| With the discovery of the quantum Hall effect,the research on topological phases have gradually become the most vibrant field in condensed matter physics.As a representative topological phase,the quantum anomalous Hall effect with quantized Hall conductance under zero magnetic field was first theoretically proposed by physicist Duncan Haldane in 1988.However,the harsh realization conditions make it stay at the theoretical research level.With the discovery of new topological materials,the quantum anomalous Hall effect was finally observed experimentally.So far,the quantum anomalous Hall effect has been observed in three systems:(ⅰ)magnetically doped topological insulators;(ⅱ)intrinsic magnetic topological insulators;(ⅲ)morie superlattice heterojunctions.Since the quantum anomalous Hall effect has extremely robust dissipationless chiral edge states,its realization is of great significance for the construction of next-generation low-power electronic devices.In addition,on the basis of the quantum anomalous Hall effect,important physical effects such as chiral topological superconductivity and topological magnetoelectric effect can be realized.As a result,the research on the quantum anomalous Hall effect is extremely important.Although a series of important breakthroughs have been made in the study of the quantum anomalous Hall effect,its experimental observation temperature has remained at the order of tens of Kelvin,which is still far from practical application.At present,the systems that have realized the quantum anomalous Hall effect show no potential for raising the observation temperature.To make the quantum anomalous Hall effect practicable,a deeper understanding of the underlying mechanisms and the exploration of new material systems are necessary.Aside from raising the observation temperature,achieving the quantum anomalous Hall effect with a high Chern number is also a key topic.Because a high Chern number indicates that the system can provide more chiral edge states without dissipation,which plays a key role in improving the performance of quantum anomalous Hall devices.Up to now,most of the realized quantum anomalous Hall effects possessing an Chern number is limited to C=1.The major work of this dissertation will focus on these two themes.In the first part of the work,we designed a two-dimensional halide perovskite material family A3B2C9(A=Rb,Cs;B=Pd,Pt;C=Cl,Br)by using first-principles calculations.By electronic structure and magnetic analysis,it is found that they are all XY ferromagnetic half metal,and there is a spin-polarized Dirac point at the K point in first Brillouin zone.When all the mirror reflection symmetries are broken by the in-plane magnetization,the spin-orbit coupling can open up band gaps possessing quantum anomalous Hall effect in Rb3Pt2Cl9,Cs3Pd2Cl9 and Cs3Pt2Cl9.The topologically nontrivial band gaps in these three materials reach 93 meV,69 meV and 103 meV,respectively.If the magnetization direction of the system is tuned to the z direction,their topologically nontrivial band gaps will further increase to 100 meV,99 meV and 137 meV,respectively.In addition,their Berezinskii-Kosterlitz-Thouless critical temperatures are all above 248 K.As a result,it is hopeful to realize quantum anomalous Hall effect with an observation temperature near to room temperature in this type of material.In the second part of the work,we designed two monolayer transition metal oxides NiAsO3 and PdSbO3 by using first-principles calculations.These two materials are also XY ferromagnetic half metal.Furthermore,along Γ-M high symmetry line in the first Brillouin zone,a spin-polarized Dirac point appears at the Fermi level.Due to the threefold rotation symmetry(C3)and inversion symmetry,six spin-polarized Dirac points emerge.After taking spin-orbit coupling into consideration,we found the quantum anomalous Hall effect with tunable Chern numbers can be realized via manipulating their magnetization orientation.When the magnetization lies in the x-y plane and breaks all mirror reflection symmetries,the system is in low-Chern-number phase with C=±1.When the magnetization deviates from the x-y plane by a certain angle,the system will enter the high-Chern-number phase with C=±3 from the low-Chern-number phase.By using Wannier-based tight-binding model,we understand the entire process of phase transition,and establish the phase diagram of magnetization induced topological phase transition.The work in this part provides an ideal platform for realizing the quantum anomalous Hall effect with tunable Chen number for practical applications. |
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