  The two inequivalent valleys in graphene preclude the protection against
inter-valley scattering offered by an odd-number of Dirac cones characteristic
of Z2 topological insulator phases. Here we propose a way to engineer a chiral
single-valley metallic phase with quadratic crossover in a honeycomb lattice
through tailored \sqrt{3}N *\sqrt{3}N or 3N *3N superlattices. The possibility
of tuning valley-polarization via pseudo-Zeeman field and the emergence of
Dresselhaus-type valley-orbit coupling are proposed in adatom decorated
graphene superlattices. Such valley manipulation mechanisms and metallic phase
can also find applications in honeycomb photonic crystals.
