Haidan Wen: Emergent Ultrafast Structural Dynamics in Quantum MaterialsArgonne National Laboratory
New properties emerge when material systems “scale up” via uniquely connected individual element, or “scale down” by reducing dimensionality and resulting in symmetry breaking. In this talk, I will show recent efforts to use light pulses at terahertz and x-ray frequencies to stimulate and track emergent dynamical properties of quantum materials on ultrafast time scales. In the “scale-up” example, we observed a new set of collective excitations in polar vortices, named vortexons. A unique soft mode is identified as a pair of oscillating vortex cores that can be significantly tuned by strain around room temperature. The discovery of tunable vortexons opens a new avenue for high-frequency dielectrics and optoelectronics applications. In the “scale-down” example, I will show the distinct structural dynamics of monolayer crystals WSe2 from their bulk counterparts. We found the absorbed optical photon energy is preferably coupled to the in-plane lattice vibrations within one picosecond whereas the out-of-plane lattice vibration amplitude remains unchanged during the first ten picoseconds, marking the distinct structural dynamics of monolayer crystals from their bulk counterparts. Looking into the future, the recent progress of developing multimodal, multiscale x-ray imaging platform will be discussed to go beyond the ensemble average for studying nanoscale ultrafast dynamics.