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.
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.
