Orthorhombic bismuth halides: Novel topological insulators*

Quasi-one-dimensional bismuth halides, Bi₄Br₄ and Bi₄I₄, are emerging as pivotal platforms for investigating a plethora of topological phases, such as weak and higher-order topological insulators and room-temperature quantum spin Hall insulators. Recent experiments have unveiled new structural phases in Bi₄Br_xI_4-x. By using first-principles calculations and topological band theories, we theoretically show that the new orthorhombic crystal structures host a variety of topological boundary states including Dirac surface states, helical hinge states, and hourglass-like states.