Universal Spin-Induced Magnetoresistance in the Variable-Range Hopping Regime

The magnetoresistance in the variable-range hopping regime due to Zeeman spin-splitting and intra-impurity interactions is calculated analytically and shown to be a universal function of $\mu H / kT \log R$. Good agreement with numerical calculations in one and two dimensions is observed. With the inclusion of quantum interference effects, excellent agreement with recent experiments is obtained.