层状关联磁性半导体及其范德华异质结的磁性机理和量子调控

Two-dimensional (2D) magnetic semiconductors, which can manipulate electronic charge and spin degrees simultaneously, are essential to the development of low-dimensional spintronics. Unfortunately, most of the available 2D materials are nonmagnetic. Seeking novel two-dimensional intrinsic magnetic semiconductors thus has great significance. The ferromagnetism observed in single-layer material very recently has been inspiring the extensive studies of 2D magnetic materials. Due to the strong electron correlation of d electrons, the layered transition metal compounds (TMC) exhibit rich electronic and magnetic properties, and are suggested as potential candidates for 2D magnetic materials. However, magnetic mechanism of low-dimensional layered TMC is still not clear and the magnetic transition temperature of the available 2D magnetic TMC is much lower than room temperature. The present project will combine the First-principles calculation, the spin Hamiltonian and Monte Carlo simulation to study and turn the electronic and magnetic properties of the layered magnetic semiconducting TMC and their vdW Heterostructures. We will consider the vdW interaction, electron correlation and spin-orbital coupling to investigate the structural and magnetic stability of layered magnetic TMC in 2D limit. The novel 2D correlated magnetic semiconductor will be identified and their electronic and magnetic properties with different atomic layers will be investigated systemically. Moreover, the corresponding intrinsic mechanism of layer-dependent physical properties will be analyzed and the underlying mechanism of magnetic exchange coupling and magnetic anisotropy will be revealed in details. Furthermore, the magnetic vdW heterostructures will be built by combing the novel 2D magnetic materials and other layered materials such as 2D Dirac system. The role of proximity effect in the electronic and magnetic properties of magnetic vdW Heterostructures will be revealed, and the exotic phenomenon such as quantum anomalous Hall effect induced by broken time-reversal symmetry will be explored. Moreover, we will also reveal the relationship between the physical properties of 2D TMC and their vdW heterostructures and the extrinsic methods such as strain and electric field. Finally, the properties of 2D magnetic materials and their vdW heterostructure will be optimized rationally. The present project will provide a theoretical reference for designing the low-dimensional spintronic materials.

二维磁性半导体能同时操控电荷和自旋，是发展低维自旋电子学的基础。现有二维材料大多没有磁性，寻找二维本征磁性半导体意义重大。近来铁磁单层的发现激起了二维磁性材料的研究热潮。由于d电子的强关联效应，层状过渡金属化合物(TMC)物性丰富，是潜在的二维磁性材料，但低维条件下其磁性机理不清楚且磁转变温度低。项目拟结合第一性原理、自旋哈密顿量和蒙特卡罗研究和调控层状磁性TMC半导体及范德华异质结的电磁性质。我们将考虑范德华力、电子关联和自旋轨道耦合探索二维极限下TMC的结构和磁稳定性，发掘二维磁性半导体，研究电磁性质随层数的变化机理，分析磁性耦合和磁各向异性的微观机制；并与二维狄拉克体系等层状材料构建范德华异质结，研究邻近效应对电磁性质的影响，探讨时间反演对称破缺诱发的量子反常霍尔效应等新奇现象；揭示应力电场等对磁性TMC及异质结物性的调控规律，优化电磁性能，并为低维自旋电子学材料的设计提供理论参考。

近来铁磁单层CrI3的实验发现激起了二维磁性材料的研究热潮。但现有二维铁磁单层候选材料少、磁转变温度低以及磁性机理不清楚，急需进一步研究。本项目结合第一性原理、自旋哈密顿量、Wannier函数、机器学习和蒙特卡罗方法，发现系列新型磁性单层，分析层数依赖的物性演化机理，探讨了时间反演对称破缺导致的新奇物性，并提出了多种增强铁磁单层磁性的方法。具体包括：.（1）预言了ReOBr和YX2(X=I,Br,Cl)等多种新型磁性单层，研究了稳定性、磁基态、能带结构、磁各向异性及其机理，发现了YX2具有铁谷性，解决铁磁VI3单层的结构和磁基态存在的冲突，同时还发展了一种新的描述符显著提升了机器学习效率，并预言近1432种二维新型MXene铁磁材料。.（2）研究CrX3体系层数和堆垛依赖的Raman谱，建立了可用于理解所有CrX3层间磁序的定量超-超交换作用机理。结果表明拉曼光谱波数和强度随层数增加而增加并发现了二种可用于表征不同的堆垛结构的拉曼光谱，而层间磁性可以基于局域坐标系对称匹配原理，以及不同超-超交换相互作用相互竞争来解释。并提出了一种基于轨道重叠的超-超交换理论，解释了一系列过渡金属三卤化物堆垛依赖的层间磁序。.（3）同时还提出通过卤素吸附和构建vdW异质结提升CrX3居里温度的新方法,探讨了异质结时间反演对称破缺诱发的新奇物性。结果发现卤素元素修改增强Cr-d和I-p态之间的跃迁，导致磁交换相互作用增强。我们还发现p电子的退局域化导致了由p电子和d电子磁性层组成的范德华异质结存在数百meV的超大层间磁交换能。改变层间磁序能开/关层间p-d电子耦合，导致体系的电子结构显著变化。另外，我们也探讨了铁磁单层对MX2能谷电子学的影响，并借助于细致的能量等高线计算和能带反折叠方法分析了不同磁性衬底对MX2能带结构的影响。.项目研究为低维自旋电子学的发展提供材料基础，并为下一步研究提供了重要参考。