In general, the surface state of the TI is one dimension lower than the bulk structure. The Dirac electrons move on its surface. For 2D TI, electrons move along the boundary of the material. Bi2Se3, Bi2Te3 and Sb2Te3 only have one Dirac point that can pass through the band gap so ...
The unit cell becomes a unit super-cell in the GPSC, as opposed to the dual lattice in the second-generation photonic crystals. "Period 1" is the period for the traditional photonic crystal, while "Period 2" is the second period in the x-direction for the GPSC, as shown in Figure 1....