Random Rashba spin-orbit coupling at the quantum spin Hall edge

We study a one-dimensional helical system with random Rashba spin-orbit coupling. Using renormalization group methods, we derive a consistent set of flow equations governing the important control parameters of the backscattering process. Thereby, we prove the existence of disorder-induced two-particle backscattering that can even be nonlocal in space. This analysis allows us to derive the scaling form of the conductance at low temperatures. We find that two-particle backscattering due to random spin-orbit coupling differs from the one off a single Rashba impurity by both the scaling of the conductance with the temperature and the relevance of the backscattering operators.