Magnetic impurities on the surface of a topological insulator

TL;DR

This paper investigates how magnetic impurities affect the surface states of topological insulators, revealing local gap opening, ferromagnetic RKKY interactions, and implications for experimental detection and disorder effects.

Contribution

It demonstrates that magnetic impurities induce a local gap and mediate ferromagnetic RKKY interactions on topological insulator surfaces, with analysis of disorder effects.

Findings

Magnetic impurities open a local gap in surface states.

RKKY interaction among impurities is always ferromagnetic near the Dirac point.

Effects are detectable via STM experiments.

Abstract

The surface states of a topological insulator are described by an emergent relativistic massless Dirac equation in 2+1 dimensions. In contrast to graphene, there is an odd number of Dirac points, and the electron spin is directly coupled to the momentum. We show that a magnetic impurity opens up a local gap and suppresses the local density of states. Furthermore, the Dirac electronic states mediate an RKKY interaction among the magnetic impurities which is always ferromagnetic, whenever the chemical potential lies near the Dirac point. These effects can be directly measured in STM experiments. We also study the case of quenched disorder through a renormalization group analysis.