  Honeycomb structures of group IV elements can host massless Dirac fermions
with non-trivial Berry phases. Their potential for electronic applications has
attracted great interest and spurred a broad search for new Dirac materials
especially in monolayer structures. We present a detailed investigation of the
\beta 12 boron sheet, which is a borophene structure that can form
spontaneously on a Ag(111) surface. Our tight-binding analysis revealed that
the lattice of the \beta 12-sheet could be decomposed into two triangular
sublattices in a way similar to that for a honeycomb lattice, thereby hosting
Dirac cones. Furthermore, each Dirac cone could be split by introducing
periodic perturbations representing overlayer-substrate interactions. These
unusual electronic structures were confirmed by angle-resolved photoemission
spectroscopy and validated by first-principles calculations. Our results
suggest monolayer boron as a new platform for realizing novel high-speed
low-dissipation devices.
