An edge index for the Quantum Spin-Hall effect

TL;DR

This paper introduces a new topological invariant for Quantum Spin-Hall systems, demonstrating that a specific edge current observable is quantized and related to the index of a Fredholm operator, aiding the analysis of edge channels.

Contribution

It defines a novel observable and edge current whose quantization is linked to a Fredholm index, providing a new tool for studying edge states in Quantum Spin-Hall systems.

Findings

The edge current associated with the new observable is quantized.

The quantization corresponds to the index of a Fredholm operator.

Edge conducting channels are robust against random edge perturbations.

Abstract

Quantum Spin-Hall systems are topological insulators displaying dissipationless spin currents flowing at the edges of the samples. In contradistinction to the Quantum Hall systems where the charge conductance of the edge modes is quantized, the spin conductance is not and it remained an open problem to find the observable whose edge current is quantized. In this paper, we define a particular observable and the edge current corresponding to this observable. We show that this current is quantized and that the quantization is given by the index of a certain Fredholm operator. This provides a new topological invariant that is shown to take same values as the Spin-Chern number previously introduced in the literature. The result gives an effective tool for the investigation of the edge channels' structure in Quantum Spin-Hall systems. Based on a reasonable assumption, we also show that the edge conducting channels are not destroyed by a random edge.