# Interplay of chiral and helical states in a Quantum Spin Hall Insulator lateral junction

## TL;DR

This study investigates electronic transport in a HgTe quantum well junction, revealing how magnetic fields and carrier density influence chiral and helical edge states, including fractional quantum Hall effects and interference phenomena.

## Contribution

It demonstrates the tunability of edge modes in a topological insulator junction under magnetic fields and carrier density control, highlighting the interplay between chiral and helical states.

## Key findings

- Observation of fractional quantum Hall plateaus due to edge mode equilibration
- Detection of Fabry-Perot interference oscillations in helical states
- Disappearance of oscillations at high magnetic fields when band inversion is lifted

## Abstract

We study the electronic transport across an electrostatically-gated lateral junction in a HgTe quantum well, a canonical 2D topological insulator, with and without applied magnetic field. We control carrier density inside and outside a junction region independently and hence tune the number and nature of 1D edge modes propagating in each of those regions. Outside the 2D gap, magnetic field drives the system to the quantum Hall regime, and chiral states propagate at the edge. In this regime, we observe fractional plateaus which reflect the equilibration between 1D chiral modes across the junction. As carrier density approaches zero in the central region and at moderate fields, we observe oscillations in resistance that we attribute to Fabry-Perot interference in the helical states, enabled by the broken time reversal symmetry. At higher fields, those oscillations disappear, in agreement with the expected absence of helical states when band inversion is lifted.