Xiaojian Bai

(presenting on September 10, 2025)

Beyond Magnons: Unconventional Excitations in Large-Spin Quantum Magnets

Abstract

Just as a pebble creates ripples across the surface of a pond, disturbances in a magnet generate ripples of spin alignment—elementary excitations known as spin waves, or magnons. For decades, magnons have provided the textbook description of collective dynamics in magnets. Yet recent experiments show that this picture is incomplete: in certain large-spin quantum magnets, strong anisotropy and correlations give rise to entirely new types of excitations. To explore these hidden dynamics, we turn to neutron scattering, a powerful probe that can directly measure how spins fluctuate and interact over space and time. Using this technique, along with theoretical modeling, we uncover hybrid modes involving higher-order spin fluctuations and bound states formed from multiple magnons in triangular-lattice magnets such as FeI₂. These discoveries demonstrate how even well-studied materials can harbor surprising forms of collective dynamics, establishing large-spin magnets as a fertile platform for exploring emergent phenomena in quantum materials.

Speaker Bio

Xiaojian Bai received his Ph.D. in Physics from the Georgia Institute of Technology in 2019, followed by a postdoctoral fellowship at Oak Ridge National Laboratory. In 2022, he joined the Department of Physics and Astronomy at Louisiana State University as an assistant professor. His research centers on understanding the fundamental properties of quantum materials through combined experimental and theoretical approaches, with particular emphasis on using neutron scattering to probe correlated phenomena and emergent phases of matter.