Visualizing quantum Hall ferromagnets, their 1D topological edge modes and their interplay with superconductivity

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Non-technical AbstractResearch on topological phases has connection to both concepts in quantum field theory and proposed approaches for creation of fault tolerant quantum computers. The idea that low energy, emergent quasiparticles in condensed matter systems can provide a laboratory to test field theory concepts follows the long tradition of generalizing concepts in physics across energy and length scales. Equally important to the study of topological materials, topological phases and associated emergent quasiparticles may well provide fundamental new approaches to electronics. For example, valley degree of freedom for electronic wavefunction in solids, a property examined here, is a feature that has been proposed to create new types of electronic devices. The graduate and undergraduate students trained on this project are learning state-of-the-art scanning probe microscopy, cryogenics, and materials physics experimental techniques that are of strong interest to both industry and academia. Lab research is being moved into the classroom with the development of a Princeton freshman seminar course introducing students to some basics of quantum mechanics, quantum computing, and exposing them to 'tabletop' discoveries in quantum condensed matter physics such as superconductivity, superfluidity, laser cooling and Bose condensation. The course is designed to introduce quantum phenomena to those with minimal training, with only high school courses in science and math. This approach will allow students from various academic backgrounds and interests to be exposed to the excitement of condensed matter physics and quantum phenomena early in their career.Technical AbstractThis project is focused on the study of electronic phases in which interaction between electrons and topology of their wavefunctions must be treated on equal footing. These are quantum Hall ferromagnetic phases in which electrons' valley degree of freedom makes the topological quantum Hall states also display broken symmetry driven by the exchange interaction between electrons. To accomplish its goals, the program brings the power of high energy and spatial resolution spectroscopic mapping with a scanning tunneling microscope (STM) to not only visualize valley quantum Hall ferromagnets in real space, but also to probe their topological boundary modes and to tune their properties by changing their carrier density, or using strain, and proximity with superconductors and magnets. A new class of interacting 1D Luttinger liquids that form at the topological boundary between different quantum Hall valley phases is examined during this program. In addition, the program explores the physics of interactions at low carrier concentrations to drive stripe, bubble, or fractional quantum Hall phases in an electronic system (Bi and other surface states) accessible to STM, and visualize these phases for the first time. The interplay between quantum Hall valley-polarized phases with superconductivity in special hybrid structures are also studied. These efforts will explore ways in which Majoranas' "lattices" and topological superconductivity may emerge when a vortex lattice from a conventional superconductor is coupled with a two-dimensional (2D) electron gas with strong spin-orbit coupling in the presence of a quantized Landau level (LL). A number of different materials will be explored to extend our ability to visualize Landau wavefunction to other 2D systems. The program will bring together a wide range of experiments, novel creation of materials and thin film structures to realize topological electronic phases and associated novel phenomena that can be directly studied with high spatial and energy resolution using the STM. The research is made possible by high-resolution STM instrumentation that have been designed and constructed by the principal investigator and his research team.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.