The search for metallic boron allotropes has attracted great attention in the past decades and recent theoretical works predict the existence of metallicity in monolayer boron. Here, we synthesize the ${\beta }_{12}$-sheet monolayer boron on a Ag(111) surface and confirm the presence of metallic boron-derived bands using angle-resolved photoemission spectroscopy. The Fermi surface is composed of one electron pocket at the $\overline{S}$ point and a pair of hole pockets near the $\overline{X}$ point, which is supported by the first-principles calculations. The metallic boron allotrope in ${\beta }_{12}$ sheet opens the way to novel physics and chemistry in material science.

• Received 10 February 2016
• Revised 24 June 2016

DOI:https://doi.org/10.1103/PhysRevB.94.041408