Topological insulators (TIs) behave like insulators in their interiors but are conductors on their exteriors. In a weak TI, only four sides are conducting, while the top and bottom surfaces remain insulating. “Other researchers have claimed to have produced a weak TI, but their evidence was not as strong. Because of this, the experimental confirmation of a weak TI has been elusive.”“We believe our study has provided the smoking gun evidence to claim this bismuth iodide material is a weak TI. At room temperature — the temperature range at which the crystals are grown — the material is a weak topological insulator, where surface conduction is robust.