CAREER: Nanoscale Synthesis and Imaging of Novel Topological Phases

非技术摘要：材料通常可以根据其内部电子的行为进行分类。例如，在金属中，电子可以自由移动并导电，而在绝缘体中则不能。“拓扑材料”是一种新的材料家族，不能用这种简单的方式进行分类。例如，一种被称为“拓扑绝缘体”(TI)的典型拓扑材料在整体上是绝缘体，但在表面上表现出金属行为。换句话说，只有TI的表面才能导电。此外，TI表面的电子可以为零质量，其行为类似于相对论粒子。近年来，出现了几种新型拓扑材料的理论框架，每一种都具有新的奇异性质，但实验一直难以完全赶上这些预测。该项目结合了两种先进的原子尺度技术来创造和表征新的拓扑材料：(1)分子束外延，一次只产生一个原子层；(2)扫描隧道显微镜，显示它们的原子和电子结构。该项目旨在从根本上促进对拓扑材料的理解，并为其最终在技术上的应用制作新材料，如自旋电子学和量子计算。该项目的教育目标利用首席研究员在材料生长和显微成像方面的专业知识，目标是影响广泛的学生，包括初中生、高中生、本科生和研究生。具体的努力包括在当地的K-12学校建立外联活动，参加研究院的暑期项目，在大学层面组织科学讲座，并开发专注于最先进的合成和显微技术的课程。技术摘要：在过去的几十年里，出现了几类具有特殊物理性能的材料，如高温超导体、巨磁电阻材料和石墨烯等2D系统。拓扑材料-拥有新的电子态的系统，其存在和性质由拓扑不变量指定-是最新添加到列表中的。典型的拓扑材料是拓扑绝缘体，系统的特征是由电子能带结构计算出的奇数Z2拓扑不变量。尽管拓扑绝缘体是体绝缘体，但能带结构的拓扑结构决定了它们边界上存在无间隙的金属电子态，这些电子态被受对称性保护的无质量狄拉克费米子占据。最近，出现了几类新的拓扑系统的理论框架，包括拓扑晶体绝缘体、拓扑超导体和Weyl半金属。然而，由于合成和表征方面的瓶颈，实验一直难以完全赶上这些预测。该项目罕见地使用了分子束外延和光谱成像扫描隧道显微镜的组合来探索发现和操纵拓扑相的新途径。具体地说，该项目旨在通过探索不同的薄膜厚度、衬底、掺杂和应变，在(铅，锡)Te系列半导体薄膜中创造新的拓扑相。在可变磁场中，纳米尺度的光谱表征可以低至约400mK的基温，从而可以探索具有更高空间和能量分辨率的拓扑相。该项目的教育目标是通过在当地初中和高中建立外展活动，参加科学研究所的暑期计划，在大学层面为学生组织科学讲座，以及开发专注于最先进的合成和显微技术的课程，来影响广泛的学生。

Nontechnical Abstract:Materials can often be categorized based on how electrons within them behave. For example, in metals, electrons are free to move around and conduct electricity, while in insulators, they cannot. "Topological materials" are a new family of materials that cannot be classified in such a simple manner. For instance, a prototypical topological material called a "topological insulator" (TI) is an insulator in the bulk, but exhibits metallic behavior on the surface. In other words, only the surface of a TI is allowed to conduct electricity. Moreover, electrons on the surface of a TI can have zero mass and behave like relativistic particles. A theoretical framework for several novel types of topological materials has emerged in recent years, each one hosting new exotic properties, but experiments have struggled to fully catch up with these predictions. This project combines two advanced atomic-scale techniques to create and characterize new topological materials: (1) molecular beam epitaxy to create the materials a single atomic layer at a time, and (2) scanning tunneling microscopy to visualize their atomic and electronic structure. The project aims to provide a fundamental advancement in the understanding of topological materials, as well as to craft new materials for their eventual use in technology, such as in spintronics and quantum computing. The education goals of this project utilize the principal investigator's expertise in materials growth and microscopy imaging, and are targeted to impact a wide range of students, including middle school, high school, undergraduate and graduate students. The specific efforts include establishing outreach events in local K-12 schools, participating in Research Science Institute summer program, organizing science talks at the university level, and developing courses focused on state-of-the-art synthesis and microscopy techniques.Technical Abstract:The past few decades have seen the emergence of several classes of materials with extraordinary physical properties, such as high-temperature superconductors, colossal magnetoresistance materials and 2D systems such as graphene. Topological materials - systems hosting novel electronic states whose existence and properties are specified by a topological invariant - are the most recent addition to the list. Prototypical topological materials are topological insulators, systems characterized by an odd Z2 topological invariant calculated from the electronic band structure. Even though topological insulators are bulk insulators, the topology of the band structure dictates the existence of gapless metallic electronic states at their boundary, occupied by massless Dirac fermions that are protected by symmetry. Recently, a theoretical framework for several new classes of topological systems has emerged, including topological crystalline insulators, topological superconductors and Weyl semimetals. However, experiments have struggled to fully catch up with these predictions, due to both synthesis and characterization bottlenecks. This project uses a rare combination of molecular beam epitaxy and spectroscopic-imaging scanning tunneling microscopy to explore new pathways for discovering and manipulating topological phases. Specifically, the project aims to create new topological phases in thin films of (Pb,Sn)Te family of semiconductors, by exploring different film thicknesses, substrates, doping and strain. Nanoscale spectroscopic characterization down to ~400 mK base temperature in variable magnetic field allows the explorations of topological phases with superior spatial and energy resolution. The education goals of this project are targeted to impact a wide range of students via establishing outreach events in local middle school and high schools, participating in Research Science Institute summer program, organizing science talks for students at the university level, and developing courses focused on state-of-the-art synthesis and microscopy techniques.