EFRI 2-DARE Proposal: Spin-Valley Coupling for Photonic and Spintronic Devices

非技术：目前固态技术的关键性能和能力，如发光二极管、太阳能电池和其他电光元件，最终受到传统半导体（如硅和砷化镓）基本特性的限制。层状过渡金属二硫族化合物单层（化学式为MX2，如MoS2和WSe2）最近被发现是在原子薄极限下有效的一类新半导体。它们具有截然不同的基本特性，可以突破以前的限制，允许观察和发现新的物理现象，并提供前所未有的固态器件可能性的潜力。MX2中的基本新性质包括电子自旋之间的关系，“谷指数”指定电子在二维晶格中占据两种等效动量状态中的哪一种，“层指数”指定当两个单层形成双层时电子在哪一层。这种材料中的自旋、谷和层量子数是相互联系的，可以共同操纵，正如研究人员已经从理论上和实验上证明的那样。提出的工作旨在研究这种“自旋谷层耦合”的各个方面，从它在电传输中的作用到它与光腔的相互作用，以期开发具有电、磁和光控制的新颖实用的设备技术。这项工作将由一个由物理学和工程师组成的混合团队完成，并将为大量研究生和本科生提供跨学科的研究教育。技术性：在单层MX2中，强自旋-轨道耦合将带边缘的实自旋锁定在谷指数上，从而产生自旋-谷耦合。同时，利用圆偏振光学选择规则可以对谷和自旋进行寻址。此外，在双层和异质结构中，谷指数与给定谷指数的层指数进一步耦合，导致新的磁电效应。因此，MX2s提供了第一个固态系统，在该系统中，可以对谷伪自旋和实自旋的组合进行电气和动态控制，从而开辟了大量混合自旋和谷器件的可能性。本文的工作重点是研究MX2单层、双层和异质结构中独特的自旋、谷和层伪自旋耦合，并考虑到光子和自旋电子器件的应用。具体目标和方法如下：(1)发展自旋谷-电荷耦合输运的微观理论，指导和模拟单层、双层、异质结构和杂化体系的实验工作和合成；(2)利用可控的带电和磁性掺杂剂，实现高质量、大面积的晶体生长；(3)结合电学和光学对本征自旋和谷特性的研究，确定混合自旋谷电子不同器件几何形状下单层和双层的基本参数；(4)研究并演示了异质结构和混合光子器件中自旋和谷极化的操纵，以及将MX2s与光子晶体腔等结构结合在一起的混合光子器件，解决了包括界面上的自旋谷电荷传输、层间激子物理、集成腔结构中的激子-极化以及纳米腔内强光子谷激子相互作用等方面的问题；(5)开发MX2横向结器件，用于高效、可控、自旋和谷特定的光发射器、调制器和探测器。

Non-Technical: The performance and capabilities of key present solid-state technologies, such as light-emitting diodes, solar cells, and other electro-optic components, are ultimately constrained by the fundamental properties of conventional semiconductors such as silicon and gallium arsenide. Monolayers of the layered transition-metal dichalcogenides (with formula MX2, such as MoS2 and WSe2) have recently been discovered to be a new class of semiconductors effectively at the atomically thin limit. They have dramatically different fundamental properties which can defy previous constraints, allowing observation and discovery of new physical phenomena and offering potential for unprecendented solid-state device possibilities. Fundamentally new properties in MX2 include the relationship between electron spin, the "valley index" specifying which of two equivalent momentum states the electron occupies in the 2D lattice, and the "layer index" specifying which layer the electron is in when two monolayers form a bilayer. The spin, valley and layer quantum numbers in this material are linked and can be manipulated jointly, as has already been demonstrated theoretically and experimentally by the investigators. The proposed work aims to study all aspects of this "spin-valley-layer coupling", ranging from its role in electrical transport to its interplay with optical cavities, with a view to novel yet practical device technologies with electrical, magnetic, and optical control. The work will be done by a mixed team of physics and engineers and will provide interdisciplinary research education to a large number of graduate and undergraduate students.Technical: In monolayer MX2 the strong spin-orbit coupling locks the real spin at the band edges to the valley index to produce spin-valley coupling. At the same time the valleys, and hence spins, are addressable using circularly polarized optical selection rules. Additionally, in bilayers and heterostructures the valley index is further coupled to the layer index for a given valley index, leading to new magnetoelectric effects. MX2s thus provide the first solid-state system in which electric and dynamical control of combined valley pseudospin and real spin are possible, opening up a wealth of hybrid spin and valley device possibilities. The focus of the proposed work is on the studying the unique spin, valley and layer pseudospin couplings in MX2 monolayers, bilayers and heterostructures with photonic and spintronic device applications in mind. The specific goals and methods are as follows: (1) develop microscopic theories for coupled spin-valley-charge transport to guide and model the experimental efforts and synthesis of monolayers, bilayers, heterostructures, and hybrid systems; (2) develop highest quality, large-area crystal growth with controllable charged and magnetic dopants; (3) combine electrical and optical investigation of intrinsic spin and valley properties to determine the fundamental parameters of monolayers and bilayers in various device geometries for hybrid spin-valleytronics; (4) investigate and demonstrate manipulation of spin and valley polarizations in heterostructures and hybrid photonic devices combining MX2s with structures such as photonic-crystal cavities, addressing aspects including spin-valley-charge transport across interfaces, interlayer exciton physics, exciton-polaritons in integrated cavity structures, and strong photon-valley exciton interactions within a nanocavity; and (5) develop MX2 lateral junction devices for efficient, controllable, and spin and valley specific light emitters, modulators and detectors.