Disorder and diffuse scattering in single-chirality ( TaSe4)2I crystals

The chiral charge density wave material (TaSe₄)₂I contains a half-filled Ta d-orbital, which creates Weyl points in a metallic band structure. However, historical measurements show insulating electrical transport. The authors use single-crystal diffraction and transmission electron microscopy to show that large single crystals have pristine chirality: they can exhibit a single enantiomer over millimeters. They resolve the contradiction in transport by showing a consistent iodine deficiency of more than 10%. These vacancies produce structural modulations visible in diffuse scattering and represent a new competing interaction in a class of materials with fragile electronic states.