  The field of topological insulators (TI) was sparked by the prediction of the
quantum spin Hall effect (QSHE) in time reversal invariant systems, such as
spin-orbit coupled monolayer graphene. Ever since, a variety of monolayer
crystals have been proposed as two-dimensional (2D) TIs exhibiting the QSHE,
possibly even at high temperatures. However, conclusive evidence for a
monolayer QSHE is still lacking, and systems based on semiconductor
heterostructures operate at temperatures close to liquid helium. Here we report
the observation of the QSHE in monolayer WTe2 at temperatures up to 100 Kelvin.
The monolayer exhibits the hallmark quantized transport conductance, ~ e2/h per
edge, in the short edge limit. Moreover, a magnetic field suppresses the
conductance, and the observed Zeeman-type gap indicates the existence of a
Kramers degenerate point, demonstrating the importance of time reversal
symmetry for protection from elastic backscattering. Our results establish the
high-temperature QSHE and open a new realm for the discovery of topological
phases based on 2D crystals.
