The First-Principle Study Of Quantum Anomalous Hall Effect In Tow-dimensional Materials

The successful synthesis of both quantum spin Hall materials and graphene lay the foundation for the development of topological materials.Topological materials have attracted much extensive attention due to their lots of novel physical properties.The most unique property that the bulk is insulating with gapless edge states.Hence,they possess the broad practical applications in dissipationless spintronics.The successful fabrication of two-dimensional magnetic materials pushes the study of quantum anomalous Hall effect When a longitudinal electric field is applied,a quantized plateau of transverse conductance will be observed,which is due to the combined effect of intrinsic magnetization and spin-orbit coupling.However,the quantum anomalous Hall effect currently can only be experimentally detected at extremely low temperatures.The practical applications are seriously limited in spintronic devices due to the extreme-temperature condition.Searching some special materials with the quantum anomalous Hall effect at the high temperature,even at the room temperature,becomes very important.In this thesis,based on the density functional theory,we have studied the quantum anomalous Hall effect by designing several typical systems.In order to help experimental design of new sipntronic devices,we try to obtain some general rules on the design of quantum anomalous Hall materials.Firstly,we predict a new quantum anomalous Hall material based on Mn2Cl3Br3 with a two-dimensional hexagonal Janus structure.The first-principle calculation shows that the ground state is ferromagnetic state.The phonon spectra show the absence of imaginary phonon modes of all the Brillouin zone,indicating the dynamic stability.Moreover,the electronic structure exhibits the Dirac half-metallic character.The up-spin bands are metallic with Dirac cone at the high symmetric K point near the Fermi level,while down-spin bands are insulating with a gap of 4.09 eV.Notably,the magnetic anisotropic energy reaches up 11.89 meV/cell in xz plane and the x axis is the easy magnetization direction.Furthermore,the non-trival band gap is opened by the spin-orbital coupling and the Chern number is calculated as-1.More interestingly,the Chern number changes to 1 when the magnetization direction is tuned perpendicular to the xy plane.It implies that the tunable magnetization direction could be used to reverse the direction of chiral edge current.In addition,the existence of the quantum anomalous Hall state is further confirmed by the Berry curvature and edge state calculations.Then,we propose that the quantum anomalous Hall effect can be realized in Mn2O3 with a two-dimensional Kagome lattice.Different from Y2O3,V2O3 and Nb2O3,the most stable structure of Mn2O3 is not the high-symmetric P6/mmm structure,but a lower-symmetric Cmm2 and C222 phase with the relative shift of oxygen atoms along the z axis shift.Our calculations show that Mn2O3 exhibits many fascinating properties,including Dirac half-metal,massless Dirac fermions,100%spin polarization and large magnetic moments.Moreover,the calculations of anomalous Hall conductance and edge states calculation shows that the Chem number are-2 and two chiral edge state are present in the bulk's gap.More interestingly,the biaxial strain can cause the structural phase transition from Cmm2 and C222 phase to P6/mmm,accompanied with a topological phase transition.The Curie temperature of three Mn2O3 sturture are larger than 586 K,indicating a high-temperature quantum anomalous Hall effect.Finally,without the addition of the magnetic ions,BaX(X=Si,Ge,Sn)is discovered to be a candidate of the quantum anomalous Hall effect.The broken time reversal symmetry is due to the polaried X-p orbitals.The results show that the topological properties originate from quadratic px,y non-Dirac bands.Considering the spin-orbital coupling interaction,a band gap is opened with a quadratic non-Dirac point.The calculated anomalous Hall conductance,Chern number,Berry curvature and edge state indicate the nontrivial topology.More interestingly,this calculated non-trival topological state is robust against the biaxial strain with structural phase transitions.These findings provide new thought for the investigation of the quantum anomalous Hall effect at the high temperatures.