Spatial Fluctuations of Helical Dirac Fermions on the Surface of Topological Insulators

TL;DR

This study investigates how topological insulator surface states respond to magnetic and non-magnetic doping, revealing resilience to backscattering far from the Dirac point but significant fluctuations near it, impacting their electronic manipulation.

Contribution

It provides the first detailed spectroscopic mapping of the nanoscale response of topological insulator surface states to various dopants near the Dirac energy.

Findings

Helicity resists backscattering far from Dirac energy.

Nanoscale fluctuations increase near the Dirac point due to doping.

Reducing charge defects is crucial for tuning and mobility.

Abstract

Surfaces of topological insulators host a new class of states with Dirac dispersion and helical spin texture. Potential quantum computing and spintronic applications using these states require manipulation of their electronic properties at the Dirac energy of their band structure by inducing magnetism or superconductivity through doping and proximity effect. Yet, the response of these states near the Dirac energy in their band structure to various perturbations has remained unexplored. Here we use spectroscopic mapping with the scanning tunneling microscope to study their response to magnetic and non-magnetic bulk dopants in Bi2Te3 and Bi2Se3. Far from the Dirac energy helicity provides remarkable resilience to backscattering even in the presence of ferromagnetism. However, approaching the Dirac point, where the surface states' wavelength diverges bulk doping results in pronounced nanoscale spatial fluctuations of energy, momentum, and helicity. Our results and their connection with similar studies of Dirac electrons in graphene demonstrate that while backscattering and localization are absent for Dirac topological surface states, reducing charge defects is required for both tuning the chemical potential to Dirac energy and achieving high electrical mobility for these novel states.