Extended Data Fig. 1: Crystal structure and experimental X-ray diffraction data of α-Bi4Br4.

a, Side view of the crystallographic structure of α-Bi4Br4, showing the interlayer AB stacking. b, Top view of the A-type and B-type monolayers. c, X-ray diffraction data obtained from a single crystal of α-Bi4Br4, featuring the (00 l) preferred peaks. The refined lattice parameters are: a=\(13.0667\)(12) Å, b=4.3359(4) Å, c=20.0676(19) Å, and β=107.336(2)°. The inset displays an optical microscopy image of a typical α-Bi4Br4 bulk single crystal, placed on millimeter grid paper, with the top surface corresponding to the (001) plane. This surface was used for the mechanical exfoliation process to fabricate the devices utilized in our transport measurements.