Ultrafast electron imaging and diffraction of non-equilibrium structural dynamics in the charge-density wave system 1T-TaS₂

Time-resolved electron imaging, diffraction and spectroscopy are exceptional laboratory-based tools to trace non-equilibrium phenomena in materials with a sensitivity to structural, electronic and electromagnetic degrees of freedom. The capabilities of these approaches are largely governed by the quality of the beam of electrons used.
This talk will discuss recent advances made by employing high-coherence ultrashort electron pulses from nanoscale field emitters, which substantially enhance the achievable image resolution in both real and reciprocal space. We have recently developed two complementary experimental techniques with ultimate surface sensitivity and spatial resolution, respectively, namely Ultrafast Low-Energy Electron Diffraction (ULEED) [1] and Ultrafast Transmission Electron Microscopy (UTEM) [2].
These methods are aplied to study non-equilibrium dynamics in the prominent charge-density wave (CDW) material 1T-TaS₂ after femtosecond optical excitation. Specifically, we study structural phase transitions between different CDW phases and the optical excitation of fluctuation modes. In one set of experiments, ULEED is used to investigate the transient population of phonons and phase modes (phasons). For the transition between the domain-like nearly-commensurate (NC) CDW phase and the incommensurate (IC) phase, we identify the importance of dislocation-type topological defects in the periodic lattice distortion for establishing long-range phase-order. Finally, a new contrast mechanism for UTEM is developed that allows for ultrafast real-space domain imaging upon the structural phase transition.

[1] S. Vogelgesang et al., Nature Physics 14, 184–190 (2018).
[2] A. Feist et al., Ultramicroscopy 176, 63-73 (2017).