  The boundary modes of topological insulators are protected by the symmetries
of the non-trivial bulk electronic states. Unless these symmetries are broken,
they can give rise to novel phenomena, such as the quantum spin Hall effect in
one-dimensional (1D) topological edge states, where quasiparticle
backscattering is suppressed by time-reversal symmetry (TRS). Here, we
investigate the properties of the 1D topological edge state of bismuth in the
absence of TRS, where backscattering is predicted to occur. Using spectroscopic
imaging and spin-polarized measurements with a scanning tunneling microscope,
we compared quasiparticle interference (QPI) occurring in the edge state of a
pristine bismuth bilayer with that occurring in the edge state of a bilayer,
which is terminated by ferromagnetic iron clusters that break TRS. Our
experiments on the decorated bilayer edge reveal an additional QPI branch,
which can be associated with spin-flip scattering across the Brioullin zone
center between time-reversal band partners. The observed QPI characteristics
exactly match with theoretical expectations for a topological edge state,
having one Kramer's pair of bands. Together, our results provide further
evidence for the non-trivial nature of bismuth, and, in particular, demonstrate
backscattering inside a helical topological edge state induced by broken TRS
through local magnetism.
