Quantum Spin Hall Effect in Graphene Nanoribbons: Effect of Edge Geometry

TL;DR

This paper investigates how edge geometry affects the quantum spin Hall effect in graphene nanoribbons, revealing that edge structure and passivation influence the stability and emergence of spin-filtered edge states.

Contribution

It provides a detailed analysis of edge geometry effects on QSHE in graphene nanoribbons, including the role of spin-orbit interaction and edge passivation.

Findings

QSHE does not occur in pristine zigzag nanoribbons without high carrier density.

Hydrogen passivation can restore QSHE in zigzag nanoribbons.

QSHE remains stable in armchair nanoribbons under studied conditions.

Abstract

There has been tremendous recent progress in realizing topological insulator initiated by the proposal of Kane and Mele for the graphene system. They have suggested that the odd $Z_2$ index for the graphene manifests the spin filtered edge states for the graphene nanoribbons, which lead to the quantum spin Hall effect(QSHE). Here we investigate the role of the spin-orbit interaction both for the zigzag and armchair nanoribbons with special care in the edge geometry. For the pristine zigzag nanoribbons, we have shown that one of the $\sigma$ edge bands located near E=0 lifts up the energy of the spin filtered chiral edge states at the zone boundary by warping the $\pi$-edge bands, and hence the QSHE does not occur. Upon increasing the carrier density above a certain critical value, the spin filtered edge states are formed leading to the QSHE. We suggest that the hydrogen passivation on the edge can recover the original feature of the QSHE. For the armchair nanoribbon, the QSHE is shown to be stable. We have also derived the real space effective hamiltonian, which demonstrates that the on-site energy and the effective spin orbit coupling strength are strongly enhanced near the ribbon edges. We have shown that the steep rise of the confinement potential thus obtained is responsible for the warping of the $\pi$-edge bands.