过渡族金属硫属化物二维体系中激子的运动、转化、湮灭过程的理论研究

Graphene, silicene, molybdenum disulfide, and black phosphorus are two-dimensional materials that have been attracted great attentions due to their promising application in future ultrahigh speed transistor devices and new quantum optoelectronic devices. As the dimension reduced, the optical properties of molybdenum disulfide and other two-dimensional transition-metal dichalcogenides are dominated by excitonic effects. In order to choose suitable transition-metal dichalcogenides in optoelectronic devices, a comprehensive study of the excitonic effects in these materials is needed. The studying of excitonic motion, excitonic transformation, excitonic annihilation and other exciton dynamics are especially important because they direct affect the performance of various optoelectronic devices. However, the traditional method that based on density functional theory and employing the GW-Bethe-Salpeter equation fails to capture these exciton dynamics. In this project, by considering electron-electron interactions and electron-hole interactions in tight-binding mothed, we will study the motion laws of different excion states in transition-metal dichalcogenides. Exciton and charged carriers will combined to form trion in transition-metal dichalcogenides, thus we will carefully study the combination processes and calculate the binding energies of trions. Exciton-exciton interactions will induce excitonic annihilation through Auger recombination, will induce the formation of two new excitons through exchange spin of electrons or holes, and also will induce the formation of biexcitons by through exciton-exciton combination. We will calculate all these three exciton-exciton interaction processes in this project. The performance of this project is helpful to choose suitable transition-metal dichalcogenides for construction of optoeletronic devices and also can be helpful to understand many body effects in two-dimensional materials.

石墨烯、硅烯、二硫化钼、黑磷等新型二维材料是构建超高速电子器件和光电量子器件的理想材料。由于维度限域效应，二硫化钼等过渡族金属硫属化物二维体系中光学性质由激子的性质决定。体系中激子的运动、转化、湮灭等动力学过程是影响基于过渡族金属硫属化物二维体系构筑的光电器件工作性能的决定性因素。常用的基于密度泛函理论上考虑激子的Bethe-Salpeter方程的计算方法只能获得激子能级和激子光谱，难以得到激子动力学行为。本项目拟在紧束缚模型上考虑电子-电子、电子-空穴相互作用，计算过渡族金属硫属化物二维体系中激子在Berry相位作用下的运动，激子-载流子束缚形成带电激子的束缚能，各种激子-激子散射矩阵。从而理解这些二维体系中激子的运动、转化、湮灭规律。这一基础研究有助于选择合适的过渡族金属硫属化物二维体系于新型光电量子器件中，也有助于理解多体效应在二维材料中特殊的表现形式。

摘要. 半导体中，导带中的电子与价带中的空穴由于库仑吸引作用束缚在一起的状态称为激子。在二维材料中，由于维度限域效应，电子波函数和空穴波函数的重叠几率增大，并且由于二维材料中具有更弱的库仑屏蔽效应，导致体系中具有增强的激子效应。研究二维材料中激子效应有助于正确理解这些材料中的电学和光学特性，以及这些电学和光学性质中多体效应的表现形式。本项目在紧束缚模型基础上，考虑了电子-电子相互作用，在平均场近似下求解体系的多体哈密顿方程，得到了二维材料的准粒子能带结构。进一步用单电子-空穴对的线性组合构造体系的激子波函数，计算了激子的本征值方程，得到了激子能级和相应波函数，分析不同激子之间的相互作用机制。我们发现了不同边界的二维材料中诱导出不同激子态，特别是发现其中的自旋三态激子能量不再简并。我们研究了普通激子与磁性激子对应的光谱的特点，以及这些光谱随边界和尺寸的变化关系。研究了边缘态激子、体态激子、半边缘态半体态激子之间的相互竞争作用，确定了它们之间相互转化的变化关系。这些研究表明了在二维材料中具有丰富的新奇激子效应，为进一步研究二维材料电学和光学性质中的多体效应提供理论基础。