Low-Energy Spectroscopy of Moire Quantum Matter

Nontechnical description: This research project will develop a new way to study unusual behaviors in advanced materials, called moiré materials, made from stacking ultra-thin layers. These moiré materials can create unique "quantum phases," which may lead to future breakthroughs like energy-efficient electronics or powerful quantum computers. However, scientists have not had the right tools to closely examine these tiny layered devices. The research team plans to solve this problem by developing a new kind of light-based tool using terahertz radiation —in between microwave and infrared light. This method, based on a recent breakthrough by the team in generating terahertz radiation, can be used right on the chip and can detect both the energy and structure of these quantum phases. Besides advancing science, the project also includes outreach programs to inspire students of a broad range of backgrounds in New York City to explore careers in science and technology through hands-on research and educational activities. Technical description: The formation of two-dimensional moiré interfaces has created unprecedented opportunities in the exploration of quantum phases of matter, but there is a lack of spectroscopic tools in probing moiré quantum matter, due to a mismatch in sample sizes of moiré devices (~1-10 µm) and the wavelengths of electromagnetic radiation (0.1-3 mm) relevant to the low-energy excitations in these quantum phases. The proposed research aims to fill the critical gap in current research. The research team will develop in-situ terahertz spectroscopy to directly access low energy excitations in moiré quantum matter. This approach is based on the discovery by the team of intense and broadband terahertz generation from the van der Waals ferroelectric semiconductor niobium oxydiiodide with record-setting efficiency. This terahertz emitter can be integrated into van der Waals heterostructures for on-chip near-field terahertz spectroscopy of a target moiré material/device. In addition to the low energy scales characterized by the correlation gaps, this experiment will also quantify the degree of correlation and topologies. The former will be deduced from terahertz photo-conductivity, while the latter will be determined from the connection of band topologies to terahertz optical selection rules or Kerr/Faraday rotation. The research team will i) probe the correlation gaps; ii) measure terahertz photo-conductivity of correlated charge carriers; iii) determine topologies from selection rules of resonant excitation with circularly polarized terahertz field, and iv) determine the dynamics of electronic excitations with optical pump and terahertz probe. Such spectroscopic insights will complement prevalent transport measurements and motivate/guide theoretical efforts in searching and understanding of quantum phases in the broadly tunable moiré material systems. 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.