First-Principles Study Of Noval Two-Dimensional Dirac Materials And Half-Metals

In recent years,two-dimensional(2D)materials have attracted attention in many applications such as condensed matter physics and material science.It also provided great opportunities in the research fields,including insulators,semiconductors,metals,topological insulators,superconductors.2D materials consisting of different elements with different atomic structures exhibit electric,chemical,mechanical properties and so on.Among these 2D materials,graphene,a monolayer of carbon atoms,is most famous due to their unique atomic structures and excellent properties.Graphene consists of sp2-hybridized carbon atoms,which are connected by C-C covalent bands,forming a honeycomb lattice.The electronic states near the Fermi level,as well as the linear energy-momentum dispersion relation(characterized by the so-called Dirac cones),are mainly contributed by the pz orbitals.These interesting properties of graphene can be well reproduced by using a single pz-orbital tight-binding(TB)model.The electrons and holes near the Dirac point behave as massless relativistic particles,which obey the Dirac equation.Graphene has a high Fermi velocity that is about 1/300 of speed of light,leading to ultrahigh carrier mobility.As result,graphene can serve as a candidate material of high-speed electronic devices.Therefore searching for the 2D materials with Dirac-type electronic band structure becomes a hot topic of condensed matter physics and material science.Using by tight-binding models,which have been demonstrated that beside hexagonal lattice,Dirac cones can also exist in kagome lattice and ruby lattice.For kagome and ruby lattices,Dirac cones are also accompaned by flat bands,which are almost dispersionless.These two models open avenue to the design of 2D Dirac materials.Half-metals which can host 100%spin-polarized electrons are crucial for sprintronics.Two-dimensional organic half-metals have longer spin-relaxation time compared with tradional transition metal.For the half-metallic Dirac materials,the coexistence of electron spin-polarization and Dirac cones will lead to interesting properties,such as spin-polarized Dirac fermions and even quantum anomalous Hall effect as the electron spin-orbit coupling is taken into account.These intriguing properties are crucial for spintronics devices.In this thesis,we focus on metal-free ternary 2D materials with Dirac cones or half-metallicity,by using first-principles calculations.We proposed a novel metal-organic framework with spin-polarized Dirac cones and topological nontriviality due to spin-orbit coupling,which are quite promising for achieving quantum anomalous Hall effect and dissipationless edge states.The main results are summarized as follows.(1)We propose a new family of 2D ternary covalent organic frameworks(COFs),C2N6O3,C2N6Se3 and C2N6Te3.On behave of them,the sawtooth-like lingages in the frameworks of C2N6S3 make it soft and sustain a biaxial tensile strain up to 24%and even more.The Dirac cones and the flat band near the Fermi level,which can be well reproduced using the TB model of a Kagome lattice.The Fermi velocity is comparable to that in graphene,and could tune by applying biaxial tensile strain.The topological nontriviality and chiral edge states due to spin-orbit coupling are confirmed by using maximally localized Wannier functions(MLWFs)method.(2)We predict two 2D ternary COFs,namely C2S6N3 and C2O6N3 monolayer,with stable electron spin-polarization and half-metallicity.The spin-polarization of the pz electrons leads to stable ferromagnetism.Monte Carlo(MC)simulations on the basis of an Ising model the Curie temperature of C2S6N3 monolayer can attain 413K.We evaluated the magnetic anisotropy of C2N3S6 monolayer by taking the spin-orbit coupling(SOC)into account,the magnetic anisotropy energy(MAE)is 13?eV per unit cell.The stable ferromagnetism and half-metallicity of the metal-free 2D COFs may find applications in spintronics.(3)We propose a new family two-dimensional(2D)metal-organic framework(MOF),Ni2C24S6H12,Ni2C24O6H12 Ni2C24N6H18 and V2C24S6H12 monolayer,with spin-polarized Dirac cones setting at six corners of the Brillouin zone.In this MOF,the transition metal atoms(Ni or V)are jointed together by thiophene/pyrrole/furan ligands,forming a honeycomb lattice For Ni2C24S6H12,the Fermi velocity of one spin channel is about 12%of that of graphene,while a large band gap of 1.29 eV exists in another spin channel.The ferromagnetic ordering mediated by the thiophene oligomers has a Cuire temperature of about 630 K as revealed by the Monte Carlo simulation within Ising model.Both Ni2C24O6Hi2 and Ni2C24n6H18 show the same properties,although the ligands connecting the transition metal atoms are different.As spin-orbit coupling is taken into account,a band gap will be opened up at the Dirac cones.For Ni2C24S6H12 monolayer,the topological nontriviality of the band gap can be verified by a nonzero Chern number which calculating from the Berry curvature.The band gap due to spin-orbit coupling is about 5.9 meV.This band gap is available for achieving quantum anomalous Hall effects at low temperature.