  Evidence for the quantum spin Hall (QSH) effect has been reported in several
experimental systems in the form of approximately quantized edge conductance.
However, the most fundamental feature of the QSH effect, spin-momentum locking
in the edge channels, has never been demonstrated experimentally. Here, we
report clear evidence for spin-momentum locking in the edge channels of
monolayer WTe2, thought to be a two-dimensional topological insulator (2D TI).
We observe that the edge conductance is controlled by the component of an
applied magnetic field perpendicular to a particular axis, which we identify as
the spin axis. The axis is the same for all edges, situated in the mirror plane
perpendicular to the tungsten chains at 40$\pm$2{\deg} to the layer normal,
implying that the spin-orbit coupling is inherited from the bulk band
structure. We show that this finding is consistent with theory if the band-edge
orbitals are taken to have like parity. We conclude that this parity assignment
is correct and that both edge states and bulk bands in monolayer WTe2 share the
same simple spin structure. Combined with other known features of the edge
states this establishes spin-momentum locking, and therefore that monolayer
WTe2 is truly a natural 2D TI.
