Ideal weak topological insulator and protected helical saddle points

Here we report the design and observation of an ideal weak topological insulator in a quasi-one-dimensional bismuth halide, Bi₄I_1.2Br_2.8. Via angle-resolved photoemission spectroscopy, we identify that Bi₄I_1.2Br_2.8 hosts topological surface states on the (100)′ side surface in the form of two anisotropic π-offset Dirac cones while topologically dark on the (001) top surface. The results fully determine a unique side-surface Hamiltonian and thereby identify two pairs of nondegenerate helical saddle points. The fact that both the surface Dirac and helical saddle points are in the global bulk band gap of 195 meV suggests the potential of this system to be a fertile ground for topological many-body physics.