Energy band of the ruby lattice in the first Brillouin zone. (a) The energy dispersion of the anisotropic ruby lattice ( λ = t 1 / t 2 , λ = 0.5). (b) The energy dispersion of the isotropic ruby lattice while λ = 1.0 (c) and (d) The energy dispersions of the anisotropic ruby lattice for λ = 2.0 and λ = 10, respectively. t 1 = 1 for all cases.