单原子层d轨道电子体系中自旋光电流的产生和调控

Generation and manipulation of a spin current is one of the most critical steps in developing semiconductor spintronics applications. In a two-dimensional electronic system with spin degeneracy lifted, irradiation with circularly polarized light can result in a non-uniform distribution of photo-excited carriers in k-space following optical selection rules and energy/momentum conservation, finally leading to a spin current. For example, the valley degree of freedom in layered transition-metal dichalcogenides (MX2) provides the opportunity to extend functionalities of novel spintronics and valleytronics devices. Due to spin splitting induced by spin-orbit coupling, the non-equilibrium charge carrier imbalance between two degenerate and inequivalent valleys to realize valley/spin polarization has been successfully demonstrated theoretically and supported by optical experiments. However, the generation and an electrical control of a valley/spin current by the valley polarization in MX2 remains elusive, and also the observation of the spin/valley Nernst effect based on the electric field generated from the Seebeck effect is still a great challenge. .Here, this project proposes, based on the light matter interaction, the initiation of an exploratory research for understanding and controlling the spin/valley photocurrent and related phenomena, in particular on the atomically thin sample and nano-scale sample area as well as with electric-double-layer gating. Exploiting well-defined nature of the valley/spin current, the focus of the proposal will primarily be the following four directions: 1) to theoretically search new emergent materials and hetero-structures with large spin-orbit coupling and spin splitting, which are good for spin current generation; to identify the detailed electronic structure and their evolution with an electric field or an optical field; to analyze the optical selection rules under irradiation. 2) to investigate the generation and electrical control of the spin photogalvanic current and see how the electric field tunes the surface band bending, spatial inversion asymmetry, the spin orbit coupling and thus the magnitude of the spin/valley current. 3) to exploit the possibility of an experimental observation on the spin/valley Nernst effect and its electric tuning. 4) to investigate the generation and electrical control of the spin photogalvanic current in the nano-scale surface area between two tips of scanning tunneling microscopy..The proposed research may provide important exploitation along the way to achieve scientific and technical breakthroughs in spin current generation that is already central for spintronics and valleytronics; and we expect many follow-up activities in each of the four aspects of research conveyed in the proposal once it is finished three years later.

后摩尔时代的电子器件接近物理极限尺寸工作，其热耗散问题一直是电路大规模集成化的瓶颈和障碍。作为新型电子器件可能的信息传输介质，电子自旋在输运过程中却是几乎没有热耗散的，因此近十几年来对电子自旋的新物理和新原型器件的科学探索正迅速发展成为一个新兴学科。作为自旋电子学的核心要素之一，自旋极化电子和自旋流的产生和调制成为构筑高速低功耗自旋原型器件的前决条件。本项目旨在通过d轨道电子异质结构体系的理论设计和对光场与物质相互作用过程的精密控制，研究圆偏振光光场对单原子层d轨道电子体系中自旋和能谷光电流的产生和调控的影响，构筑高性能自旋电子器件，探索实验观测能谷能斯特效应的可能性，并探索使用四探针扫描隧道显微镜来探测纳米/原子尺度自旋/能谷光电流的可能性。本项目的实施有希望探索全新的物理现象，推动光学在自旋电子学、表界面物理和半导体材料等领域的实际应用，具有重要的基础科学意义和实际应用价值。

在自旋电子学的三大基本课题（自旋的注入/操控/探测）当中，自旋电流的高效产生和调控是实现自旋电子原型器件的核心前提条件。利用光场与物质的相互作用，尤其是圆偏振光在半导体材料体系上所产生的自旋光电效应是注入自旋电流的一种高效便捷的方法。在光与物质的相互作用下所引起的自旋流的产生和调控过程中，通常希望研究的体系有足够强的自旋轨道耦合且在能带结构中可以产生比较大的自旋劈裂。在众多强自旋轨道耦合的材料体系中，具有d轨道电子的二维半导体因其独特的层状晶体结构和电子能带结构，被认为是实现载流子自旋极化注入的最佳候选材料。因此，在具有d轨道电子的二维半导体及其异质结构中，在光与物质相互作用下实现自旋极化的操控和自旋流的产生，对于探讨自旋流产生的新机制、实现自旋流的外场调控具有重要意义和广阔的应用前景。.通过“晶体对称性工程”的全新概念设计，在单原子层d轨道电子的二维半导体及其异质结构中实现范德华界面的电子极化和自发光电流效应，通过组合具有不同旋转对称性的晶体WSe2（C3对称性）和BP（C2对称性），在WSe2/BP范德华界面通过面内电子极化的实现诱导产生沿极化方向的自发光电流效应（沿极性方向实现自发的光电流而在垂直方向上不存在）。通过圆偏光的控制，实现了自发光电流生成自旋极化电流，并可以通过外加电场直接调控。这项工作中的“对称性工程”概念适用于多种范德华材料，更多其他前所未有的新兴光电流特性有待研究发现。进一步，我们找寻到一种全新的具有C2对称性的材料体系，在全新各向异性SiP2二维材料体系中，通过不同层厚材料的偏振依赖光致发光PL谱和拉曼光谱等光谱学测试手段，观测到A激子与声子多体作用产生的声子边带，并发现各向异性的荧光和吸收，证实了强各向异性激子态，通过低温、掺杂、施加应力、施加压力等技术手段进行调控，研究线偏振激子和激子声子多体过程。进而制备成不同转角堆叠的过渡金属硫族化合物（TMDCs）与SiP2的范德华异质结器件，研究了不同对称性的TMDCs/SiP2人工异质界面对材料电子能态结构的调控。利用PL谱和光电流扫描测试，探究不同堆叠角度时一维摩尔条纹不同于二维摩尔条纹的调制作用，研究摩尔条纹对沿极性方向自发光电流和自旋电流的调控作用。.本项目探索了全新的物理现象，有助于推动光学在自旋电子学、表界面物理和半导体材料等领域的实际应用，具有重要的基础科学意义和实际应用价值。

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