CAREER:Visualization and Control of Chiral Bound States and Ballistic Chiral Electrons in Two-Dimensional Material Heterostructures

非技术摘要：本CAREER项目由一项研究和推广计划组成，该计划解决了有关纳米尺度材料中非常快（狄拉克）电子控制的基本问题。这些奇异粒子为材料中的电子运动提供了一个令人兴奋的新测试平台，因为它们跨越障碍的行为与正常电子不同。对于狄拉克电子在这些界面上的行为有许多理论预测，但直到现在还不可能直接测试它们。PI最近开发了一种技术，可以创建具有任意形状的理想纳米级屏障，从而实现受限狄拉克电子的首次成像。狄拉克电子运动的这种控制和可视化将产生深刻的基础见解，为全新的电子设备铺平道路，例如新型传感器和可调谐纳米级电子透镜。该职业项目的外展部分将通过使用该研究项目和加州大学圣克鲁斯分校（UCSC）现有的美国国家科学基金会本科生研究经验，增加拉美裔在STEM专业中的代表性。来自当地西班牙裔服务机构的西班牙裔高中学生和成绩优异的社区大学学生将在他们的教育发展的三个阶段工作并得到指导：大学预科、大学早期和大学后期。这些阶段将为大学预科组提供科学职业意识，为大学预科组提供在PI研究工作中进行研究经验的机会，并为大学预科组提供基于职业的指导和网络机会。这些拓展工作不仅将使参与的学生受益，也将使加州大学圣迭戈分校的学生和更广泛的社区受益。技术摘要：这个职业项目解决了关于石墨烯和双层石墨烯的手性束缚态和轨迹的基本问题。由这些二维材料承载的奇异准粒子——狄拉克费米子，由于其手性和负能态，为电子运动提供了一个新的测试平台。PI最近开发了一种多功能技术来创建具有任意形状的原始纳米级PN结，这使得首次成像受限狄拉克费米子成为可能。本项目研究了势形的作用以及在这些约束势中无质量和有质量狄拉克费米子之间的行为差异。由于狄拉克费米子表现出克莱因隧穿（100%势垒传输）和反克莱因隧穿（100%势垒反射），本项目开展的研究揭示了Schrödinger费米子没有发现的新的束缚态。此外，通过PN和石墨烯/超导体结及其响应的定向狄拉克费米子轨迹被映射并直接与屏障传输和超导的微观模型进行了比较。关于狄拉克费米子在这些界面上的行为的理论预测是丰富的，但直到现在还不可能直接测试它们。具体来说，为了揭示手性结合态和轨迹，本CAREER项目包括三个主要研究工作：(1)可视化和控制由石墨烯和双层石墨烯组成的二维材料异质结构中的手性结合态。本研究在二维材料异质结构中创建了原始的纳米级PN结，并使用扫描隧道显微镜来绘制这些PN结内的量子干涉和约束。研究了不同的结形状（圆形与非圆形）和载流子类型（无质量与有质量狄拉克费米子）；(2)可视化和控制二维材料异质结构中的弹道手性电子轨迹。本研究使用扫描门显微镜结合纳米级PN结和磁场来绘制和引导无质量和有质量狄拉克费米子的弹道轨迹；(3)可视化和控制石墨烯/超导体界面上的手性轨迹。这项研究使用扫描门显微镜在纳米尺度上映射超导体界面上的狄拉克费米子反射。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Nontechnical abstract:This CAREER project is composed of a research and outreach plan that addresses fundamental questions regarding the control of very fast (Dirac) electrons in materials at the nanoscale. These exotic particles provide an exciting new testbed for electron motion in materials because their behavior across barriers is distinct from normal electrons. There are many theoretical predictions for the behavior of Dirac electrons at these interfaces, but it has not been possible to directly test them - until now. The PI recently developed techniques to create ideal nanoscale barriers with arbitrary shapes that enable the first imaging of confined Dirac electrons. Such control and visualization of Dirac electron motion will generate deep fundamental insights, paving the way toward entirely new electronic devices, such as novel sensors and tunable nanoscale electronic lenses. The outreach component of this CAREER project will increase Hispanic representation among STEM majors by using this research project with an existing NSF Research Experience for Undergraduates at the University of California Santa Cruz (UCSC). Hispanic high school students and high achieving community college students from Hispanic Serving Institutions from the local area will work and be mentored at three phases in their educational development: pre-college, early-college, and late-college. These stages will provide science career awareness to the pre-college group, provide opportunities for research experiences within the PI's research effort to the early-college group, and provide career-based mentoring and networking opportunities to the late-college group. These outreach efforts will benefit not only the participating students, but also students at UCSC, and the broader community.Technical abstract:This CAREER project addresses fundamental questions regarding chiral bound states and trajectories in graphene and bilayer graphene. The exotic quasiparticles-Dirac fermions, hosted by these two-dimensional materials provide a new testbed for electron motion because of their chiral nature and negative energy states. The PI recently developed a versatile technique to create pristine nanoscale PN junctions with arbitrary shapes, which enabled the first imaging of confined Dirac fermions. This project investigates the role of the potential shape and the differences in behavior between massless and massive Dirac fermions within these confinement potentials. Because Dirac fermions exhibit Klein tunneling (100% barrier transmission) and anti-Klein tunneling (100% barrier reflection), the research carried out by this project reveals novel bound states not found with Schrödinger fermions. Moreover, steered Dirac fermion trajectories across and in response to PN and graphene/superconductor junctions are mapped and compared directly to microscopic models for barrier transmission and superconductivity. Theoretical predictions for the behavior of Dirac fermions at these interfaces are abundant, but it has not been possible to directly test them - until now. Specifically, to reveal chiral bound states and trajectories this CAREER project contains three main research efforts: (1) Visualize and control chiral bound states in two-dimensional material heterostructures composed of graphene and bilayer graphene. This study creates pristine nanoscale PN junctions in two-dimensional material heterostructures and use scanning tunneling microscopy to map quantum interference and confinement within these PN junctions. Different junction shapes (circular vs. non-circular) and charge carrier types (massless vs. massive Dirac fermions) are investigated; (2) Visualize and control ballistic chiral electron trajectories in two-dimensional material heterostructures. This study uses scanning gate microscopy in conjunction with nanoscale PN junctions and magnetic fields to map and steer ballistic trajectories of massless and massive Dirac fermions; (3) visualize and control chiral trajectories at a graphene/superconductor interface. This study uses scanning gate microscopy to map Dirac fermion reflections off a superconductor's interface at the nanoscale.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.