Modeling electrical detection of current generated spin in topological insulator surface states

Current generated spin in topological insulator (TI) surface states due to spin-momentum locking has been detected recently using FM/tunnel barrier contacts, where the projection of the TI spin onto the magnetization of the FM is measured as a voltage. However, opposing signs of the spin voltage have been reported, attributed to the coexistence of trivial 2DEG states on the TI surface that may exhibit opposite current-induced polarization. Models based on electrochemical potential have been presented to determine the sign of the spin voltage expected for the TI surface states, however neglecting critical experimental parameters. Here we present a Mott two-spin current resistor model which takes into account parameters such as spin-dependent interface resistances, and show that such inclusion can lead to a crossing of the voltage 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.