  Condensed matter realization of a single Dirac cone of fermions in two
dimensions is a long-standing issue. Here we report the discovery of a single
gapless Dirac cone of half-quantized Hall conductance in a magnetically-doped
topological insulator heterostructure. It demonstrates that the Hall
conductance is half-quantized in the unit e^{2}/h when the parity symmetry is
invariant near the Fermi surface. The gapless Dirac point is stable and
protected by the local parity symmetry and the topologically nontrivial band
structure of the topological insulator. The one-half Hall conductance observed
in a recent experiment [Mogi et al, Nat. Phys. 18, 390 (2022)] is attributed to
the existence of the gapless Dirac cone. The results suggest a condensed matter
realization of a topological phase with a one-half topological invariant.
