  CsV$_3$Sb$_5$ is a newly discovered Kagome superconductor that attracts great
interest due to its topological nontrivial band structure and the coexistence
of superconductivity and charge-density-wave (CDW) with many exotic properties.
Here, we report the detailed characterization of the CDW gap in high-quality
CsV$_3$Sb$_5$ single crystals using high-resolution angle-resolved
photoemission spectroscopy. We find that the CDW gap is strongly momentum
dependent. While gapped around the $M$ point, the electronic states remain
gapless around the $\Gamma$ point and along the $\Gamma$-$K$ direction. Such
momentum dependence indicates that the CDW is driven by the scattering of
electrons between neighboring $M$ points, where the band structure hosts
multiple saddle points and the density of state diverges near the Fermi level.
Our observations of the partially gapped Fermi surface and strongly
momentum-dependent CDW gap not only provide a foundation for uncovering the
mechanism of CDW in CsV$_3$Sb$_5$, but also shed light on the understanding of
how the CDW coexists with superconductivity in this topological Kagome
superconductor.
