  Three-dimensional topological insulators (TIs) exhibit time-reversal symmetry
protected, linearly dispersing Dirac surface states. Band bending at the TI
surface may also lead to coexisting trivial two-dimensional electron gas (2DEG)
states with parabolic energy dispersion that exist as spin-split pairs due to
Rashba spin-orbit coupling (SOC). A bias current is expected to generate spin
polarization in both systems arising from their helical spin-momentum locking.
However, their induced spin polarization is expected to be different in both
magnitude and sign. Here, we compare spin potentiometric measurements of bias
current-generated spin polarization in Bi2Se3(111) films where Dirac surface
states coexist with trivial 2DEG states, with identical measurements on
InAs(001) samples where only trivial 2DEG states are present. We observe spin
polarization arising from spin-momentum locking in both cases, with opposite
signs of the spin voltage. We present a model based on spin dependent
electrochemical potentials to directly derive the signs expected for the TI
surface states, and unambiguously show that the dominant contribution to the
current-generated spin polarization measured in the TI is from the Dirac
surface states. This direct electrical access of the helical spin texture of
Dirac and Rashba 2DEG states is an enabling step towards the electrical
manipulation of spins in next generation TI and SOC based quantum devices.
