  We investigate Dirac fermions in the antifferomagnetic metallic state of
iron-based superconduc- tors. Deriving an effective Hamiltonian for Dirac
fermions, we reveal that there exist two Dirac cones carrying the same
chirality, contrary to graphene, compensated by a Fermi surface with a
quadratic energy dispersion as a consequence of a non-trivial topological
property inherent in the band structure. We also find that the presence of the
Dirac fermions gives the difference of sign- change temperatures between the
Hall coefficient and the thermopower. This is consistent with available
experimental data.
