Linear and Nonlinear Magnetotransport on a Quantum Spin Hall Edge

We model the magnetotransport on a helical edge of a quantum spin Hall insulator, in the presence of bulk midgap states side-coupled to the edge. In the presence of a magnetic field, the midgap levels are spin split, and hybridization of these levels with the itinerant edge states leads to backscattering, and the ensuing increase in the resistance. We show that there is a singular cusplike contribution to the positive magnetoresistance stemming from resonant midgap states weakly coupled to the edge. Based on it, we develop a theory of nonlinear low-magnetic-field magnetotransport on a helical edge of a quantum spin Hall insulator due to the edge state coupling to bulk midgap states. We focus on the part of the nonlinear I-V characteristic that is odd in the applied magnetic field and quadratic in the applied bias voltage. This part of the I-V characteristic corresponds to the resistance of the sample being dependent on the relative orientation of the current and an external magnetic field, hence representing a type of edge magnetochiral anisotropy. We identify a mechanism of the magnetochiral anisotropy related to the Hubbard interaction on the midgap state, which leads to the dependence of the scattering characteristics on the current flowing on the edge. This, in turn, leads to bias-voltage-dependent resistance, or equivalently conductance; hence, a nonlinear I-V. We compare the developed theory to the experiments on monolayer WTe2 and find good agreement with the developed theory.