  The fractional quantum Hall (FQH) state is a topological state of matter
resulting from the many-body effect of interacting electrons and is of vast
interest in fundamental physics. The experimental observation of topological
surface states (SSs) in three-dimensional bulk solids has allowed the study of
a correlated chiral Dirac fermion system, which can host a single Dirac valley
without spin degeneracy. Recent theoretical studies suggest that the fractional
quantum Hall effect (FQHE) might be observable in topological insulators.
However, due to the dominant bulk conduction it is difficult to probe the
strong correlation effect in topological insulators from resistivity
measurements. Here we report the discovery of FQH states in Bi2Te3 from
thermopower measurements. The surface thermopower is ten times greater than
that of bulk, which makes possible the observation of fractional-filled Landau
levels in SSs. Thermopower hence provides a powerful tool to investigate
correlated Dirac fermions in topological insulators. Our observations
demonstrate that Dirac topological SSs exhibit strongly correlated phases in a
high magnetic field, and would enable studies of a variety of exotic fractional
quantum Hall physics and other correlated phenomena in this newly discovered
chiral Dirac system.
