TITLE: Direct electrical detection of spin-momentum locking in topological insulators
ABSTRACT: Topological insulator (TI) is a new quantum state of matter characterized by an insulating bulk with metallic surface states populated by massless Dirac fermions. One of its most striking properties is that of spin-momentum locking - the spin of the TI surface state lies in-plane, and is locked at right angle to the carrier momentum. An unpolarized charge current should thus create a net spin polarization whose amplitude and orientation are controlled by the charge current. This remarkable property has been anticipated by theory, but never accessed in a simple transport structure. Here we show that a bias current indeed produces a net surface state spin polarization via spin-momentum locking in molecular beam epitaxially grown Bi2Se3 films, and this polarization is directly manifested as a voltage on a ferromagnetic metal contact. This voltage is proportional to the projection of the TI spin onto the contact magnetization, is determined by the direction and magnitude of the bias current, with a sign expected from spin-momentum locking rather than Rashba effects. Further demonstration of this direct electrical detection of spin-momentum locking in a p-type TI, as well as the spin-to-charge conversion via the inverse Elderstein effect will also be briefly discussed. These results demonstrate direct electrical access to the TI surface state spin system and enable utilization of its remarkable properties for future technological applications.