  A quantum spin Hall (QSH) insulator is a novel two-dimensional quantum state
of matter that features quantized Hall conductance in the absence of magnetic
field, resulting from topologically protected dissipationless edge states that
bridge the energy gap opened by band inversion and strong spin-orbit coupling.
By investigating electronic structure of epitaxially grown monolayer 1T'-WTe2
using angle-resolved photoemission (ARPES) and first principle calculations, we
observe clear signatures of the topological band inversion and the band gap
opening, which are the hallmarks of a QSH state. Scanning tunneling microscopy
measurements further confirm the correct crystal structure and the existence of
a bulk band gap, and provide evidence for a modified electronic structure near
the edge that is consistent with the expectations for a QSH insulator. Our
results establish monolayer 1T'-WTe2 as a new class of QSH insulator with large
band gap in a robust two-dimensional materials family of transition metal
dichalcogenides (TMDCs).
