Spin Coherence and Magnetism in Graphene

*非技术摘要*电子具有一种称为自旋的基本性质，它使电子相当于一块小磁铁。该项目的长期目标是开发一种功能强大的新型计算机，被称为“自旋计算机”，它使用电子自旋来存储和处理数据。这种类型的计算机的设计是为半导体中的电子自旋而开发的，但这种努力受到小信号和对低温(非常低)操作温度的需求的阻碍。一种名为石墨烯的新电子材料的出现，使自旋计算机的前景变得更加现实，因为已经在室温下展示了大的自旋信号。石墨烯是一种单原子碳片。这个项目将通过实验来研究石墨烯薄片中的杂质、空位和涟漪等缺陷的作用，从而推进石墨烯中电子自旋的基础知识。这些研究是至关重要的，因为缺陷被认为是电子自旋所持有的信息丢失(“退相干”)的原因，尽管目前尚不清楚哪种类型的缺陷是问题的主要来源。这些实验将系统地解决这一关键问题，并探索利用缺陷来控制自旋对齐(“磁性”)的新方法。该项目将支持使用最先进的仪器对一名博士生、一名本科生和一名高中生进行凝聚态物理方面的培训。在这些研究中获得的知识将极大地扩大自旋电子学的科学和技术影响。*技术摘要*石墨烯中自旋散射的起源是石墨烯自旋电子学的中心问题，而可调磁是一种有趣的集体现象，预测了掺杂和/或缺陷的石墨烯。本项目通过在石墨烯自旋阀和霍尔棒器件中系统地引入杂质、空位和波纹来研究自旋散射和可调谐磁性。该实验方法以一种独特的方式结合了分子束外延(MBE)和磁输运测量技术。杂质将通过分子束外延沉积逐个原子系统地引入，而空位将通过超高真空室中的Ar离子溅射产生。它们对电荷和自旋输运的影响将在同一腔内通过原位磁输运测量来测量。波纹将通过使用分子束外延生产的原子平面基板来控制。这些类型的无序对自旋寿命的影响将通过自旋阀的自旋进动(Hanle)测量来测量，并将阐明石墨烯中自旋散射的机制。通过磁输运测量和磁化强度测量相结合的方法来研究掺杂和/或缺陷石墨烯中的近藤效应和栅极可调磁有序。这些实验将极大地扩展固态系统中与自旋相关的相互作用的知识，并为凝聚态物理的博士生、本科生和高中生提供出色的培训。

****NON-TECHNICAL ABSTRACT****Electrons possess a fundamental property known as spin, which makes the electron equivalent to a small magnet. The long term goal of this project is to develop a powerful new type of computer known as a "spin computer" that uses the electron spin to store and process data. The design of this type of computer has been developed for electron spins in semiconductors, but such efforts have been hampered by small signals and the need for cryogenic (very low) operating temperatures. The advent of a new electronic material known as graphene, a single atomic sheet of carbon, is making the prospects of a spin computer more realistic because large spin signals have been demonstrated at room temperature. This project will advance the fundamental knowledge of electron spin in graphene by performing experiments that investigate the role of imperfections such as impurities, vacancies, and ripples in the graphene sheet. These studies are crucial because imperfections are believed to be responsible for the loss of information held by the electron spin ("decoherence"), although it is currently unclear which type of imperfection is the main source of the problem. The experiments will systematically address this critical issue and also explore new methods of using the imperfections to control the alignment of the spins ("magnetism"). This project will support the training of a PhD student, an undergraduate student, and a high school student in condensed matter physics using state-of-the-art instrumentation. The knowledge gained in these studies will greatly broaden the scientific and technological impact of spintronics.****TECHNICAL ABSTRACT****The origin of spin scattering in graphene is a central issue of graphene spintronics, while tunable magnetism is a fascinating collective phenomena predicted for doped and/or defective graphene. This project investigates spin scattering and tunable magnetism by systematically introducing impurities, vacancies, and ripples into graphene spin valves and Hall bar devices. The experimental approach combines the techniques of molecular beam epitaxy (MBE) and magnetotransport measurements in a unique manner. Impurities will be systematically introduced atom-by-atom through MBE deposition while vacancies will be generated by Ar-ion sputtering in an ultrahigh vacuum chamber. Their effect on charge and spin transport will be measured in the same chamber via in situ magnetotransport measurements. Ripples will be controlled through the use of atomically flat substrates produced by MBE. The effects of these types of disorder on spin lifetimes will be measured by spin precession (Hanle) measurements on spin valves and will elucidate the mechanism of spin scattering in graphene. The Kondo effect and gate tunable magnetic ordering in doped and/or defective graphene will be investigated through a combination of magnetotransport measurements and magnetization measurements. These experiments will greatly expand the knowledge of spin-dependent interactions in solid-state systems and provide excellent training for a PhD student, an undergraduate student, and a high school student in condensed matter physics.