  Quantum spin Hall (QSH) materials are two-dimensional systems exhibiting
insulating bulk and helical edge states simultaneously. A QSH insulator
processes topologically non-trivial edge states protected by time-reversal
symmetry, so that electrons can propagate unscattered. Realization of such
topological phases enables promising applications in spintronics,
dissipationless transport and quantum computations. Presently, realization of
such QSH-based devices are limited to complicated heterostructures. Monolayer
1T'-WTe2 was predicted to be semimetallic QSH materials, though with a negative
band gap. The quasi-particle spectrum obtained using hybrid functional approach
shows directly that the quantum spin Hall gap is positive for monolayer
1T'-WTe2. Optical measurement shows a systematic increase in the interband
relaxation time with decreasing number of layers, whereas transport measurement
reveals Schottcky barrier in ultrathin samples, which is absent for thicker
samples. These three independent pieces of evidence indicate that monolayer
1T'-WTe2 is likely a truly 2-dimensional quantum spin Hall insulator.
