The effect of magnetic impurity scattering on transport in topological insulators

TL;DR

This paper investigates how magnetic impurities affect charge transport in topological insulators, revealing non-monotonic conductance responses and differences in magneto-transport signatures depending on impurity density.

Contribution

It introduces a detailed analysis of magnetic impurity effects on helical edge states using NEGF and semi-analytic methods, highlighting defect interactions and conductance behavior.

Findings

Magnetic impurities cause back-scattering via spin-flip processes.

Conductance shows non-monotonic dependence on impurity density.

Differences in magneto-transport signatures between dilute and dense impurity regimes.

Abstract

Charge transport in topological insulators is primarily characterised by so-called topologically projected helical edge states, where charge carriers are correlated in spin and momentum. In principle, dissipation-less current can be carried by these edge states as backscattering from impurities and defects is suppressed as long as time-reversal symmetry is not broken. However, applied magnetic fields or underlying nuclear spin-defects in the substrate can break this time reversal symmetry. In particular, magnetic impurities lead to back-scattering by spin-flip processes. We have investigated the effects of point-wise magnetic impurities on the transport properties of helical edge states in the BHZ model using the Non-Equilibrium Green's Function formalism and compared the results to a semi-analytic approach. Using these techniques we study the influence of impurity strength and spin impurity polarization. We observe a secondary effect of defect-defect interaction that depends on the underlying material parameters which introduces a non-monotonic response of the conductance to defect density. This in turn suggests a qualitative difference in magneto-transport signatures in the dilute and high density spin impurity limits.