  Electronic correlation is believed to play an important role in exotic
phenomena such as insulator-metal transition, colossal magneto resistance and
high temperature superconductivity in correlated electron systems. Recently, it
has been shown that electronic correlation may also be responsible for the
formation of unconventional plasmons. Herewith, using a combination of
angle-dependent spectroscopic ellipsometry, angle resolved photoemission
spectroscopy and Hall measurements all as a function of temperature supported
by first-principles calculations, the existence of low-loss high-energy
correlated plasmons accompanied by spectral weight transfer, a fingerprint of
electronic correlation, in topological insulator
(Bi$_{0.8}$Sb$_{0.2}$)$_2$Se$_3$ is revealed. Upon cooling, the density of free
charge carriers in the surface states decreases whereas those in the bulk
states increase, and that the newly-discovered correlated plasmons are key to
explaining this phenomenon. Our result shows the importance of electronic
correlation in determining new correlated plasmons and opens a new path in
engineering plasmonic-based topologically-insulating devices.
