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Structure factor of interacting one-dimensional helical systems

We calculate the dynamical structure factor S(q, ω) of a weakly interacting helical edge state in the presence of a magnetic field B. The latter opens a gap of width 2B in the single-particle spectrum, which becomes strongly nonlinear near the Dirac point. For chemical potentials |μ| > B, the system then behaves as a nonlinear helical Luttinger liquid, and a mobile-impurity analysis reveals interaction-dependent power-law singularities in S(q,ω). For |μ| < B, the low-energy excitations are gapped, and we determine S(q,ω) by using an analogy to exciton physics.