  A two-dimensional (2D) topological insulator (TI) exhibits the quantum spin
Hall (QSH) effect, in which topologically protected spin-polarized conducting
channels exist at the sample edges. Experimental signatures of the QSH effect
have recently been reported for the first time in an atomically thin material,
monolayer WTe2. Electrical transport measurements on exfoliated samples and
scanning tunneling spectroscopy on epitaxially grown monolayer islands signal
the existence of edge modes with conductance approaching the quantized value.
Here, we directly image the local conductivity of monolayer WTe2 devices using
microwave impedance microscopy, establishing beyond doubt that conduction is
indeed strongly localized to the physical edges at temperatures up to 77 K and
above. The edge conductivity shows no gap as a function of gate voltage, ruling
out trivial conduction due to band bending or in-gap states, and is suppressed
by magnetic field as expected. Interestingly, we observe additional conducting
lines and rings within most samples which can be explained by edge states
following boundaries between topologically trivial and non-trivial regions.
These observations will be critical for interpreting and improving the
properties of devices incorporating WTe2 or other air-sensitive 2D materials.
At the same time, they reveal the robustness of the QSH channels and the
potential to engineer and pattern them by chemical or mechanical means in the
monolayer material platform.
