The Properties Of Two-dimensional Dirac Materials Protected By Structural Symmetry

Two-dimensional(2D)Dirac materials and 2D magnetic materials are two cutting-edge problems and research hotspots in the field of spintronics.Since the successful preparation of graphene,the 2D Dirac material has attracted extensive attention from physical researchers.In particular,the Dirac-cone structure gives graphene massless fermions,leading to half-integer/fractional quantum Hall effects(QHE),ultrahigh carrier mobility,and many other novel phenomena and properties.In addition to Dirac cone,a series of 2D Dirac materials with novel band structure,such as Dirac Nodal-line(NL),have also been found.The valence band and conduction band of NL material intersect in a crossing line near the Fermi level,as a consequence the symmetry requirements of the system are more strict.Among all these Dirac materials,the Dirac magnetic semi-half-metal is a kind of material with 100%spin-polarized Dirac band structure,which has a linear dispersion relationship near the Fermi level in one spin channel,while insulating with a large band gap in another spin channel.This unique property could be used to realized massless fermion in spin transport,and indicates its potential applications in spintronics devices.Limited by the lower dimension,the gapless Dirac nodal points/lines are easily affected by spin-orbit coupling(SOC)interactions,resulting in the opening of band gap.It has become a key issue in the studies of 2D Dirac materials to deeply understand the mechanism of influences of different effects such as structural symmetry,SOC and direction of spins.Previous studies on 2D Dirac NL materials were focus on the configurations with horizontal mirror symmetry.Here we first propose an alternative gapless NL material,namely Mn NF monolayer,with nonsymmorphic horizontal glide mirror symmetry.On the basis of comprehensive first-principles calculations and symmetry analysis,we found that it has a fully spin-polarized NL near the Fermi level with the Fermi velocity as high as 8.19×10⁵m/s.The results on formation energy,phonon spectrum and molecular dynamics show it stability in kinetics and thermodynamics.The ground states of Mn NF has a ferromagnetic configuration with the easy axis out-of-plane and magnetic moments contributed mainly by Mn.Based on the analysis on the real space wave functions,it is found that Mn NF has gapless NL under SOC due to the protection by the horizontal glide mirror symmetry together with the out-of-plane spin polarization,and in another hand the opening of gap could be controlled by applying external electric fields or rotating spin directions in order to break the structural symmetry.The results from Monte Carlo simulations show a Curie temperature higher than room temperature,and the origin of magnetism could be understood by super-exchange interactions between Mn.Moreover,we predict that the Mn NF monolayer has strong anisotropic characteristics in mechanism,magnetism and transport properties,and interesting negative differential resistance,which implies its applications in anisotropic spintronic devices.Our work not only provides a novel 2D half-metallic semimetal with strong anisotropy but also broadens the scope of 2D nodal-loop materials.Owing to the structural polymorphic,the recently high-profile metal-boride films have hold great advantages and potential for the implementation of NL.Herein,a 2D Ni B₂ monolayer with anisotropic NL state is proposed and investigated by first principles calculations.It shows that the Ni B₂ monolayer has excellent kinetic and thermodynamic stability,suggesting the possibility of synthesis in experiments.Remarkably,the NL,accompanied by considerable Fermi velocity,is demonstrated to be protected by nonsymmorphic glide mirror symmetry.A strain-induced self-doping phenomenon can be observed with the NL maintained,which is characterized by the effective modulation of carrier type and concentration.Moreover,the NL state is robust against the correlation effect.These findings pave the way for exploring nonsymmorphic symmetry enabled NL materials in 2D metal-borides.Our study not only predicts a new 2D Dirac NL material,but also provides a new reversible and controllable method for the doped with p-type carriers in 2D Dirac materials.In addition,a 2D hexagonal honeycomb Fe Si O₃ thin film with a configuration similar as Cr Ge Te₃ is designed and systematically investigated based on the first principles calculations.The structure and energy stability of the system were verified by cohesion energy,phonon spectrum and molecular dynamics simulations.The magnetic calculation results show that the ground state has a ferromagnetic configuration with easy axis perpendicular to the 2D structure plane,and the magnetic moments are mainly provided by Fe atoms.The angle between Fe atoms and bridge atoms is close to 90 degrees,which indicates that the magnetic origin can be understood through the mechanism of the super-exchange interaction between Fe atoms,and the Curie temperature is estimated to be higher than room temperature.Without the effect of SOC taken into account,the Fe Si O₃ film has a single-spin polarized Dirac cone structure at the point K with the Fermi velocity as high as 1.27×10⁵m/s,and it can preserve on the BN substrate.Symmetry analysis shows that Dirac is protected by the vertical mirror symmetry along the direction of?-K.Under the influence of SOC,the out-of-plane spin polarization breaks the vertical mirror symmetry that protects the Dirac cone,resulting in the band gap opening of a maximum of 33me V at point K.On this basis,we further calculate the topological number,edge states and Berry curvature of the band-gap system,and confirm that Fe Si O₃ thin films are topological non-trivial materials that can realize the quantum anomalous Hall effect.This result indicates its potential application in the field of spintronic devices.This thesis carries out in-depth research and discussion on three different kinds of Dirac materials,in order to deeply understand the mechanism of the formation of Dirac band structures in 2D,to broaden the approaches of designing 2D Dirac materials,and to promote the development and applications of 2D Dirac materials.