Probing moire flat bands with optical spectroscopy

Nontechnical description When two similar, semi-transparent, repeating patterns are overlaid with a slight rotation offset, they create large scale wavy structures called moiré patterns. Analogous effects can occur at the atomic level, when two slightly different atomic layers are stacked together, or when two identical layers are twisted at a small angle. These moiré patterns produce cells that are much larger than the actual spacing between the atoms. Electrons in these cells interact with each other more strongly than they would in atomic stacks without this patterning. As charge carriers enter the moiré cells, their interactions produce a multitude of novel electronic phases. Because of this, atomic moiré systems show great potential for developing advanced technologies such as quantum computing. This project will use optical measurements to study these electronic phases. A major benefit of this approach is that it allows a direct examination of bulk properties of these phases. The insights gained in this project will clarify how these electronic phases behave and help discover new states of matter for future quantum technologies. This project will support materials research outreach activities for K–12 students. Rigorous training for college and graduate students in modern materials physics will be provided, including the creation of a summer research experience for undergraduate students. Together, these research and educational efforts will help motivate and train the next generation of scientists and engineers, strengthen the U.S. workforce, and maintain leadership in the field of quantum science. Technical description Moiré superlattices, created by artificially constructed semiconductor atomic junctions, are fascinating platforms that host almost dispersion-less electronic bands, where the electronic kinetic energy is strongly suppressed, giving way to Coulomb interactions that drive rich, doping dependent, strongly correlated phases. This project will use optical spectroscopy to investigate the moiré flatlands and probe evolution of the quantum phases when the displacement field and charge density in the system are continuously tuned. A particular focus will be on the integer and fractional Chern insulators with non-trivial band topology, that exhibit integer and fractionally quantized Hall resistance in the absence of external magnetic field. The principal investigator’s team will optically probe their bulk to address several key questions including: the integer and fractional quantum anomalous Hall gaps, the nature of the bulk state wavefunction, and the similarity/difference of the states when compared with traditional quantum Hall insulators under a strong external magnetic field. Fundamental excitations at distinct momentum regimes will be explored, including the lowest energy roton excitation associated with thermodynamic gap of the correlated phases, the short wavelength mode associated with single particle excitations, and the long wavelength mode mimicking a graviton excitation, that provides insight on the geometric nature of the states. By systematically probing and tuning these topologically nontrivial states, the team will gain knowledge in fundamental physics, and pave way for future quantum device applications such as fault-tolerant quantum computing. 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. NSF Award ID: 2607790 | Program: 01002627DB NSF RESEARCH & RELATED ACTIVIT | Principal Investigator: Jun Yan | Institution: University of Massachusetts Amherst, AMHERST, MA | Award Amount: $444,056 View on NSF Award Search: https://www.nsf.gov/awardsearch/show-award/?AWD_ID=2607790 View on Research.gov: https://www.research.gov/awardapi-service/v1/awards/2607790.html