Magnetoresistance in semiconductor structures with hopping conductivity: Effects of random potential and generalization for the case of acceptor states

We reconsider the theory of magnetoresistance in hopping semiconductors. First, we have shown that the random potential of the background impurities affects significantly the pre-exponential factor of the tunneling amplitude which becomes short range in contrast to the long-range one for purely Coulomb hopping centers. This factor to some extent suppresses the negative interference magnetoresistance and can lead to its decrease with temperature decrease which is in agreement with the earlier experimental observations. We have also extended the theoretical models of positive spin magnetoresistance, in particular, related to presence of doubly occupied states (corresponding to the upper Hubbard band) to the case of acceptor states in two-dimensional structures. We have shown that this mechanism can dominate over the classical wave-shrinkage magnetoresistance at low temperatures. Our results are in semiquantitative agreement with the experimental data.