  Monolayer WTe2 is predicted to be a quantum spin Hall insulator (QSHI) and
electron transport along its edges has been experimentally observed. However,
the 'smoking gun' of QSHI, spin momentum locking of the edge electrons, has not
been experimentally demonstrated. We propose a model to establish the
relationship between the anisotropic magnetoresistance (AMR) and spin
orientation of the helical electrons in WTe2. Based on the predictions of the
model, angular dependent magnetoresistance measurements were carried out. The
experimental results fully supported the model and the spin orientation of the
helical edge electrons was determined. Our results not only demonstrate that
WTe2 is indeed a QSHI, but also suggest a convenient method to determine the
spin orientation of other QSHIs.
