Optical Studies of Spin in 2D Crystals

*技术摘要*二维(2D)晶体是一类令人着迷的新型材料，具有新颖的电子、自旋和光学性质。在这个项目中，我们将使用光学技术研究两种类型的2D晶体的自旋相关性质：MoS_2(及其相关的金属二卤化物)和石墨烯。单层MoS_2是一种直接带隙半导体，在K/K‘谷价带有巨大的自旋分裂。有趣的是，据预测，这将抑制大多数类型的自旋弛豫，并产生更长的自旋寿命。此外，巨大的自旋分裂使MoS_2成为实现本征自旋霍尔效应的有希望的候选者。我们的目标是用时间分辨克尔显微镜直接测量MoS_2中的自旋极化和自旋动力学来观察这些新的性质。石墨烯在室温下具有较长的自旋扩散长度，是一种很有前途的自旋电子学材料。石墨烯自旋电子学领域的前沿涉及自旋弛豫和感应磁性的性质。几乎所有的研究都基于自旋输运，但这种方法有其局限性，如铁磁接触引起的自旋弛豫和对Hanle分析的依赖。我们将利用时间分辨光学技术来克服这些限制。总之，这些关于MoS_2和石墨烯的研究处于2D晶体中与自旋相关的物理的前沿。该项目将支持两名博士生的教育，他们将在工业和学术界的职业生涯中接受出色的培训。*非技术摘要*二维(2D)晶体是一类非凡的材料，具有令人着迷的新特性，并有可能使传统硅技术以外的电子技术发生革命性变化。特别是对于自旋电子器件，二维晶体表现出比三维晶体更好的性能，并预测了与其二维结构相关的特殊自旋相关性质。为什么2D表现更好，可以进一步改进，预测的新属性能否演示？为了回答这些关键问题，我们将使用结合超快脉冲激光和光学显微镜的强大成像技术来直接可视化2D晶体中电子自旋的运动和旋转。自旋可以被认为是一块微小的磁铁，当电子流经设备时，它附着在电子上。在自旋电子器件中，电子磁极的方向(“北”和“南”)被用来在整个设备中携带信息，而这种信息可以通过旋转磁极的方向来操纵。通过使用激光和光学成像这些磁极(即自旋)的运动和旋转，我们可以研究它们是如何保存信息的，它们是如何丢失信息的，以及磁极的方向如何与电子的运动有关。了解这些问题将有助于开发用于硅以外电子产品的先进自旋电子器件。该项目将支持两名博士生的教育，他们将在工业和学术界的职业生涯中接受出色的培训。

****Technical Abstract****Two-dimensional (2D) crystals are a fascinating new class of materials that exhibit novel electronic, spintronic, and optical properties. In this project, we will use optical techniques to investigate the spin-dependent properties of two types of 2D crystals: MoS2 (and related metal dichalcogenides) and graphene. Monolayer MoS2 is a direct gap semiconductor with a giant spin splitting of the valence band at the K/K' valleys. Interestingly, this is predicted to suppress most types of spin relaxation and generate long spin lifetimes for holes. Further, the giant spin splitting makes MoS2 a promising candidate for realizing the intrinsic spin Hall effect. Our aim is to observe these novel properties using time-resolved Kerr microscopy to directly measure the spin polarization and spin dynamics in MoS2. Graphene is a promising material for spintronics due to its long spin diffusion length at room temperature. The forefront of the field of graphene spintronics concerns the nature of spin relaxation and induced magnetism. Nearly all studies are based on spin transport, but this approach has its limitations such as spin relaxation induced by ferromagnetic contacts and reliance on Hanle analysis. We will utilize time-resolved optical techniques to overcome these limitations. Together, these studies on MoS2 and graphene lie at the forefront of spin-dependent physics in 2D crystals. This project will support the education of two PhD students, who will receive excellent training for careers in industry and academia.****Non-Technical Abstract****Two-dimensional (2D) crystals are a remarkable class of materials that exhibit fascinating new properties and have the potential to revolutionize electronics beyond conventional silicon technologies. In particular for spintronic devices, 2D crystals are exhibiting much better performance than their three-dimensional counterparts and special spin-dependent properties related to their 2D structure are predicted. Why does 2D perform better, can it be further improved, and can the predicted new properties be demonstrated? To answer such key questions, we will use powerful imaging techniques combining ultrafast pulsed lasers and optical microscopes to directly visualize the motion and rotation of electron spins in 2D crystals. The spin can be thought of as a tiny magnet that is attached to an electron as is flows through a device. In a spintronic device, the direction of an electron's magnetic poles ("north" and "south") is used to carry information throughout the device, and this information can be manipulated by rotating the direction of the poles. By using lasers and optics to image the motion and rotation of these magnetic poles (i.e. spin), we can investigate how they hold information, how they lose information, and how the direction of the poles is related to the electron's motion. Understanding these issues will enable the development of advanced spintronic devices for electronics beyond silicon. This project will support the education of two PhD students, who will receive excellent training for careers in industry and academia.