  Dispersionless flat bands are proposed to be a fundamental ingredient to
achieve the various sought after quantum states of matter including
high-temperature superconductivity1-4 and fractional quantum Hall effect5-6.
Materials with such peculiar electronic states, however, are very rare and
often exhibit very complex band structures. Here, we report on the emergence of
a flat band with a possible insulating ground state in the sub-monolayer VSe2 /
Bi2Se3 heterostructure by means of angle-resolved photoemission spectroscopy
and scanning tunneling microscopy. The flat band is dispersionless along the
kll and kz momenta, filling the entire Brillouin zone, and it exhibits a
complex circular dichroism signal reversing the sign at several points of the
Brillouin zone. These properties together with the presence of a Moir\'e
patterns in VSe2 suggest that the flat band is not a trivial disorder or
confinement effect and could even be topologically non-trivial. Another
intriguing finding is that the flat band does not modify the Dirac cone of
Bi2Se3 around the Dirac point. Furthermore, we found that the flat band and the
Dirac surface states of Bi2Se3 have opposite energy shifts with electron
doping. This opens a novel way of controlling the spin texture of photocurrents
as well as the transport properties of the heterostructure. These features make
this flat band remarkably distinguishable from previous findings and our
methodology can be applied to other systems opening a promising pathway to
realize strongly correlated quantum effects in topological materials.
