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 ${\mathrm{WTe}}_2$, and find good agreement with the developed theory.

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• Quantum spin Hall effect
• Transport phenomena
• Topological materials