Backscattering of topologically protected helical edge states by line defects

TL;DR

This paper investigates how a line defect of impurities can break the topological protection of helical edge states in two-dimensional topological insulators, enabling backscattering and conductance suppression.

Contribution

It introduces a novel mechanism where a line of impurities disrupts topological protection, supported by theoretical modeling and analysis of impurity-induced bound states.

Findings

Line defects can induce backscattering in helical edge states.

Impurity channels form due to overlapping in-gap bound states.

Conductance suppression occurs near the bulk gap edge.

Abstract

The quantization of conductance in the presence of non-magnetic point defects is a consequence of topological protection and the spin-momentum locking of helical edge states in two-dimensional topological insulators. This protection ensures the absence of backscattering of helical edge modes in the quantum Hall phase of the system. However, our study focuses on exploring a novel approach to disrupt this protection. We propose that a linear arrangement of on-site impurities can effectively lift the topological protection of edge states in the Kane-Mele model. To investigate this phenomenon, we consider an armchair ribbon containing a line defect spanning its width. Utilizing the tight-binding model and non-equilibrium Green's function method, we calculate the transmission coefficient of the system. Our results reveal a suppression of conductance at energies near the lower edge of the bulk gap for positive on-site potentials. To further comprehend this behavior, we perform analytical calculations and discuss the formation of an impurity channel. This channel arises due to the overlap of in-gap bound states, linking the bottom edge of the ribbon to its top edge, consequently facilitating backscattering. Our explanation is supported by the analysis of the local density of states at sites near the position of impurities.