Alfred Zong [Stanford]

Abstract: Engineering phases of matter with ultrafast light pulses is at the forefront of materials research. Besides enabling access to exotic nonequilibrium states, time-resolved measurements provide a powerful way to study equilibrium properties by teasing out materials responses to external stimuli. In this talk, I will present two recent experiments in this direction on (i) a candidate excitonic insulator and (ii) a van der Waals magnet. Using ultrafast electron scattering, we directly reconstruct femtosecond atomic trajectories with sub-picometer precision. These experiments allowed us to (i) disentangle a putative excitonic instability from a concomitant structural transition, and (ii) establish a clear signature of spin–lattice coupling even in the absence of long-range magnetic order. More broadly, these works showcase how quantitative tracking of ultrafast atomic trajectories offer a mechanistic understanding of a diverse array of electronic and magnetic phenomena, in which interactions with the lattice structure plays a pivotal role but are otherwise difficult to detect under equilibrium conditions.

Research interest (key words): Strongly correlated materials, time-resolved diffraction, microscopy, and spectroscopy, nonequlibrium phase transitions