Visualizing Novel Electronic Orders in Bilayer Graphene Systems

Non-technical Abstract: Quantum materials play a key role in establishing the future of technology. The discovery of and research advances based on single-atom-thick materials, such as graphene, and their stacks is creating a new arena for exploration of electronic phenomena in which interaction between electrons can be controlled and analyzed with modern techniques such as the scanning tunneling microscopy (STM). This research project is focused on visualization of quantum phenomena in materials with density of electrons controlled by application of a magnetic field or twisting of layers relative to each other. By bringing the power of imaging electronic states to study these exciting materials, this project reveals the microscopic mechanism by which these quantum states are formed and their properties. The graduate and undergraduate students trained in this project will constitute the future workforce for quantum science and technology in academia and industry. This research is supplemented with an educational agenda, the continued improvement of a Princeton undergraduate course “Physics for Future Leaders”, that introduces physics (from basics to quantum phenomena) to non-science majors who are interested in pursuing leadership roles in government and policy arenas. Technical Abstract: This project is focused on electronic phases in novel two dimensional materials in which electronic correlation is induced by electrons occupying a flat band. The research team explores a wide range of quantum phenomena for electrons in flat bands in unique ways using atomic scale imaging and spectroscopy with the scanning tunneling microscope (STM). The information obtained from these experiments is nearly impossible to obtain from other types of macroscopically averaged methods and hence will add an important new perspective to the study of interacting phases of electrons. New quantum phases with complex broken symmetry and ordering are being created in ultra clean devices and the complex correlations underlying their formation is being explored. Exotic electronic phases with fractionalized charges that may host non-abelian anyons are the focus of investigation. In particular, the project supports examination of fractional quantum Hall phases (FQH) in bilayer graphene, examining ways in which such states can be probed locally with the STM. This system also provides a unique setting to visualize field-induced Wigner crystals, bubble and stripe phases of electrons and their competition with FQH phases. Another flat band system being explored is that of magic-angle twisted bilayer graphene in which STM measurements are used to understand the nature of broken symmetry states that give rise to correlated and topological insulators found in this system. Local STM imaging and its analysis are being used to characterize broken symmetries and to study the influence of strain, twist angle, and alignment with subtracted boron-nitride. Several new experimental techniques based on scanning tunneling microscopy is also being developed in this program. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.