Imaging Coherent Acoustic Phonons in LaFeAsO with Ultrafast Electron Microscopy

Several iron pnictide materials, such as LaFeAsO and BaFe2As2, display high-temperature superconductivity when lightly doped. Such compounds often undergo temperature-dependent structural, magnetic, and electronic transitions, which are generally thought to be linked in some way to the superconducting phase. In order to resolve the interdependencies of the various degrees of freedom, ultrafast spectroscopic methods have frequently been used to determine timescales associated with charge-carrier and switching dynamics in the superconducting materials, and the more-recent advent of ultrafast structural probes (femtosecond X-ray and electron scattering) has led to insights into both the cuprates and the pnictides (both the doped and parent compounds). As with dedicated reciprocal-space methods, ultrafast transmission electron microscopy (UEM) provides a method for directly probing impulsively-excited structural dynamics, with the added benefit of real-space imaging with combined nanometer and sub-picosecond spatiotemporal resolutions. In this work we report the direct imaging of coherent acoustic-phonon dynamics in the 1111-type iron-pnictide parent compound LaFeAsO via UEM.