First-principles Study On The Quantum Anomalous Hall Effect Of Two-dimensional Magnetic Materials

Since the discovery of quantum anomalous Hall effect,it has been widely concerned by researchers.The effect is that there is a chiral edge state at the boundary of the sample without external field.The realization of QAHE promotes the research and application of quantum Hall physics,which has great application prospects in low-power electronic devices.Due to the strict requirements of this effect on material system,its realization temperature is very low(only about 1K)so far,which greatly limits its application in practice.Therefore,improving the observation temperature of QAHE is an urgent problem for researchers.In this thesis,we propose to realize QAHE in 2D magnetic material system by surface modification and stress,so as to improve the observation temperature.The specific research contents are as follows:1.The electrical,magnetic and topological properties of ten kinds of van der Waals heterojunctions constructed by 2D FM insulators(CrI₃,CrBr₃)and TMDs(MoS₂,MoSe₂,MoTe₂,WS₂,WSe₂)monolayers were systematically studied.The results show that the valley splitting of MoTe₂/CrBr₃ heterojunction is about 28.7 meV,which is larger than the 3.5 meV Valley splitting observed in WSe₂/CrI₃ heterostructure.The WSe₂/CrBr₃ is characterized as a valley-polarized QAHE system,and further analysis of the low-energy k·p model shows that the topological mechanism is spin-orbital coupling(SOC)acting to induce d-d energy to flip in MoTe₂/CrBr₃ and WSe₂/CrBr₃ heterogeneous.We further find that QAHE can be readily realized in hole-doped MoTe₂/CrI₃ heterostructure.The present study provides a guideline for exploiting exotic valley-polarized states in 2D FM vd W heterostructures.2.We have studied the electronic structure,magnetic and topological properties of CrBr₃monolayer and Na doped CrBr₃ monolayer.We first reveal that a FM CrBr₃ monolayer harbors topologically nontrivial conduction bands with a high Chern number of C=2.Then,we reveal that the interesting conduction bands can be moved downwards to the Fermi levels by electron and alkali–metal-doping.Most strikingly,the Na-doped CrBr₃ system possesses a higher Chern number of C=-4 with a transition temperature of?54 K,which is attributed to the constructive coupling effect of the quadratic non-Dirac and linear Dirac band dispersions.This study provides a feasible experimental scheme for the realization of high temperature multichannel QAHE in 2D ferromagnetic system.3.Here we design a physically realistic system for realizing QAHE by expanding the recently discovered two-dimensional ferromagnetic insulators as a new class of candidate materials.Based on first-principles calculations,we predict that a CrMnI₆ monolayer is energetically stable and can be readily exfoliated.This system is further shown to be a ferromagnetic insulator,with a transition temperature of?87 K,higher than that of CrI₃.Most strikingly,such a monolayer is characterized as an intrinsic QAHE system with a high Chern number of C=2,and the underlying mechanism for the nontrivial topology is attributed to the two inequivalent subset sites of the Crand Mnatoms.The present study thus provides an ideal platform for realizing high-temperature QAHE beyond the prevailing materials class of magnetically doped topological insulators.4.We have systematically investigated the electronic,magnetic,and topological properties of layered MnSb₂Te₄,MnBi₂Se₄,and MnSb₂Se₄,where those materials host similar crystal structure as the MnBi₂Te₄,and that these materials can easily peel off to a single layer,which can also be stable.Our calculations show that each bulk system with antiferromagnetic order can be converted into a 3D topologically nontrivial insulator by applying small pressures.And we notice that QAHE can also be achieved in layered FM MnSb₂Te₄ and MnBi₂Se₄.All those findings demonstrate that the MnBi₂Te₄-like materials of MnSb₂Te₄,MnBi₂Se₄,and MnSb₂Se₄are promising candidates for exploring abundant topological quantum states.