  In many realistic topological materials, more than one kind of fermions
contribute to the electronic bands crossing the Fermi level, leading to various
novel phenomena. Here, using momentum-resolved inelastic electron scattering,
we investigate the plasmons and their evolution across the phase transition in
a type-II Weyl Semimetal MoTe$_2$, in which both Weyl fermions and trivial
nonrelativistic fermions contribute to the Fermi surface in the Td phase. One
plasmon mode in the 1T' phase at high temperature and two plasmon modes in the
topological T$_d$ phase at low temperature are observed. Combining with
first-priciples calculations, we show that all the plasmon modes are dominated
by the interband correlations between the inverted bands of MoTe$_2$.
Especially in the T$_d$ phase, since the electronic bands split due to
inversion symmetry breaking and spin-orbit coupling, the plasmon modes manifest
the interband correlation between the topological Weyl fermions and the trivial
nonrelativistic electrons. Our work emphasizes the significance of the
interplay between different kinds of carriers in plasmons of topological
materials.
