Upper panels: Tight-binding model of conventional graphene (a). The red (blue) circles represent the sublattices with on-site energy m (- m) and the black lines indicate the nearest-neighbor couplings with strength t. Lower panels: Schematics of the Dirac cone dispersion of the graphene model (a). For graphene, the K and K' valleys are related to each other by the time-reversal symmetry (TRS).
