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

非技术：目前的关键固态技术，如发光二极管、太阳能电池和其他电光组件，其性能和能力最终受制于传统半导体的基本特性，如硅和砷化镓。层状过渡金属二卤代化合物(式为MX2，如MoS2和WSe2)的单分子膜最近被发现是一类新的半导体，在原子厚度极限下是有效的。它们具有截然不同的基本性质，可以打破以前的限制，允许观察和发现新的物理现象，并为前所未有的固态设备的可能性提供了可能性。MX2的基本新性质包括电子自旋之间的关系，指定电子在2D晶格中占据两个等效动量状态中的哪一个的“谷指数”，以及当两个单分子层形成双层时电子处于哪一层的“层指数”。这种材料中的自旋、山谷和层量子数是相互关联的，可以联合操纵，研究人员已经从理论和实验上证明了这一点。这项拟议的工作旨在研究这种“自旋-谷层-层耦合”的所有方面，从它在电传输中的作用到它与光腔的相互作用，以期开发出具有电、磁和光控制的新颖而实用的器件技术。这项工作将由一个由物理和工程师组成的混合团队完成，并将为大量的研究生和本科生提供跨学科的研究教育。技术：在单层MX2中，强烈的自旋-轨道耦合将带边缘的真实自旋锁定到山谷指数，以产生自旋-山谷耦合。同时，可以使用圆偏振光学选择规则来寻址山谷，从而可以寻址自旋。此外，在双层和异质结中，对于给定的谷指数，谷指数进一步耦合到层指数，导致新的磁电效应。因此，MX2提供了第一个固态系统，在该系统中，可以对组合的谷假自旋和真实自旋进行电子和动力学控制，从而打开了丰富的混合自旋和谷子装置的可能性。这项工作的重点是研究MX2单层、双层和异质结中独特的自旋、谷态和层状赝旋耦合，并考虑到光子和自旋电子器件的应用。具体的目标和方法如下：(1)发展自旋-山谷-电荷耦合输运的微观理论，以指导和模拟单层、双层、异质结构和杂化系统的实验工作和合成；(2)利用可控的电荷和磁性掺杂，发展高质量、大面积的晶体生长；(3)结合本征自旋和山谷性质的电学和光学研究，确定用于混合自旋-山谷电子学的各种器件几何形状的单层和双层的基本参数；(4)研究和演示在异质结构和混合光子器件中对自旋和谷极化的操纵，将MX2与光子晶体腔等结构相结合，解决包括自旋-谷-电荷跨界面传输、层间激子物理、集成腔结构中的激子-极化子以及纳米腔内强光子-谷激子相互作用等方面的问题；以及(5)开发MX2横向结器件，用于高效、可控且具有自旋和山谷特性的光发射器、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.