Seminar FO: F. Yang: Observation of different types of charge-current-induced spin polarization in BiSbTeSe2 topological insulators

The surface states of 3D topological insulators (TIs) possess a helical 
spin texture in which the spin and momentum are perpendicularly locked 
to each other. Due to this spin-momentum locking, a net spin 
polarization can be induced by a charge current and vice versa. However, 
topological surface states are expected to give rise to only one type of 
spin polarization for a given current direction, which has been a 
limiting factor for spin manipulations. In this talk we report that in 
devices based on the bulk-insulating topological insulator BiSbTeSe$_2$, 
an unexpected switching of spin polarization was observed upon changing 
the chemical potential. The spin polarization expected from the 
topological surface states was detected in a heavily electron-doped 
device, whereas the opposite polarization was reproducibly observed in 
devices with low carrier densities. We propose that the latter type of 
spin polarization stems from topologically-trivial two-dimensional 
states with a large Rashba spin splitting, which are caused by a strong 
band bending at the surface of BiSbTeSe$_2$ beneath the ferromagnetic 
electrode used as a spin detector. This finding paves the way for 
realizing the "spin transistor" operation in future topological 
spintronic devices.