  We study magneto-transport phenomena in two rare-earth doped topological
insulators, SmxFexSb2-2xTe3 and SmxBi2-xTe2Se single crystals. The
magneto-transport behaviours in both compounds exhibit a systematic crossover
between weak anti-localization (positive magnetoresistance) and weak
localization (negative magnetoresistance) with changes in temperatures and
magnetic fields. The weak localization is caused by rare-earth-doping induced
magnetization, and the weak anti-localization originates from topologically
protected surface states. The transition between weak localization and weak
anti-localization demonstrates a gap opening at the Dirac point of surface
states in the quantum diffusive regime. This work demonstrates an effective way
to manipulate the magneto-transport properties of the topological insulators by
rare-earth element doping. Magnetometry measurements indicate that the
Sm-dopant alone is paramagnetic, whereas the co-doped Fe-Sm state has
short-range antiferromagnetic order. Our results hold potential for the
realization of exotic topological effects in gapped topological insulator
surface states.
