  Topological insulators (TI) are a new class of quantum materials with
insulating bulk enclosed by topologically protected metallic boundaries. The
surface states of three-dimensional TIs have spin helical Dirac structure, and
are robust against time reversal invariant perturbations. This extraordinary
property is notably exemplified by the absence of backscattering by nonmagnetic
impurities and the weak antilocalization (WAL) of Dirac fermions. Breaking the
time reversal symmetry (TRS) by magnetic element doping is predicted to create
a variety of exotic topological magnetoelectric effects. Here we report
transport studies on magnetically doped TI Cr-Bi2Se3. With increasing Cr
concentration, the low temperature electrical conduction exhibits a
characteristic crossover from WAL to weak localization (WL). In the heavily
doped regime where WL dominates at the ground state, WAL reenters as
temperature rises, but can be driven back to WL by strong magnetic field. These
complex phenomena can be explained by a unified picture involving the evolution
of Berry phase with the energy gap opened by magnetic impurities. This work
demonstrates an effective way to manipulate the topological transport
properties of the TI surface states by TRS-breaking perturbations.
