Coexistence of open and closed type nodal line topological semimetals in two dimensional B2C

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 the 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 a two dimensional (2D) B2C monolayer. 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 the B2C sheet can be described using a tight-binding (TB) model that relies on the basis of B-p(y), p(z) and C-p(y), p(z) states. One of the important merits of the system is that the energy windows of these four types of topological semimetals are different which can be easily distinguished in experiments. Moreover, the system provides an excellent platform for studying the interplay between different type semimetals.