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

Kenyon College, Gambier OH

Investigators

Abstract

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.