Single atomic layer allotrope of bismuth with rectangular symmetry

We report the observation of a new allotrope of two-dimensional bismuth. Our scanning tunneling microscopy experiments show that the structure is clearly different than the previously synthesized allotropes $\ensuremath{\beta}$- and $\ensuremath{\alpha}$-bismuthene. It has a rectangular symmetry similar to that of $\ensuremath{\alpha}$-bismuthene, but is composed of a puckered single monolayer of Bi atoms ($\ensuremath{\alpha}$-bismuthene is intrisincally a paired layer material similar to black phosphorous). Atomic resolution images and an observed moir\'e pattern show that the new allotrope has a significantly contracted surface unit cell. The electronic structure is dominated by high density of states at the Fermi level as measured using scanning tunneling spectroscopy (STS) and confirmed by calculations based on density functional theory (DFT) which reveal Dirac cones at three different points in the Brillouin zone.