Fig. 2: ARPES spectra of the cleaved (001) planes of Bi₄(Br_1-xI_x)₄ crystals.

a E-k_x-k_y dispersion spectra measured at the in-situ-cleaved (001) surfaces of Bi₄(Br_1-xI_x)₄ crystals with x ranging from 0 to 1(from left to right). The (001) projected time-reversal-invariant (TRI) momenta at the \(\bar{{{{\rm{M}}}}}\) point and the \(\bar{\Gamma }\) point are labeled by a blue dot and a red dot, respectively. The inset shows a schematic diagram of the experimental setup for the (001) surface. b The E-k_y dispersion at \(\bar{{{{\rm{M}}}}}\) along the chain direction. The edge-to-edge gap of bulk bands is indicated by the white dashed lines, where the bulk gap firstly shrinks, and then reopens with a transition point around x ~ 0.7. c Energy distribution curves (EDCs) of the corresponding spectra in panel b. The EDCs of k_y = 0 are marked in red. The blue and red dots are obtained by the peak fitting of EDCs according to the Lorentz-type distribution equation (see Supplementary Note 4 for details), indicating the half-maximum edge locations of the valence band and the conduction band, respectively. The bottom arrow is used to indicate the trend for the I content in different crystals.