  Van Hove singularity (VHS) has been considered as a driving source for
unconventional superconductivity. A VHS in two-dimensional (2D) materials
consists of a saddle point connecting electron-like and hole-like bands. In a
rare case, when a VHS appears at Fermi level, both electron-like and hole-like
conduction can coexist, giving rise to an enhanced density of states as well as
an attractive component of Coulomb interaction for unconventional electronic
pairing. However, this van Hove scenario is often destroyed by an incorrect
chemical potential or competing instabilities. Here, by using angle-resolved
photoemission measurements, we report the observation of a VHS perfectly
aligned with the Fermi level in a kagome superconductor CsV$_{3-x}$Ta$_x$Sb$_5$
(x~0.4), in which a record-high superconducting transition temperature is
achieved among all the current variants of AV$_3$Sb$_5$ (A=Cs, Rb, K) at
ambient pressure. Doping dependent measurements reveal the important role of
van Hove scenario in boosting superconductivity, and spectroscopic-imaging
scanning tunneling microscopy measurements indicate a distinct superconducting
state in this system.
