Upcoming Lectures
November 22, 2024
11:00 a.m. - 12:00 p.m.
W-203 Millennium Science Complex
University Park, PA
Probing Phonon and Magnon Dynamics with Fast Electrons
Dr. Yimei Zhu
Senior Physicist and Group Lead
Brookhaven National Laboratory
Abstract:
Recent advancements in meV EELS have sparked momentum in using spectroscopy to measure phonons and even magnons. In this presentation, Dr. Yimei Zhu will demonstrate how ultrafast electron diffraction can be used to probe photoinduced phonon dynamics through quantitative diffuse scattering analysis, tracking the evolution of both optical and acoustic phonon populations in a 2-dimensional charge-density-wave (CDW) system, TiSe₂. It is revealed that 1-dimensional CDW-related defects emerge within 1 ps after photoexcitation. This defect growth is not governed by the amplitude of the order parameter but rather mediated by a non-thermal population of longitudinal optical phonons.
Additionally, Dr. Yimei Zhu will introduce a novel electron-pulse-based imaging method to directly visualize magnon dynamics. By integrating microwave into TEM, topological spin textures is created and stabilized in permalloy, featuring vortex-antivortex pairs and various types of domain walls within a magnetic single-layer geometry. The generation, propagation, reflection, and interference of spin waves are visualized under RF-field excitation, demonstrating that spin waves are preferentially generated at topological anti-vortices and the spin wave emission is associated with the oscillatory motion of specific domain walls. The study opens new avenues for spintronic and multiferroic applications, including spin transfer torque oscillators through selective coupling and reconfigurable spin wave interactions. These insights into phonon and magnon dynamics are beyond the reach of static EELS methods.
Short Bio:
Yimei Zhu is a Senior Physicist and Group Leader at Brookhaven National Laboratory (BNL). He received his PhD from Nagoya University and joined BNL as Assistant Scientist in 1988, advancing to Tenured Senior Scientist in 2002.
His research interests include developing electron microscopy instrumentation and methodology to study charge, spin, orbital and lattice interactions in quantum materials. Zhu has authored over 700 peer-reviewed journal articles and delivered more than 300 invited talks at international conferences. He is the recipient of numerous honors, including the Distinguished Scientist Award from MSA, the Peter Duncumb Award from MAS, and the Vernon Cosslett Medal from IFSM. Zhu is a Fellow of APS, MSA, AAAS, MRS and MAS.