Reports

Topological crystalline insulators (1–5) (TCIs) are a class of materials that possess a unique type of electronic topology that arises from crystal-line symmetry; this gives rise to surface states with an unusual band dispersion and spin texture. In particular, the (001)-oriented surfaces of IV-VI semiconductor TCIs (Fig. 1A) harbor two generations of Dirac cones at different energies (Fig. 1B). The band structure of (001) sur-face states can be visualized by starting from a pair of Dirac points lo-cated at the X points at the edge of the surface Brillouin zone (BZ) with the Dirac point energies close to the conduction and valence band edge, respectively (6). The hole-branch of the upper Dirac cone and the elec-tron-branch of the lower Dirac cone coexist inside the band gap. The hybridization between these two branches leads to an avoided crossing in all directions except along the Γ X line where a band crossing is dictated by electronic topology of TCI and protected by the (110) mirro