First Principles Study On Two-dimensional Dirac Semimetals TPH-graphene And C₄N₄

Since graphene was successfully synthesized in 2004,due to its excellent performance and potential applications,it has attracted wide attention of scientists.The research shows that numerous excellent properties of graphene are mainly induced by the Dirac cones.Therefore,scientists have been searching for two-dimensional（2D）materials with Dirac cones.However,due to the extremely harsh conditions for the formation of Dirac cones,the materials with Dirac semi-metallic properties are still rare.Moreover,a few Dirac cones of materials have been experimentally confirmed so far.Therefore,searching for 2D Dirac semimetals with good properties and high preparation feasibility is still a subject worthy of research.Based on the first-principles calculations,we have explored two 2D Dirac semimetals:TPH-graphene and C₄N₄.The main contents and conclusions are as follows:1.Based on the first-principles calculations,we studied a Dirac semimetal TPH-graphene composed of tetragonal,pentagonal and heptagonal carbon rings.The material exhibits not only well thermodynamic and kinetic stability but also mechanic stability.The results indicate that the structure possesses nonisotropic mechanical performance,and its Young’s modulus is up to 291 N/m,which is close to that of graphene.The Dirac points on high symmetry pathsΓ-X and X-S are protected by the nonsymmorphic mirror operations{m₀₁₀|0,1/2,0}and{m₁₀₀|0,1/2,0},respectively.The maximum Fermi velocity of this material is 6.55×10⁵m/s,which is at the same order with graphene.By cutting 2D TPH-graphene into 1D nanobelts,we found that the edge states obtained from semi-infinite structure confirm the nontrivial topological nature of these cones.The nanoribbons of this structure have obvious magnetism,and the ground state of the structure is ferromagnetic state.Our work not only provides a good candidate for studying the nonsymmorphic-symmetry-protected Dirac cones,but also proves that TPH-graphene is a potential application in spintronics and nanoelectronics.2.Based on the first-principles calculations and the RG² search method,we proposed a novel 2D carbon-nitrogen material C₄N₄.Unlike common 2D carbon-nitrogen materials,which are usually insulators or semiconductors,C₄N₄exhibits Dirac semi-metal properties.The material has good kinetic,mechanical and thermal stability,and is expected to be prepared by chemical synthesis of C₄N₄H₄molecules.Moreover,this material possesses nonisotropic mechanical performance.By analyzing the band structure of C₄N₄,we find that the crossing points D₁ and D₂are formed along the high symmetric pathΓ-M andΓ-X,respectively.Further study of its three-dimensional band structure shows that the material has eight Dirac cones in the first Brillouin zone,which can be divided into two categories.One is equivalent to Dirac cone D₁ and the other is equivalent to Dirac cone D₂.Moreover,both of them are jointly protected by σ _v（xz）and σ _v（yz）mirrors.The maximum Fermi velocity of Dirac cones is as high as 1.04×10⁶ m/s,which is an order higher than that of graphene（9.00×10⁵ m/s）.The edge states prove its nontrivial topological nature.Our research shows that C₄N₄ has excellent application value in nanometer electronics and is expected to play an important role in experimental and theoretical research.