Figure 4.

Light lines are shown in the vicinity of the Dirac point at $k=k_{D2}=(2\pi /d_x)\sqrt{28/3}$ with k−k_D2= 0.01. The isofrequency contours for the triangular lattice correspond to kd_x=19.20. The circular contours are of radius 0.01γ_i, with γ_i given in table 1, and bound the isofrequency contours. (a–c) Correspond to increasing ranges of Δk. (Online version in colour.)