We are excited to share our latest preprint. Utilizing our unique cryogenic attosecond beamline, we identified distinct core-level signatures of CDW formation in time-resolved measurements that were undetectable in equilibrium photoemission and absorption measurements in the putative excitonic insulator TiSe2. We also observe excitonic correlations in the normal-state of the material above the transition temperature. These findings provide crucial insights into the mechanism of exciton condensation in this material, highlighting the interplay between short-range excitonic fluctuations and long-range order. Our work underscores the importance of attosecond spectroscopy in exploring both equilibrium phase diagrams and novel nonequilibrium states in strongly correlated materials.

Preprint available here: https://arxiv.org/abs/2407.00772

Our latest research explores into the controversial intermediate liquid phase of carbon, a topic debated for decades. Using time-resolved resonant inelastic X-ray scattering (RIXS) and X-ray emission spectroscopy (XES), we studied amorphous carbon and ultrananocrystalline diamond under laser irradiation. Our findings indicate no evidence of a liquid state, instead revealing crystalline properties even under extreme conditions. Differences in signal behavior are attributed to variations in sample thickness and incomplete melting. This study enhances our understanding of carbon’s behavior under high-energy conditions, with significant implications for material science.

The measurements were done at the PAL free-electron laser. This research was done in a larger collaboration led by Saykally and Schwartz.

Journal publication in the Journal of Physical Chemistry B here: https://pubs.acs.org/doi/full/10.1021/acs.jpcb.4c02862

Photoexcitation by ultrashort laser pulses plays a crucial role in controlling reaction pathways, creating nonequilibrium material properties, and offering a microscopic view of complex dynamics at the molecular level. The photo response following a laser pulse is, in general, non-identical between multiple exposures due to spatiotemporal fluctuations in a material or the stochastic nature of dynamical pathways. However, most ultrafast experiments using a stroboscopic pump-probe scheme struggle to distinguish intrinsic sample fluctuations from extrinsic apparatus noise, often missing seemingly random deviations from the averaged shot-to-shot response. Leveraging the stability and high photon-flux of time-resolved X-ray micro-diffraction at a synchrotron, we developed a method to quantitatively characterize the shot-to-shot variation of the photoinduced dynamics in a solid-state electrolyte. By analyzing temporal evolutions of the lattice parameter of a single grain in a powder ensemble, we found that the sample responses after different shots contain random fluctuations that are, however, not independent. Instead, there is a correlation between the nonequilibrium lattice trajectories following adjacent laser shots with a characteristic “correlation length” of approximately 1,500 shots, which represents an energy barrier of 0.38~eV for switching the photoinduced pathway, a value interestingly commensurate with the activation energy of lithium ion diffusion. Not only does our nonequilibrium noise correlation spectroscopy provide a new strategy for studying fluctuations that are central to phase transitions in both condensed matter and molecular systems, it also paves the way for discovering hidden correlations and novel metastable states buried in oft-presumed random, uncorrelated fluctuating dynamics.

The experimental work was done at the Advanced Photon Source at Argonne National Laboratory. This research is conducted in collaboration with the Cushing group at Caltech.

Arxiv pre-print available here: https://arxiv.org/abs/2406.06832