Large-Spin Magnetic Impurity near a 2D Topological Edge

2D topological insulators have attracted much attention lately, due to their gapless helical edge modes, which should display maximal charge-spin entanglement and be protected from time-reversal-invariant backscattering at low temperatures. However, significant low-energy backscattering was measured in recent experiments, an observation that has hitherto remained elusive. In this work we study the possible role that may be played by magnetic impurities with spin larger than 1/2, such as the ubiquitous S=5/2 Mn in HgTe. For the first time we treat the case of arbitrary isotropy in the impurity-edge exchange, as well as the self-exchange (local anisotropy) of the impurity. We find the latter may strongly enhance backscattering, not only at low temperatures and voltages (where it can exponentially suppress Kondo screening), but also at relatively high energies. The resulting rich behavior of the current-voltage characteristics may allow experiments to reveal the complex internal structure of the magnetic impurities.