Materials Science Research Lecture
***Refreshments at 3:45pm in Noyes lobby
Abstract:
Chirality, the property of handedness, is a fundamental concept in physics, chemistry, and biology, yet its manifestation in the collective excitations of solids remains poorly understood and experimentally underexplored. This talk presents efforts to understand chirality and topology of bosonic excitations, magnons and phonons in particular, across a range of quantum materials, with momentum-resolved inelastic scattering employed as the primary probe.
Altermagnetism is a recently identified magnetic phase in which spin sublattices are related by crystal rotation rather than translation, producing spin-split bands without spin-orbit coupling or net magnetization. Using inelastic neutron scattering on single-crystal α-Fe₂O₃, the first direct observation of chiral magnon band splitting consistent with g-wave altermagnetic symmetry is reported, with a 3 meV splitting at ~100 meV magnon energy. The alternating handedness of magnon branches across nodal surfaces in the Brillouin zone is confirmed by neutron chiral factor analysis. Chiral phonons are also probed using momentum-resolved IXS across the full Brillouin zone. In elemental tellurium, whose helical trigonal structure provides a natural platform for phonon angular momentum, pronounced circular polarization mixing of the longitudinal acoustic branch approaching the zone boundary is revealed — the first momentum-resolved detection of chiral acoustic phonons. A circular-polarization IXS capability is further developed using a diamond X-ray phase retarder and applied to non-centrosymmetric materials; anomalous scattering intensities near chiral phonon modes are observed, and a discrepancy with standard IXS theory is traced to phase information carried by complex phonon eigenvectors.
These findings open pathways toward chirality-enabled phononic and magnonic technologies. The symmetry-based framework developed here is expected to inform the design of quantum materials with engineered Berry curvature topology, with longer-term implications for topological transport and chiral-excitation-based quantum information platforms.
More about the Speaker:
Chen Li is an Associate Professor in the Department of Mechanical Engineering and Materials Science and Engineering Program, and a Cooperating Faculty in the Department of Physics and Astronomy, at the University of California, Riverside, where he has been a faculty member since 2016. His research focuses on quantum materials, high pressure, and thermal transport using both experimental and computational tools. Prior to joining UCR, he was a research scientist at the Carnegie Institution of Washington. He received his B.Sc. in Physics from Peking University and his Ph.D. in Materials Science from the California Institute of Technology, followed by postdoctoral appointment at the Materials Science and Technology Division at Oak Ridge National Laboratory. Dr. Li is a recipient of the NSF CAREER Award.