Robustness of helical edge states in 2D topological insulators (TI) againststrong interactions remains an intriguing issue. Here, by performing the firstsign-free quantum Monte Carlo (QMC) simulation of the Kane-Mele-Hubbard-Rashbamodel which describes an interacting 2D TI with two-particle backscattering onedges, we verify that the gapless helical edge states are robust against afinite range of two-particle backscattering when the Coulomb repulsion is notstrong. However, when the Coulomb repulsion is strong enough, the helical edgestates can be gapped by infinitesimal two-particle backscattering, resulting inedge magnetic order. We further reveal universal properties of the magneticedge quantum critical point (EQCP). At magnetic domain walls on edges, we findthat a fractionalized charge of e/2 emerges. Implications of our results torecent transport experiments in the InAs/GaSb quantum well, which is a 2D TIwith strong interactions, will also be discussed.
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