First-principles Study Of Quantum Anomalous Hall Insulator Based On Magnetic Van Der Waals Heterostructure

Quantum anomalous Hall(QAH)insulators provide a reliable platform to realize next-generation spintronic devices at ultra-low power dissipation due to the co-existence of magnetic order and topological band structure.Therefore,the exploration of QAH effect is one of the most attractive research topics in condensed-matter physics and materials science.Since the discovery of QAH effects in magnetically doped topological insulators,researchers have proposed a large number of potential magnetically doped QAH systems.However,it is difficult to obtain long-range ordered magnetic doping in topological materials,so the QAH effect caused by magnetic doping can only be observed at extremely low temperatures.Recently,intrinsic QAH effect has been observed in the van der Waals(vd W)layered magnetic topological insulator Mn Bi₂Te₄.In addition,the intrinsic QAH effect has also been observed in vd W superlattice composed of Mn Bi₂Te₄ and Bi₂Te₃.In particular,the two dimensional(2D)intrinsic ferromagnetic semiconductors Cr I₃ and Cr₂Ge₂Te₆ have been successfully fabricated.These 2D ferromagnets provide potential platforms to design2D vd W heterostructures with long-range magnetic order based on proximity effects.Such 2D vd W heterostructures enable coupling between the magnetism and topology,potentially giving rise to intrinsic QAH effect.Therefore,considerable research interest has been attracted and it has been demonstrated that intrinsic QAH effect can be realized in graphene-based vd W heterostructures.However,due to the weak spin-orbit coupling(SOC)in graphene,the topological nontrivial band gap of the heterostructure is very small,severely hindering the experimental observation of QAH effect at high temperatures.Therefore,based on first-principles calculations and low-energy efficient model analysis,this thesis explored the potential intrinsic QAH phase in the vd W heterostructure composed of Germanene and 2D ferromagnetic semiconductor Cr₂Ge₂Te₆.The main results are as follows:The vd W heterostructure composed of Germanene and Cr₂Ge₂Te₆ can achieve a large topological nontrivial band gap and a robust QAH effect.Germanene is a common quantum spin Hall(QSH)insulator protected by time inversion.It is a monolayer composed of Ge atoms in 2D honeycomb hexagonal lattice with low-buckled geometry.It is found that the heterostructure possesses high thermodynamic stability,indicating a high feasibility to synthesize such structure in experiments.The calculated results show that there are strong orbital hybridization and charge transfer at the interface.The magnetic proximity effect gives rise to a quadratic node at the Fermi level without considering the SOC.When SOC is considered and the direction of magnetization is along the easy magnetization axis,a large topologically nontrivial band gap of 29 me V appear at the quadratic node,rendering the heterostructure a QAH insulator.In addition,we show that applying pressure in the vertical plane increases the band gap to 72 me V,and gives a clear inter-band chiral edge state and quantized hall conductivity(-0)0)²??).These findings indicate that the Germanene/Cr₂Ge₂Te₆ vd W heterostructure manifests a reliable platform for experimental study of QAH states and the further realization of high-temperature nontrivial quantum transport.