Visualizing Surface States of Topological Insulators with Scanning Tunneling Microscopy

Abstract

Topological insulators are materials that have a bulk band gap similar to commonly known insulators, but have conducting states on their edge or surface. The bulk band gap is generated because of the strong spin-orbit coupling inherent to these systems, which also modifies the band in a fundamental way, leading to unconventional spin-polarized Dirac fermions on the boundary of the insulator. In this thesis, we present our scanning tunneling spectroscopy studies of topological surface states in Bi_{1- x}Sb_x, Sb, Bi ₂Te₃ and Bi₂ Se₃. Due to their helical spin texture, the surface states of topological insulators are distinct from the well-known conventional surface states of noble metals. A key predicted feature of these metallic surface states is their immunity to localization and ability to overcome barriers caused by material imperfections. These predictions can be examined by studying the scattering of surface states from random alloying disorder. We have mapped the interference of the surface states in Bi_0.9 Sb_0.1 and demonstrated that despite strong atomic scale disorder, backscattering between states of opposite momentum and opposite spin is absent, resulting from the helical spin texture of the surface states. Furthermore, we have measured transmission and reflection of the topological surface states of Sb by atomic terraces. In contrast to surface states of noble metals, these surface states penetrate such barriers with high probability. These results experimentally demonstrate the fundamental difference of these surface states in comparison to other known surface states, and show their potential to be used for spin-based electronics and nano-scale devices.

In recent years, the topological surface states of Bi ₂Te₃ and Bi₂ Se₃, the "second generation" topological insulators, has become the focus of intense research. The single Dirac cone surface states on these compounds constitutes the simplest manifestation of 3D topological insulators. Many of the interesting theoretical proposals that utilize topological insulator surfaces require the chemical potential to lie at or near the surface Dirac point, and consequently bulk doping is commonly used to tune the chemical potential to the Dirac point. We have studied the surface states of Bi₂Te₃ and Bi ₂Se₃ in the presence of magnetic and non-magnetic dopants. Bulk doping results in strong nano-scale spatial fluctuations of the surface states' energy and momentum. In spite of these fluctuations, Dirac electrons show a remarkable robustness to backscattering that can be understood based on their helical spin texture, which is preserved even in the presence of magnetic dopants or bulk magnetism. While we show that these strong spatial fluctuations influence the transmission of topological surface states, we find no evidence for their localization by bulk or surface disorder. In the vicinity of the Dirac point, the energy and momentum fluctuations we observed would result in spatially alternating spin helicity. This could possibly limit the mobility of topological surface state near the Dirac point. Our findings suggest that utilization of helical Dirac fermions on topological insulators requires methods of tuning the chemical potential which do not involve chemical doping.