Observation of the Quantum Spin Hall Effect up to 100 Kelvin in a Monolayer Crystal

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.