Fig. 2: Overall band structures of Bi4Br4 measured on the (001) and (100) surfaces.

a Bulk and surface Brillouin zone with high-symmetry points indicated. b, c Fermi surface measured on the (001) and (100) surfaces. d Band dispersions along the chain direction on the (001) surface with ab-initio calculated results overlaid. The inset shows the schematics of ARPES measurements on the (001) surface. e Zoom-in plot of the fine band structures around the \({\bar{\Gamma }}_{001}\) (left) and \(\bar{M}\) (right) points. f Band dispersions along the chain direction measured on the (100) surface with ab-initio calculated results overlaid. The inset shows the schematics of ARPES measurements on the (100) surface. g Zoom-in plot of the fine band structures around the \({\bar{\Gamma }}_{100}\) (right) and \({\bar{\varGamma }{\prime} }_{100}\) (left) points on the (100) surface. Data were collected with synchrotron-based nano-ARPES at hν = 101 eV at 20 K.