  Spin-momentum helical locking is one of the most important properties of the
nontrivial topological surface states (TSS) in 3D topological insulators (TI).
It underlies the iconic topological protection (suppressing elastic
backscattering) of TSS and is foundational to many exotic physics (eg.,
majorana fermions) and device applications (eg., spintronics) predicted for
TIs. Based on this spin-momentum locking, a current flowing on the surface of a
TI would be spin-polarized in a characteristic in-plane direction perpendicular
to the current, and the spin-polarization would reverse when the current
direction reverses. Observing such a spin-helical current in transport
measurements is a major goal in TI research and applications. We report
spin-dependent transport measurements in spin valve devices fabricated from
exfoliated thin flakes of Bi2Se3 (a prototype 3D TI) with ferromagnetic (FM) Ni
contacts. Applying an in-plane magnetic (B) field to polarize the Ni contacts
along their easy axis, we observe an asymmetry in the hysteretic
magnetoresistance (MR) between opposite B field directions. The polarity of the
asymmetry in MR can be reversed by reversing the direction of the DC current.
The observed asymmetric MR can be understood as a spin-valve effect between the
current-induced spin polarization on the TI surface (due to
spin-momentum-locking of TSS) and the spin-polarized ferromagnetic contacts.
Our results provide a direct transport evidence for the spin helical current in
TSS.
