Topological insulators are a new class of insulators in which spin-orbit interactions are so strong that the insulating energy gap is inverted, the states that should appear at high energy above the gap actually appear below the gap. A key consequence of this band inversion is the appearance of topological surface states that are robust to the effects of disorder and changes in the chemical potential, making them of strong interest for future electronics applications. While rapid developments on both theoretical and experimental fronts have made topological insulators the subject of intense study, previous work has focused on their electronic properties. This project addresses the next frontiers of understanding topological insulators and their surface states, elucidating the influence of magnetism and superconductivity on their physical characteristics and electronic behavior. The application of magnetic fields is expected to be a powerful method by which the properties of topological surface states could be modulated, and magnetism-induced gapped topological surface states are predicted to give rise to a number of exotic magneto-optical and electron transport effects. Local spectroscopic techniques and the PI's state-of-the-art scanning tunneling microscope (STM) facilities will allow nanometer-scale examination of these effects.
Soon after the discovery of quantum mechanics it was realized why some solids are insulating (like diamond) and others are highly conducting (like graphite), even though they could be comprised of the same element. Now, 80 years later, the concept of insulators and metals is again being fundamentally revised. During the last few years, it has become apparent that there can be a distinct type of insulator, which can occur because of the topology of electronic wavefunctions in materials comprised of heavier elements. Strong interaction between the spin and the orbital angular momentum of electrons in these compounds alters the sequence in energy of their electronic states. The key consequence of this topological characteristic (and the way to distinguish a topological insulator from an ordinary one) is the presence of metallic electrons with helical spin texture at their surfaces. The proposed program will directly visualize these novel quantum states of matter and demonstrate their unusual properties through spectroscopic mapping with the scanning tunneling microscope (STM). The potential of these novel quantum states for electronic application will be examined as part of the proposed program. Undergraduate, graduate, and postdoctoral researchers will be trained as part of the program.
