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Topological insulators constitute a new quantum phase of matter distinct from the classic dichotomy of simple metals and semiconductors. This phase was predicted in 2007 to exist in certain materials with high spin-orbit interaction, and confirmed a year later by spectroscopic measurements. While the bulk of a TI material is nominally an insulator, the surface layer (approximately one nanometer thick) is occupied by metallic states that are topologically protected from perturbations to their environment. TIs are expected to exhibit new behaviors and open horizons for science previously inaccessible with "conventional" materials. One of the most striking properties is that of spin-momentum locking—the spin of an electron in the TI surface state is locked at a right angle to its momentum. This consequently implies that when an unpolarized charge current flows in the topologically protected surface states, a net electron spin polarization should spontaneously appear whose amplitude and orientation are controlled by the charge current. This remarkable property has been anticipated by theory since 2010, but never measured by a transport experiment. Detecting this spin polarization directly as a voltage is key to understanding the fundamental properties of such materials, and interfacing them to electronic circuitry for future device applications.

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