Topological insulators are a new class of insulators in which a bulk gap for electronic excitations is generated by strong spin-orbit coupling. These novel materials are distinguished from ordinary insulators by the presence of gapless metallic boundary states, akin to the chiral edge modes in quantum Hall systems, but with unconventional spin textures. A key characteristic of these spin-textured boundary states is their insensitivity to spin-independent scattering, which protects them from backscattering and localization. We use a scanning tunneling microscope (STM) to visualize the gapless surface states of the three-dimensional topological insulator Bi1-xSbx and to examine their scattering behavior from disorder caused by random alloying in this compound. Combining STM and angle-resolved photoemission spectroscopy, we show that despite strong atomic scale disorder backscattering between states of opposite momentum and opposite spin is absent. Our observation of spin-selective scattering demonstrates that the chiral nature of these states protects the spin of the carriers; they therefore have the potential to be used for coherent spin transport in spintronic devices.
Electrons in topological insulator BiSb scatter from alloying defects leaving distinct signatures in the Fourier transform of the STM measured LDOS. These FFT maps show a absence of waavevector connecting k to –k for surface wave scattering hence proving absence of backscattering