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

摘要：人们对一种名为拓扑绝缘体（TIs）的新型材料非常感兴趣，这种材料将两种看似不同的材料，即导电相和绝缘相交织在一起。虽然这些材料为解决物理学中无数的理论问题提供了一个平台，但由于它们独特的性质，它们也可以被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，密切关注其表面和体态之间的相互作用。具体来说，主要目标之一是建立一个基本的理解，如何从表面状态和体状态中分离材料的整体电导率。此外，对磁性掺杂TI样品进行了分析，以询问磁性和各种性质TI之间的相互作用。该项目主要使用一种称为光谱椭偏的光学研究技术来确定TI样品的表面和体态的贡献。此外，还进行了与温度相关的实验，以揭示控制ti物理的复杂细节。这项工作完全由本科生在文科环境中进行。这些学生接受材料表征、光学和低温学方面的培训，为研究生学习或科学技术方面的职业做好准备。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现有的物理课程。此外，还为高中生开展了一些外展活动，以培养他们对科学更广泛的兴趣。技术摘要：由于强自旋-轨道耦合和时间反转不变对称性，一种新型材料——拓扑绝缘子（TIs）具有独特的嵌入特性；它在体中有一个能隙，但具有抗无序散射的金属表面态。尽管在过去的几年里，人们一直在努力理解人工智能的物理特性，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的电导率，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，d)电子-声子耦合对拓扑保护表面态的意义。从这些方面获得的见解将有助于更深入地理解人工智能的基本物理学，这是该项目的最终目标。利用椭圆偏振光谱法在较宽的光谱范围内（即30 meV至6.2 eV）测定复合物的电导率，从而可以破译自由载流子和带电子对电导率的贡献。利用温度相关的测量来探测ti中电子-声子耦合，它对表面态的影响至关重要。由于ti受到缺陷的困扰，这些缺陷不幸地掩盖了令人兴奋和有趣的表面现象，因此缺陷状态的细节是通过评估高阶跃迁（即临界点）来获得的。对磁掺杂钛进行了探测，以确定其磁性的来源，并研究了时间反转对称性的破缺。最后，利用基于mueller矩阵的椭圆偏振光谱法得到的ti的圆二色性来探测其自旋织构。该项目包括若干活动，以加强科学方面的教育目标。由于这项工作完全由本科学生在文科环境中完成，他们接受材料表征、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准备。此外，本项目在物理课程的现有课程中注入了几个高影响力的实验活动。此外，还为高中生开展了一些外展活动，以培养他们对科学更广泛的兴趣。

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.