  The two-dimensional topological insulators (2DTI) host a full gap in the bulk
band, induced by spin-orbit coupling (SOC) effect, together with the
topologically protected gapless edge states. However, the SOC-induced gap is
usually small, and it is challenging to suppress the bulk conductance and thus
to realize the quantum spin Hall (QSH) effect. In this study, we find a novel
mechanism to effectively suppress the bulk conductance. By using the
quasiparticle interference (QPI) technique with scanning tunneling spectroscopy
(STS), we demonstrate that the QSH candidate single-layer 1T'-WTe$_2$ has a
semi-metal bulk band structure with no full SOC-induced gap. Surprisingly, in
this two-dimensional system, we find the electron interactions open a Coulomb
gap which is always pinned at the Fermi energy (E$_F$). The opening of the
Coulomb gap can efficiently diminish the bulk state at the E$_F$ and is in
favor of the observation of the quantized conduction of topological edge
states.

Mesoscale and Nanoscale Physics;Materials Science