  The band inversion of topological materials in three spatial dimensions is
intimately connected to the parity anomaly of two-dimensional massless Dirac
fermions. At finite magnetic fields, the parity anomaly reveals itself as a
non-zero spectral asymmetry, i.e., a non-zero difference between the number of
conduction and valence band Landau levels, due to the unpaired zero Landau
level. Here, we realize this two-dimensional Dirac physics at a single surface
of the three-dimensional topological insulator (Hg,Mn)Te. We observe an
unconventional re-entrant quantum Hall effect that can be directly related to
the occurrence of spectral asymmetry in a single topological surface state. The
effect should be observable in any topological insulator where the transport is
dominated by a single Dirac surface state.
