Switching spin and charge between edge states in topological insulator constrictions

TL;DR

This paper proposes a novel method for controlling spin and charge currents in topological insulator edge states using constrictions, enabling a three-state transistor function without relying on Rashba spin-orbit interaction.

Contribution

It introduces a new approach to manipulate edge state coupling in HgTe topological insulators for spin switching and charge control via constrictions, supported by numerical calculations.

Findings

Coupling in constrictions enables spin and charge control.

Demonstrates a three-state spin and charge transistor.

Control achieved through gating of the constriction.

Abstract

Since the prediction of a new topological state of matter in graphene, materials acting as topological insulators have attracted wide attention. Shortly after the theoretical proposal for a mercury telluride (HgTe)-based two-dimensional topological insulator, the observation of the quantum spin Hall effect and non-local edge transport brought compelling experimental evidence for quantized conductance due to edges states. The spin orientation and propagation direction of such helical edge states are inherently connected, providing protection against backscattering. However, these features conversely render controlled spin operations such as spin switching difficult. Here we therefore propose constrictions as connectors between opposite edge (and spin) states in HgTe. We demonstrate how the coupling between edge states, which overlap in the constriction, can be employed both for steering the charge flow into different edge modes and for controlled spin switching. This gives rise to a three-state charge and spin transistor function. Unlike in a conventional spin transistor, the switching does not rely on a tunable Rashba spin-orbit interaction, but on the energy dependence of the edge state wavefunctions. Based on this mechanism, and supported by numerical transport calculations, we present two different ways to control spin- and charge-currents depending on the gating of the constriction.