Spin polarized STM spectra of Dirac Fermions on the surface of a topological insulator

TL;DR

This paper develops a theoretical framework for understanding how spin-polarized STM spectra reveal the spin textures of Dirac Fermions on topological insulator surfaces, highlighting their unconventional dependence on tip magnetization orientation.

Contribution

It introduces a theory linking tunneling conductance to spin orientations of Dirac Fermions, including effects of symmetry breaking and magnetization direction.

Findings

G(V) depends on the tip's magnetization direction.

Broken in-plane symmetry introduces azimuthal dependence.

The theory suggests experimental tests for spin textures.

Abstract

We provide a theory for the tunneling conductance $G(V)$ of Dirac Fermions on the surface of a topological insulator as measured by a spin-polarized scanning tunneling microscope tip for low bias voltages $V$. We show that $G(V)$ exhibits an unconventional dependence on the direction of magnetization of the tip and can be used to measure the magnitude of the local out-of-plane spin orientation of the Dirac Fermions on the surface. We also demonstrate that if the in-plane rotational symmetry on the surface of the topological insulator is broken by an external field, then $G(V)$ acquires a dependence on the azimuthal angle of the magnetization of the tip. We explain the role of the Dirac Fermions in this unconventional behavior and suggest experiments to test our theory.