Monoclinic C₁₆: sp²-sp³ hybridized nodal-line semimetal protected by PT-symmetry

Based on first-principles calculations and Wannier tight-binding model, a three-dimensional (3D) carbon structure, named mC16, composed of interpenetrating graphene, is shown to be a topological nodal-line semimetal. The band structure calculations reveal that mC16 exhibits some exceptional properties. These include the Dirac band and band inversion around the Fermi level, and four nodal-lines in the entire Brillouin zone, symmetrically distributed on both sides of the mirror plane, and protected by the PT symmetry. In addition, the topological “drumhead-like” surface states appear on the (001) surface of mC16 and the double “drumhead-like” flat surface state emerges on the type-II (001) surface of the 3D mC16 structure. Moreover, the exotic surface Dirac points are located on the type-I (001) surface, implying that mC16 may process interesting surface transport properties. These new features make mC16 a new member in the topological carbon family, with promising novel applications in electronic device.