Figure 3.

Light lines are shown in the vicinity of the first Dirac point at $k=k_{D1}=(2\pi /d_x)\sqrt{4/3}$. The circular curves have the radii 2(k−k_D1), $2(k-k_{D1})/\sqrt{3}$, and they coincide to numerical accuracy with the isofrequency contours for the triangular lattice for kd_x=7.32. (Online version in colour.)