C[formula omitted]N[formula omitted]: A symmetry-protected semimetal

Abstract

In this paper, we propose a symmetry-protected semimetal C4N4, which is a two-dimensional carbon–nitrogen monolayer that can be synthesized from the moleculars of 1,3,5,7-Tetrazocine via the dehydrogenation reaction. With the help of first-principle calculations and symmetry analysis, we find the nodal line in the monolayer C4N4 are protected by horizonal mirror symmetry and they originates from the crossing between N-px(y) and C-pz orbitals. An k⋅p model is used to explain the formation of the nodal line. If this material is synthesized on a substrate, the nodal line will transform into multiple isolated Dirac points with the protection of vertical mirror symmetries. The study of the energy band structures of AA- and AB-stacking bulks reveals that there are nodal surfaces in their 3D Brillouin zone. Our research provides a possible experimental routine to synthesize 2D C4N4 and validate the symmetry-protected nature of the nodal line.