Signatures of Helical Edge Transport in Millimetre-Scale Thin Films of Na3Bi

TL;DR

This study demonstrates that millimeter-scale Na3Bi thin films exhibit helical edge transport with high spin polarization, evidenced by giant negative magnetoresistance and non-local conductance, confirming the topological nature of their edge states.

Contribution

It provides the first experimental evidence of helical edge transport in large-area Na3Bi 2DTI films using non-local measurements and magnetic field effects.

Findings

Edge mean free path ~100 nm

Giant negative magnetoresistance up to 80% at 0.9 T

>98% of scattering is helical spin scattering

Abstract

A two-dimensional topological insulator (2DTI) has an insulating bulk and helical spin-polarised edge modes robust to backscattering by non-magnetic disorder. While ballistic transport has been demonstrated in 2DTIs over short distances, larger samples show significant backscattering and a nearly temperature-independent resistance whose origin is unclear. 2DTI edges have shown a spin polarisation, however the degree of helicity is difficult to quantify from spin measurements. Here, we study 2DTI few-layer Na3Bi on insulating Al2O3. A non-local conductance measurement geometry enables sensitive detection of the edge conductance in the topological regime, with an edge mean free path ~100 nm. Magnetic field suppresses spin-flip scattering in the helical edges, resulting in a giant negative magnetoresistance (GNMR), up to 80% at 0.9 T. Comparison to theory indicates >98% of scattering is helical spin scattering significantly exceeding the maximum (67%) expected for a non-helical metal. GNMR, coupled with non-local measurements demonstrating edge conduction, thus provides an unambiguous experimental signature of helical edges that we expect to be generically useful in understanding 2DTIs.