A Novel Quasi-One-Dimensional Topological Insulator in Bismuth Iodide $\beta$-Bi$_4$I$_4$: Theoretical Prediction and Experimental Confirmation*

A new strong $Z_2$ topological insulator is theoretically predicted and experimentally confirmed in the $\beta$-phase of quasi-one-dimensional bismuth iodide Bi$_4$I$_4$. According to our first-principles calculations the material is characterized by $Z_2$ invariants (1;110) making it the first representative of this topological class. Importantly, the electronic structure of $\beta$-Bi$_4$I$_4$ is in proximity with both the weak topological insulator phase (0;001) and the trivial phase (0;000), suggesting that a high degree of control over the topological electronic properties of this material can be achieved. Experimentally produced samples of this material appears to be practically defect-free, which results in a low concentration of intrinsic charge carriers. By using angle-resolved photoemission spectroscopy (ARPES) on the (001) surface we confirm the theoretical predictions of a highly anisotropic band structure with a small band gap hosting topological surface states centered at the $\bar{M}$ point, at the boundary of the surface Brillouin zone.