Fig. 1: Low-temperature crystal and band structures of bulk PbI₂.

a Ball-and-stick model of single-layer PbI₂ shown from the top view. The black rhombus is the unit cell, in which the 3 high-symmetry sites are marked by A, B, and C. b, c Stacking structure of (b) 2H-PbI₂ and (c) 4H-PbI₂ in the conventional nomenclature of PbI₂ (note that they correspond to 1 T and 2H in the nomenclature of TMDs). Black rectangles in b, c are their respective unit cells, and the circled dot in b marks the inversion centre. The label A, B, and C in b, c are the index of atomic sites as defined in a. d HAADF-STEM image of PbI₂ taken along the y axis. Red and blue rhombuses in c, d represent the octahedron unit of PbI₆ alternatively tilted in opposite directions in different layers, while those tiled in the same directions are shown in grey in b. e, f Theoretical band structure of (e) 2H-PbI₂ and (f) 4H-PbI₂, calculated by DFT considering spin-orbit coupling (see Supplementary Fig. 1 for the fuller set of DFT calculations). CB and VB are conduction and valence bands, respectively. g Experimental band structure of bulk PbI₂, taken by ARPES along k_x with the photon energy of 96 eV (corresponding to Γ_4H in k_z). h ARPES data taken over k_x and k_y and displayed along a couple of high-symmetry directions. i k_z dispersion taken by the photon-energy dependence of ARPES data. Dotted lines show the Γ points of 4H-PbI₂. Source data are provided as a Source Data file.