Topological surface states protected from backscattering by chiral spin texture

• Published in: Nature (London) 2009-08, Vol.460 (7259), p.1106-1109
• Main Authors: Roushan, Pedram, Seo, Jungpil, Parker, Colin V., Hor, Y. S., Hsieh, D., Qian, Dong, Richardella, Anthony, Hasan, M. Z., Cava, R. J., Yazdani, Ali
• Format: Article
• Language: English
• Subjects: Algebraic topology; Band structure; Clean metal, semiconductor, and insulator surfaces; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Crystals; Electron and ion emission by liquids and solids; impact phenomena; Electrons; Exact sciences and technology; Fourier transforms; Humanities and Social Sciences; letter; multidisciplinary; Photoemission; Photoemission and photoelectron spectra; Physics; Science; Science (multidisciplinary); Single crystals; Spectroscopy; Spectrum analysis; Topology
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature08308

Topological insulators: no turning back Topological insulators are materials in which a relativistic effect known as spin–orbit coupling gives rise to a bulk insulating gap and surface states that resemble so-called chiral edge states in the quantum Hall effect. It has been theoretically suggested that the quantum mechanical spin degree of freedom of such surface edge states may be protected against scattering due to topology, which could be useful for spintronics and quantum computing. Now Roushan et al . provide the experimental confirmation of this important prediction. Using scanning tunnelling and angle-resolved photoemission microscopy they are able to demonstrate that, despite strong atomic scale disorder in their system, backscattering between surface states with opposite momentum and opposite spin is absent. Topological insulators are materials in which a relativistic effect known as spin–orbit coupling gives rise to surface states that resemble chiral edge modes in quantum Hall systems, but with unconventional spin textures. It has been suggested that a feature of such spin-textured boundary states is their insensitivity to spin-independent scattering, which is thought to protect them from backscattering. Here, scanning tunnelling spectroscopy and angle-resolved photoemission spectroscopy are used to confirm this prediction. Topological insulators are a new class of insulators in which a bulk gap for electronic excitations is generated because of the strong spin–orbit coupling 1 , 2 , 3 , 4 , 5 inherent to these systems. These materials are distinguished from ordinary insulators by the presence of gapless metallic surface states, resembling chiral edge modes in quantum Hall systems, but with unconventional spin textures. A key predicted feature of such spin-textured boundary states is their insensitivity to spin-independent scattering, which is thought to protect them from backscattering and localization. Recently, experimental and theoretical efforts have provided strong evidence for the existence of both two- and three-dimensional classes of such topological insulator materials in semiconductor quantum well structures 6 , 7 , 8 and several bismuth-based compounds 9 , 10 , 11 , 12 , 13 , but so far experiments have not probed the sensitivity of these chiral states to scattering. Here we use scanning tunnelling spectroscopy and angle-resolved photoemission spectroscopy to visualize the gapless surface states in the three-dimensional topological i