▎ 摘　　要

Along with the great interest in two-dimensional elemental materials that has emerged in recent years, atomically thin layers of bismuth have attracted attention due to physical properties on account of a strong spin-orbit coupling. Thickness dependent electronic band structure must be explored over the whole Brillouin zone in order to further explore their topological electronic properties. The anisotropic band structures along zig-zag and armchair directions of alpha-bismuthene (alpha-Bi) were resolved using the two-dimensional mapping of angle-resolved photoemission spectra. An increase in the number of layers from 1- to 2-bilayers (BLs) shifts the top of a hole band on (Gamma) over bar-(X) over bar (1) line to high wavenumber regions. Subsequently, an electron pocket on (Gamma) over bar-(X) over bar (1) line and a hole pocket centred at (Gamma) over bar point appears in the 3 BL alpha-Bi. Gapless Dirac-cone features with a large anisotropy were clearly resolved on (X) over bar (2) point in the 1-BL and 2-BL alpha-Bi, which can be attributed to the strong spin-orbit coupling and protection by the nonsymmorphic symmetry of the alpha-Bi lattice.