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

非技术摘要：人们对一种名为拓扑绝缘体(TIS)的新型材料很感兴趣，这种材料似乎具有两种不同的性质，即导电和绝缘相，交织在一种材料中。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，但由于它们独特的性质，它也可以被利用来制造有趣的设备。该项目的主要重点是研究TiS的独特性质，密切关注揭示其表面状态和体态之间的相互作用。具体地说，主要目标之一是建立一个基本的理解，即如何区分表面态和体态对材料整体导电性的贡献。此外，对磁性掺杂的TI样品进行了分析，以探讨磁性与各种性能之间的相互作用。该项目主要使用一种称为光谱椭偏测量法的光学研究技术来确定TI样品的表面和体态的贡献。此外，还进行了与温度有关的实验，以揭示支配TIS物理的复杂细节。这项工作只由文科背景下的本科生完成。这些学生接受材料表征、光学和低温方面的培训，为研究生学习或从事科学技术工作做好准备。为了促进教育目标，该项目在物理课程的现有课程中纳入了几个影响较大的实验活动。此外，还为高中生开展了几项外展活动，以培养对科学的更广泛兴趣。技术摘要：由于强烈的自旋-轨道耦合和时间反转不变对称性，一类新的材料，称为拓扑绝缘体(TIS)，具有独特的特性：它在体中具有能隙，但具有抗无序散射的金属表面态。尽管在过去的几年里，人们在理解TIS的物理方面做出了一致的努力，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TIS的导电性，b)杂质带对TIS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意义。在这些方面获得的任何见解都将使我们能够更深入地理解TIS的基本物理，这是该项目的最终目标。椭圆偏振光谱用于在很宽的光谱范围内(即30 meV到6.2 eV之间)测定复杂的电导率，这使得人们能够破译自由载流子和带电子对电导率的贡献。用随温度变化的测量方法研究了TIS中的电子-声子耦合，它对表面态的影响起着至关重要的作用。由于TI受到缺陷的困扰，不幸的是，缺陷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)来获得缺陷态的细节。对磁性掺杂的TiS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，通过基于Mueller矩阵的椭圆偏振光谱测量得到TiS的圆二向色性，探讨了TiS的自旋织构。该项目包括几项提高科学教育目标的活动。由于这项工作完全由文科背景的本科生完成，他们将接受材料表征、光学和低温方面的培训，为他们在STEM领域的研究生学习或职业生涯做准备。此外，该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，还为高中生开展了几项外展活动，以培养更广泛的对科学的兴趣。

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.