  Two-dimensional (2D) carbon materials play an important role in
nanomaterials. We propose a new carbon monolayer, named
hexagonal-4,4,4-graphyne (H4,4,4-graphyne), which is a nanoporous structure
composed of rectangular carbon rings and triple bonds of carbon. Using
first-principles calculations, we systematically studied the structure,
stability, and band structure of this new material. We found that its energy is
much lower than that of some experimental carbon materials and it is stable at
least up to 1500 K. In contrast to the single Dirac point band structure of
other 2D carbon monolayers, the band structure of H4,4,4-graphyne exhibits
double Dirac points along the high symmetry points and the corresponding Fermi
velocities (1.04~1.27 * 106 m/s) are asymmetric and higher than that of
graphene. The origin of these double Dirac points is traced back to the nodal
line states, which can be well explained by a tight-binding model. The
H4,4,4-graphyne forms a moir\'e superstructure when placed on top of a BN
substrate, while keeping the double Dirac points. These properties make
H4,4,4-graphyne a promising semimetal material for applications in high-speed
electronic devices.
