Quantum Material Research through Femtosecond Electron Crystallography, Spectroscopy and Microscopy

The major potential advantage in using electron scattering data, compared to X-ray, is that electrons can be focused using an electromagnetic lens; thus, both images and diffraction patterns can be utilized for structural analysis under the same platform. Importantly, the different modalities are controlled by the post-specimen optics and aperture placement with additional sets of lenses. The lens combination offers great flexibility in creating either image, diffraction, or even spectroscopic signals from a large field of view or data for the microanalysis of small regions. Injecting ultrafast time resolution into the typical TEM modalities will not only allow ultrafast imaging and diffraction patterns to be obtained for studying material processes, but also the delicate control of timing between the pump and probe pulses can create a new contrast mechanism at various stages of the material’s responses. Especially, the recent successful implementation of high precision control of electron probe dynamics enables the technologies to perform in the 10s femtosecond regime, opening new possibility of studying control-feedback mechanism.<br/><br/>We will discuss the uses of dynamical contrast to enhance the imaging and crystallography capabilities for the understanding of the evolutionary processes set forth from different types of laser pulse initiations in quantum materials. The evolutionary processes are probed through combined approach of ultrafast spectroscopy, diffraction, and microscopy that are conducted in situ to piece together information. Using vanadium dioxide nanocrystals and transition-metal dichalcogenide (TMDC) thin layers as examples, we demonstrate how the multi-modality ultrafast electron platform may provide useful new information regarding quantum material state evolution at different spatial and temporal scales. Through joint structure and spectroscopy probes we identify the common presence of nanoscopic transition intermediate as the cornerstone for the system’s successful descend into the eventual state and adopting its functionality.<br/><br/>In the studies of TMDC (TaS₂, TaSe₂) first focusing on the dynamics on the ps and sub-ps timescale with time-domain contrasts over the optical signals and selected Bragg peaks, we observe that under different pump conditions different types of phonon modes in the hosting lattice of the charge-density-wave materials may be activated. Upon entering the phase transition regime, these phonon modes are hybridized with the emerging density-wave state observed through the dynamical crystallographic patterns. The transition becomes stepwise with the formation of nanoscopic domains that lock the emergent order with the host lattice in various formation. Remarkably, upon varying the pump power, the timescale for the multi-stage evolutions can be tuned by more than one order of magnitude. In the case of VO₂, multi-critical behavior also emerges but here we show the thermal and non-thermal controls manifest in different types of behavior. For the nonthermal control, in which, the initial step of ultrafast spectroscopic insulator-to-metal transition is observed over sub-100 fs timescale where the emergence of a nanoscopic excitonic polaron domain is simultaneously captured by the diffraction signals. Such evolution is different from the insulator-metal transition obtained under the thermal regime of control where no such polaronic domain is identified. These results indicate that the functional controls in correlated electron crystals may be more nuanced due to their sensitivity to more than one type of control parameter and our multi-perspective results may shed insight to address some highly debated open questions regarding these materials.<br/><br/>The work was funded by the U.S. Department of Energy, Grant DE-FG0206ER46309. The experimental facility was supported by U.S. National Science Foundation, Grant DMR 1625181.