to positive magnetoresistance (PMR) can be observed. Our experimental results suggest that such a crossover corresponds to a transition from variable range hopping regime to activated electron transport. This is also consistent with the measured non-monotonic carrier density dependence on TT.