An edge index for the Quantum Spin-Hall effect

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.

4 pages, 3 figures

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• Controlled topological phases and bulk-edge correspondence
• Three-dimensional phase diagram of disordered HgTe/CdTe Quantum spin-Hall wells
• The edge spectrum of Chern insulators with rough boundaries
• Ab initio study of $Z_2$ topological phases in perovskite (111) $({\text{SrTiO}}_3{)}_7/({\text{SrIrO}}_3{)}_2$ and $({\text{KTaO}}_3{)}_7/({\text{KPtO}}_3{)}_2$ multilayers
• Topological phonon modes in filamentous structures
• Manifestly gauge independent formulations of the Z2 invariants