Fig. 1: Backscattering between helical edge states.
From: Backscattering in topological edge states despite time-reversal symmetry
a1 Schematic linear edge dispersion of a QSHI with one single Kramers pair within the topological bulk gap10. Blue and red colors mark channels of opposite spin polarizations. a2 Corresponding real-space cartoon of a right-moving spin up current (blue: positive group velocity dE/dk) and a left-moving (dE/dk < 0) spin down current (red) allocated at a single edge of a QSHI. Scattering between these channels is prohibited by TRS. b1 Schematic non-linear but \({{\mathcal{S}}}\)-shaped edge dispersion of a QSHI with energy regions of one and three Kramers pairs within the topological bulk gap, the latter indicated by purple shading. b2 Real-space cartoon of the three Kramers pairs scenario, with disparate numbers of left and right movers in the two spin channels. TRS-conserving backscattering between two Kramers pairs at non-magnetic defects produces standing waves with wavelength 2π/q.
