  Topological insulators are insulating in the bulk but possess spin-momentum
locked metallic surface states protected by time-reversal symmetry. The
existence of these surface states has been confirmed by angle-resolved
photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM).
Detecting these surface states by transport measurement, which might at first
appear to be the most direct avenue, was shown to be much more challenging than
expected. Here, we report a detailed electronic transport study in high quality
Bi2Se3 topological insulator thin films. Measurements under in-plane magnetic
field, along and perpendicular to the bias current show opposite
magnetoresistance. We argue that this contrasting behavior is related to the
locking of the spin and current direction providing evidence for helical spin
structure of the topological surface states.
