  Topological semimetals, including Dirac semimetals, Weyl semimetals, and
nodal line semimetals, receive enormous research interest due to their
intrinsic topological nature and fascinating properties. In present work, with
the help of density functional theory (DFT), we find that nodal line semimetals
and Dirac semimetals can coexist in the low energy electron of two dimensional
(2D) monolayer B2C. Intriguingly, besides type-I and type-II Dirac fermions, a
kind of open nodal line appears around the Fermi level. The low energy
electronic nature of B2C sheet can be described by a tight-binding (TB) model
relied on the basis of B-py, pz and C-py, pz states. One of the most merit of
the system is that the energy windows of these four types topological
semimetals are different that can be easily distinguished in experiments.
Moreover, the system provides an excellent platform for studying the interplay
between different type semimetals.
