Quantized Spin Transport in Magnetically-Disordered Quantum Spin Hall Systems

Helical edge states that transport electrons with opposing spins in opposite directions are formed in a quantum spin Hall (QSH) system. These states are essentially robust against time-reversal-invariant perturbations such as nonmagnetic disorder, but are vulnerable to magnetic disorder that breaks time reversal symmetry. In this study, we explore how magnetic disorder affects charge and spin transport in the QSH system on the basis of the nonequilibrium Green's function formalism. We show that quantized charge transport is destroyed by any weak magnetic disorder in an infinitely large system, whereas quantized and conserved spin transport survives magnetic disorder under a specific bias condition as long as the bulk gap remains intact. These two contrasting features are explained in terms of the spin-flip backward scattering allowed in helical edge channels and its effects on charge and spin currents.