Visualizing the Response of Helical Metals on the Surface of Topological Insulators to Disorder

Topological insulators are a recently discovered new phase of matter. Similar to the quantum Hall state, its classification stems from the topology of the bulk electronic wavefunction rather than an order parameter associated with a broken symmetry. The topology class of the insulating bulk assures the formation of unique surface states on the sample boundaries with Dirac-like dispersion and helical spin texture which are protected by time reversal symmetry. Using scanning tunneling microscopy (STM) and spectroscopic mappings we show that the helical spin texture of the surface states indeed provides protection against backscattering. Studying Fabry-Perot resonating conditions we further demonstrate that the Dirac states transmit through crystallographic defects with unusually high probability as they wrap throughout the sample surface [1]. Yet, we find that the topologically protected surface states are not immune to the underlying potential landscape induced by the crystallographic defects on the surface and charged impurities in the bulk [2]. These give rise to strong fluctuations in energy, momentum and helicity which are far more dominant than effects due to breaking of time-reversal symmetry (such as ferromagnetism [3]). Implications of our findings on various experimental observations and the hurdles they impose on the ability to manipulate the novel surface states for spintronic applications will be discussed, as well as alternative approaches we are currently devising and put to the test.

[1] J.Seo, P. Roushan, H. Beidenkopf, Y. S. Hor, R. J. Cava, and A. Yazdani, Nature 466, 343 (2010).

[2] H. Beidenkopf, P. Roushan, J. Seo, L. Gorman, I. Drozdov, Y. S. Hor, R. J. Cava, and A. Yazdani, Nature Physics (Advance Online Publication, DOI: 10.1038/nphys2108).

[3] Y. S. Hor, P. Roushan, H. Beidenkopf, J. Seo, D. Qu, J. G. Checkelsky, L. A. Wray, D. Hsieh, Y. Xia, S. Y. Xu, D. Qian, M. Z. Hasan, N. P. Ong, A. Yazdani, and R. J. Cava, Phys. Rev. B 81, 195203 (2010).