  Current generated spin polarization in topological insulator (TI) surface
states due to spin-momentum locking has been detected recently using
ferromagnet/tunnel barrier contacts, where the projection of the TI spin onto
the magnetization of the ferromagnet is measured as a voltage. However,
opposing signs of the spin voltage have been reported, which had been
tentatively attributed to the coexistence of trivial two-dimensional electron
gas states on the TI surface which may exhibit opposite current-induced
polarization than that of the TI Dirac surface states. Models based on
electrochemical potential have been presented to determine the sign of the spin
voltage expected for the TI surface states. However, these models neglect
critical experimental parameters which also affect the sign measured. Here we
present a Mott two-spin current resistor model which takes into account these
parameters such as spin-dependent interface resistances, and show that such
inclusion can lead to a crossing of the voltage potential profiles for the
spin-up and spin-down electrons within the channel, which can lead to measured
spin voltages of either sign. These findings offer a resolution of the ongoing
controversy regarding opposite signs of spin signal reported in the literature,
and highlight the importance of including realistic experimental parameters in
the model.
