  We have measured magnetoresistance of hexagonal lateral superlattices. We
observe three types of oscillations engendered by periodic potential modulation
having hexagonal-lattice symmetry: amplitude modulation of the Shubnikov-de
Haas oscillations, commensurability oscillations, and the geometric resonances
of open orbits generated by Bragg reflections. The latter two reveal the
presence of two characteristic periodicities, sqrt{3} a / 2 and a / 2, inherent
in a hexagonal lattice with the lattice constant a. The formation of the
hexagonal-superlattice minibands manifested by the observation of open orbits
marks the first step toward realizing massless Dirac fermions in semiconductor
2DEGs.
