CAREER: Nanoscale Synthesis and Imaging of Novel Topological Phases

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，不能以这种简单的方式分类。例如，一种被称为“拓扑绝缘体”（TI）的原型拓扑材料在主体上是绝缘体，但在表面上表现出金属行为。换句话说，只允许TI的表面导电。此外，TI表面上的电子可以是零质量，并且表现得像相对论粒子。近年来出现了几种新型拓扑材料的理论框架，每种材料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建和表征新的拓扑材料：(1)分子束外延，一次创建单个原子层的材料；(2)扫描隧道显微镜，可视化其原子和电子结构。该项目旨在为对拓扑材料的理解提供一个基本的进步，并为其最终在自旋电子学和量子计算等技术中的应用制作新材料。该项目的教育目标是利用首席研究员在材料生长和显微镜成像方面的专业知识，并针对影响广泛的学生，包括初中，高中，本科生和研究生。具体工作包括在当地K-12学校开展推广活动，参加科学研究所暑期项目，组织大学级别的科学讲座，以及开发以最先进的合成和显微镜技术为重点的课程。技术摘要：在过去的几十年里，出现了几类具有非凡物理性能的材料，如高温超导体、巨磁阻材料和石墨烯等二维系统。拓扑材料（Topological materials）是最近加入该列表的材料，它是一种拥有新型电子态的系统，其存在和性质由拓扑不变量指定。典型的拓扑材料是拓扑绝缘体，其特征是由电子能带结构计算得出的奇数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其边界上存在无间隙金属电子态，由对称保护的无质量狄拉克费米子占据。最近，出现了几种新型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，由于合成和表征瓶颈，实验一直在努力完全赶上这些预测。本项目利用分子束外延和光谱成像扫描隧道显微镜的罕见结合，探索发现和操纵拓扑相的新途径。具体来说，该项目旨在通过探索不同的薄膜厚度、衬底、掺杂和应变，在（Pb,Sn）Te半导体家族薄膜中创造新的拓扑相。在可变磁场下，低至~400 mK基温的纳米尺度光谱表征允许以优越的空间和能量分辨率探索拓扑相。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，参与研究科学研究所的暑期项目，为大学学生组织科学讲座，以及开发以最先进的合成和显微镜技术为重点的课程，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.