Noise in 2d topological edges and spin Hall systems

摘要：电子既携带电荷，也像陀螺一样携带被称为“自旋”的角动量，虽然普通电子设备通常只使用电荷，但人们对利用自旋进行信息存储和处理的兴趣越来越大。在某些具有特殊结构的材料中，一个方向上的电荷电流可以驱动横向方向上的自旋电流（“自旋霍尔效应”），并且自旋的净不平衡可以在材料表面建立起来。在其他材料（二维“拓扑绝缘体”）中，电荷和自旋预计只会围绕材料的周长流动。在这两个系统中，由于电荷和自旋的数量是离散的，并且耦合在一起，因此不仅可以通过测量电荷电流，还可以通过测量电荷电流的波动（称为“散点噪声”）来了解它们是如何运动的。该项目测量沿二维拓扑绝缘体边缘流动的电流在无线电频率下产生的散粒噪声，以确定边缘传输的性质和限制其流动的散射机制。这些信息通过噪声的几何形状、温度和磁场依赖性揭示出来。本项目采用相同的测量方法来寻找自旋霍尔效应在材料表面形成自旋极化时预计会产生的散粒噪声。这是在不需要涉及磁性材料的复杂设备结构的情况下确定自旋积累量的一种途径。本项目为两名研究生提供研究训练和专业发展，并为本科生提供研究机会。研究结果通过出版物、在会议和大学的演讲以及主要研究者的博客进行传播。首席研究员正在与K12教师和本科生合作，在博客上发表文章，并与《休斯顿纪事报》合作，以适合普通观众的水平解释这一研究和相关研究。首席研究员也在为科学作家和记者开发资源，以帮助向公众解释凝聚态物质和纳米级物理概念。技术摘要：越来越多的人认识到，电子自旋和轨道自由度之间的相互作用为操纵自旋和创造前所未有的电子系统提供了机会。这项工作的智力价值集中在理解两个物理系统上。在特定的材料中，能带结构预测了块体中二维“拓扑绝缘体”状态的形成，具有拓扑保护的一维螺旋边缘状态包围着周长。了解这些边缘状态对于实现它们在无耗散传输和计算中的好处是必不可少的。电子-电子相互作用、磁性杂质和无序将以可通过测量边缘态传导中的非平衡噪声来测试的方式影响这些边缘态。此外，相关的自旋霍尔效应是金属中产生非平衡自旋积累的可行手段，在信息存储和处理方面具有巨大的潜力。电荷噪声被预测为一种不需要铁磁材料就能表征自旋积累的方法。该项目测量非平衡噪声和电荷输运，以检测二维TI界面的边缘状态和金属纳米结构中she驱动的自旋积累。具体的科学问题包括：半导体异质界面的边缘状态是平凡的还是拓扑的？是什么限制了这种边缘状态下的弹道输运？自旋霍尔效应产生的自旋积累在多大程度上产生电荷电流噪声？这些科学问题与技术应用相关，包括奇异的计算架构和磁信息存储和处理。除了技术之外，这个项目更广泛的影响还包括研究生和本科生作为下一代技术劳动力的专业培训，以及包括K12教育在内的公共宣传。首席研究员与Rice K12项目和正在进行的Rice REU项目合作，重点关注休斯顿地区的社区大学生；以及基于他建立了十年的博客的公众宣传。具体产品包括为科学记者提供的凝聚态物质/纳米物理学入门/指南；向科学记者提供有关该领域新发展的“简报”；以及为《休斯顿纪事报》撰写科学文章。

Non-Technical Abstract:Electrons carry both electrical charge and, much like a top, angular momentum called "spin", and while ordinary electronic devices typically only use the charge, there is increasing interest in taking advantage of the spin for information storage and processing. In some materials with particular structures, a current of charge in one direction can drive a current of spin in a transverse direction (the "spin Hall effect"), and a net imbalance of spin can build up at the material's surface. In other materials (two-dimensional "topological insulators"), charge and spin are predicted to flow only around the perimeter of the material. In both systems, because charge and spin come in discrete amounts and are coupled together, a lot can be learned about how they move by measuring not just the charge current, but fluctuations in the charge current called "shot noise". This project measures the shot noise at radio frequencies produced by current flow along the edges of 2d topological insulators, to determine the nature of that edge transport and the scattering mechanisms that limit its flow. This information is revealed by the geometry, temperature, and magnetic field dependence of the noise. This project applies the same measurement approach to look for shot noise predicted to result when the spin Hall effect builds up spin polarization at a material surface. This is one avenue to determine the amount of spin build-up without the need for complicated device structures involving magnetic materials. This project provides research training and professional development for two graduate students, as well as research opportunities for undergraduates. Results are disseminated through publications, presentations at conferences and universities, and through the principal investigator's blog. The principal investigator is working with K12 teachers and undergraduates to explain this and related research at level appropriate for a general audience, in writings distributed in the blog and in cooperation with the Houston Chronicle. The principal investigator is also developing resources for science writers and journalists, to aid in explaining condensed matter and nanoscale physics concepts to the public.Technical Abstract:There is a growing appreciation that the interplay between spin and orbital degrees of freedom of electrons provides opportunities to manipulate spin and create previously unprecedented electronic systems. The intellectual merit of this effort focuses on understanding two physical systems. In particular materials, band structure predicts the formation of a two-dimensional "topological insulator" state in the bulk, with topologically protected, 1d helical edge states girding the perimeter. Understanding these edge states is essential for realizing their benefits in dissipationless transport and computing. Electron-electron interactions, magnetic impurities, and disorder are expected to affect these edge states in ways testable through measurements of nonequilibrium noise in the edge state conduction. In addition, the related spin Hall effect is a viable means of generating nonequilibrium spin accumulation in metals, with tremendous potential for information storage and processing. Charge noise is predicted to be a way to characterize that spin accumulation without the need for ferromagnetic materials. This project measures nonequilibrium noise and charge transport to examine edge states at 2d TI interfaces and SHE-driven spin accumulation in metal nanostructures. Specific scientific questions include: Are edge states at semiconductor heterointerfaces trivial or topological? What limits ballistic transport in such edge states? To what extent does spin accumulation from the spin Hall effect produce charge current noise? These science questions have relevance for technological applications, including exotic computational architectures and magnetic information storage and processing. Beyond technologies, the broader impacts of this project involve the professional training of graduate students and undergraduates as the next generation of technological workforce, and public outreach including K12 education. The principal investigator works with Rice K12 programs and an ongoing Rice REU program focusing on Houston-area community college students; and public outreach based on his decade-established blog. Specific products include a primer/guide to condensed matter/nano physics for science journalists; "tip-sheets" to science journalists regarding new developments in the field; and science writing contributions for the Houston Chronicle.