  The spectrum of tight binding electrons on a square lattice with half a
magnetic flux quantum per unit cell exhibits two Dirac points at the band
center. We show that, in the presence of an additional uniaxial staggered
potential, this pair of Dirac points may merge into a single one, with a
topological transition towards a gapped phase. At the transition, the spectrum
is linear in one direction and quadratic in the other one (a spectrum recently
named "hybrid" or "semi-Dirac"). This transition is studied in the framework of
a general Hamiltonian describing the merging of Dirac points. The possibility
of creating gauge fields for cold atoms in optical lattices may offer the first
opportunity to observe this merging of Dirac points and the hybrid dispersion
relation.
