RUI: Exploring the Transport Properties of Topological Insulators using Spectroscopic Ellipsometry

非技术摘要：有一个相当大的兴趣，在一种新的材料称为拓扑绝缘体（TI），似乎两个不同的性质的材料，即导电和绝缘相，交织成一个单一的材料。 虽然这些材料提供了一个平台来解决物理学中的无数理论问题，但由于其独特的性质，TI也可以用来制造有趣的设备。该项目的主要重点是研究TI的独特性质，密切关注揭示其表面和体相态之间的相互作用。具体来说，主要目标之一是建立一个基本的理解，如何分离的贡献从表面和体状态的材料的整体电导率。此外，磁掺杂的TI样品进行了分析，以询问磁性和各种性能TI之间的相互作用。该项目主要使用一种称为光谱椭圆偏振法的光学调查技术来确定TI样品的表面和体态的贡献。此外，温度相关的实验进行，以揭示复杂的细节，管理的物理TI。 这项工作完全由文科专业的本科生完成。这些学生接受材料表征，光学和低温方面的培训，为研究生学习或科学技术职业做好准备。 为了进一步实现教育目标，该项目将几个高影响力的实验活动纳入物理课程的现有课程。此外，还为高中生开展了几项外联活动，以培养他们对科学的更广泛兴趣。技术摘要：由于强自旋轨道耦合和时间反演不变对称性，一类新的材料，称为拓扑绝缘体（TI），嵌入了独特的特性;它在体中具有能隙，但具有金属表面状态，对无序散射具有鲁棒性。尽管在过去的几年里，人们为理解TI的物理特性做出了一致的努力，但仍有几个关键方面是未知的; a）体态和表面态之间的相互作用决定了TI的导电性，B）杂质带对TI的影响，c）拓扑保护态和磁性之间的相互作用，以及d）电子-声子耦合对拓扑保护表面态的重要性。对这些方面的任何见解都将使人们能够更深入地了解TI的基本物理学，这是该项目的最终目标。光谱椭圆偏振法用于在宽光谱范围内确定复合电导率（即，在30 meV到6.2 eV之间），这使得人们能够破译自由载流子和能带电子对电导率的贡献。温度依赖的测量进行探测的电子-声子耦合的TI，它起着至关重要的作用，影响表面状态。由于TI受到缺陷的困扰，这不幸地掩盖了令人兴奋和有趣的表面现象，因此通过评估高阶跃迁（即，临界点）。磁掺杂TI探测，以确定其磁性的起源，并研究时间反演对称性的破缺。最后，自旋织构的TI探测通过其圆二色性，获得基于米勒矩阵的光谱椭圆偏振。 该项目包括若干旨在加强科学教育目标的活动。由于这项工作完全由文科专业的本科生完成，他们接受了材料表征，光学和低温学方面的培训，为他们在STEM领域的研究生学习或职业生涯做好了准备。 此外，该项目将几个高影响力的实验活动注入物理课程的现有课程。此外，还为高中生开展了一些外联活动，以培养他们对科学的更广泛兴趣。

Nontechnical Abstract:There has been a considerable interest in a novel material called Topological Insulators (TIs) where seemingly two distinct properties of materials, namely conducting and insulating phases, are interwoven into a single material. While these materials provide a platform to address a myriad of theoretical problems in physics, because of their unique properties TIs can be exploited to produce interesting devices as well. The main focus of the project is to investigate the unique properties of TIs, paying close attention to uncovering the interplay between their surface and bulk states. Specifically, one of the main objectives is to establish a fundamental understanding of how to separate the contributions from surface and bulk states to the overall conductivity of the material. Additionally, magnetically doped TI samples are analyzed to interrogate the interplay between magnetism and various properties TIs. This project primarily uses an optical investigation technique known as spectroscopic ellipsometry to determine the contributions from the surface and the bulk states of TI samples. Additionally, temperature dependent experiments are conducted in order to uncover the intricate details that govern the physics of TIs. The work is performed exclusively by undergraduate students in a liberal arts setting. These students receive training in materials characterization, optics, and cryogenics, preparing them for graduate studies or careers in science and technology. To further educational goals, this project incorporates several high-impact experimental activities into existing courses in the physics curriculum. Furthermore, several outreach activities for high school students are conducted in order to foster a wider interest in the sciences. Technical Abstract:Because of strong spin-orbit coupling and time reversal invariant symmetry, a new class of materials, called topological insulators (TIs), are embedded with unique characteristics; it has an energy gap in the bulk but has metallic surface states that are robust against disorder-scattering. Although there has been a concerted effort made towards understanding the physics of TIs in the past few years, there are several key aspects that are still unknown; a) the interplay between the bulk and the surface states in dictating the conductivity of TIs, b) the impact of impurity bands on TIs, c) interplay between topologically protected states and magnetism, and d) the significance of electron-phonon coupling on topologically protected surface states. Insights gained about any of these aspects will enable a deeper understanding of the fundamental physics of TIs, which is the ultimate goal of the project. Spectroscopic ellipsometry is used to determine the complex conductivity in a wide spectra range (i.e., between 30 meV to 6.2 eV), which enables one to decipher the contributions from free carriers and band electrons to conductivity. Temperature dependent measurements are conducted to probe the electron-phonon coupling in TIs, which plays a vital role in influencing the surface states. Since TIs are plagued by defects, which unfortunately mask the exciting and intriguing surface phenomena, the details of defect-states are obtained by evaluating the higher-order transitions (i.e., critical points). The magnetically-doped TIs are probed to determine the origin of their magnetism and to study the breaking of time-reversal symmetry. Finally, the spin texture of TIs are probed via their circular dichroism, obtained by Mueller-Matrix based spectroscopic ellipsometry. This project incorporates several activities to enhance educational goals in the sciences. As the work is performed exclusively by undergraduate students in a liberal arts setting, they receive training in materials characterization, optics, and cryogenics, preparing them for graduate studies or careers in STEM-based fields. In addition, this project injects several high-impact experimental activities into existing courses in the physics curriculum. Also, several outreach activities for high school students are conducted in order to foster a wider interest in the sciences.