The Study Of Electronic Properties Of Dirac Materials Based On First-principles Method

Since successfully synthesized in 2004,graphene has extracted lots of attentions because of its unique Dirac band structure,which exhibits many unusual electric properties,such as high carrier mobility,massless carriers,half-integer quantum Hall effect,etc.Exploring underlying physical properties of Dirac electrons and finding related materials have been an advanced filed in condensed matter of physics.This article is divided into six chapters.In chapter one,we introduced Dirac equation.Besides,we also introduced some representative Dirac materials such as graphene,graphyne,graphene network and As3Cd2.In chapter two,the density functional method and calculation approaches are introduced.In chapter three,we have studied electric properties of NMLD-graphene,The line defect structures exhibit diverse band structures,depending on the size W of the armchair-edged nanoribbons between line defects.There are standard type-I Dirac bands around the Fermi level when W = 3n-1 and 3n.When W = 3n+1,type-II Dirac fermions are found,with strongly titled Dirac cones.The analysis of PDOS and a tight-binding model explains the origination of the different Dirac fermions.The two types of Dirac points leads to two kinds of edge states: the type-I Dirac points induce one Fermi arc corresponding to edge states along the armchair direction,while the type-II Dirac points induce two Fermi arcs corresponding to two sets of edge states along the zigzag direction.It is noted that this is the first time that type-II Dirac fermions are found in the light-element structures with negligible SOCIn chapter four,we identified a new boron sheet,namely hr-sB,and investigated its electric properties by first principles computations.The hr-sB monolayer consists of rhombus and hexagon strips with a hexagonal vacancy density ?=1/5.Interestingly,two types of Dirac electrons coexist in this nanosheet.One type is the electrons in the Dirac nodal lines traversing the whole BZ,whose velocities can approach 106 m/s.These electrons transport along 1D channels in the stripes.The other type is the electrons in the tilted semi-Dirac cones,where strong anisotropic electronic properties are observed.The unique electronic characteristics are originated from its special bonding patterns.Some clues also indicate that hr-sB may be a good superconductor.This newly predicted boron nanosheet has a cohesive energy comparable with the experimentally available ?6-,?12-and ?3-sheets,and also has outstanding dynamic and thermal stabilities,thus is highly feasible for experimental realization.In chapter five,we studied two new three-dimensional structures C4 H and C3 N.Our results demonstrate these two structures exhibit extremely high thermodynamically stable.More interestingly,they both possess unique Dirac fermions,and Dirac node-line properties are also explored.In chapter six,a summary is given to our study and we also explicitly described the studies in the future.