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Topological surface states protected from backscattering by chiral spin texture

Abstract

Topological insulators are a new class of insulators in which a bulk gap for electronic excitations is generated because of the strong spin–orbit coupling1,2,3,4,5 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 structures6,7,8 and several bismuth-based compounds9,10,11,12,13, 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 insulator Bi1-xSb x , and examine in detail the influence of scattering from disorder caused by random alloying in this compound. We show that, despite strong atomic scale disorder, backscattering between states of opposite momentum and opposite spin is absent. Our observations demonstrate that the chiral nature of these states protects the spin of the carriers. These chiral states are therefore potentially useful for spin-based electronics, in which long spin coherence is critical14, and also for quantum computing applications, where topological protection can enable fault-tolerant information processing15,16.

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Figure 1: STM topography and d I /d V spectroscopy of the Bi 0.92 Sb 0.08 (111) surface.
Figure 2: d I /d V maps, QPI patterns and ARPES measurements on the Bi 0.92 Sb 0.08 (111) surface.
Figure 3: Construction of JDOS and SSP from ARPES data and their comparison with FT-STS.
Figure 4: Comparison of the various parts of the QPI patterns along the direction at the Fermi energy.
Figure 5: Dispersion of various peaks from FT-STS and ARPES.

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Acknowledgements

We acknowledge K. K. Gomes and A. N. Pasupathy for suggestions about experimental procedure and initial analysis. This work was supported by grants from ONR, ARO, DOE, NSF-DMR and the NSF-MRSEC programme through the Princeton Center for Complex Materials. P.R. acknowledges an NSF graduate fellowship.

Author Contributions Y.S.H. and R.J.C. carried out the growth of the single crystals and characterized them; D.H., D.Q. and M.Z.H. performed the ARPES studies of the samples; STM measurements and data analysis were done by P.R., J.S., C.V.P., A.R. and A.Y.

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Correspondence to Ali Yazdani.

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Roushan, P., Seo, J., Parker, C. et al. Topological surface states protected from backscattering by chiral spin texture. Nature 460, 1106–1109 (2009). https://doi.org/10.1038/nature08308

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