  We study the problem of a single Dirac fermion in a quantizing orbital
magnetic field, when the chemical potential is at the Dirac point. This can be
realized on the surface of a topological insulator, such as Bi2Se3, tuned to
neutrality. We study the effect of both long range Coulomb interactions
(strength alpha=e^2/(epsilon hbar v_F).) and local repulsion U which capture
the effect of electron correlations. Interactions resolve the degeneracy of
free fermions in the zeroth Landau level at half filling, but in a manner
different from that in graphene. For weak interactions, U=0 and alpha<<1, a
composite Fermi liquid is expected. However, in the limit of strong local
correlations (large U but alpha<<1), a charge density wave phase is predicted,
which we term "axion stripe". While reminiscent of quantum Hall stripe phases,
its wavelength is parametrically larger than the magnetic length, and the
induced fermion mass term (axion) also oscillates with the charge density. This
phase is destroyed by sufficiently strong Zeeman coupling. A phase diagram is
constructed and consequences for experiments are discussed.
