Abstract:
Our world is 3D, yet electrons in van der Waals materials can behave as if they live in 2D sheets, or even along quasi-1D chains. How does this reduced electronic dimensionality shape the motion of photoexcited charge, and does it survive when a material is driven far from equilibrium? This nonequilibrium regime is central to both light-driven material function and the emergence of transient states that have no equilibrium counterpart. Answering these questions requires a probe that combines femtosecond time resolution with sensitivity to individual atomic sites. In this talk, I will introduce our development of table-top ultrafast extreme-ultraviolet (XUV) spectroscopy for single-crystalline van der Waals materials. Broadband XUV pulses access multiple elemental core-level transitions simultaneously, allowing us to distinguish the responses of electronically active metal sites from those of surrounding ligands. I will present our latest measurements of charge redistribution, screening, and coherent lattice motion in van der Waals materials, and discuss how these results open a site-resolved view of the 2D electronic world far from equilibrium.

Bio:
Yuki Kobayashi is an Assistant Professor of Chemistry and a fellow of the Quantum Research Institute at the University of Michigan. He received his Ph.D. from UC Berkeley and was a postdoctoral scholar at Stanford University. Since starting his laboratory at Michigan in 2023, his group has developed ultrafast XUV and strong-field spectroscopies to study electronic and structural dynamics in quantum materials.