Slight lattice changes caused by acoustic propagation can be directly imaged

Ultrafast electron microscopy (UEM) is a valuable technique for studying acoustic-phonon vibrational modes in materials, and a more comprehensive model of electron scattering can help improve its abilities. Motivated by this, Daniel Du and David Flannigan simulated bright-field image contrast arising from lattice distortions caused by these vibrations.

Du and Flannigan used a kinematical approximation, in which electrons encountering the material are restricted to a single scattering event, and found the approach to be insufficient for quantitatively describing the lattice deformations. They then incorporated dynamical effects into the model by removing this limitation and allowing for multiple scattering events, which improved its performance by nearly an order of magnitude and allowed for the observation of strain states on the order of 1%.

They discovered a direct comparison of the two approaches allows for the isolation of the contrast in the UEM image that arises solely from the lattice deformation. They did so by conducting simulations of a distortion wave propagating within an initially undistorted structure. They studied the scattering intensity at different positions of the structure and different times in the simulation to determine the average strain encountered by a propagating electron and the resulting contrast strength in the UEM images.

“The exciting result is that this supports the hypothesis that the very-slight changes in lattice parameters associated with coherent acoustic phonons indeed can give rise to measurable UEM image contrast, thus paving the way to quantification of the associated properties directly from nanometer-picosecond UEM imaging,” Flannigan said.

Next, the team plans to increase the complexity of their model in order to apply it to understand properties such as energy, strain and bond-length modulation associated with vibrations imaged with UEM.

Source: “Imaging phonon dynamics with ultrafast electron microscopy: Kinematical and dynamical simulations,” by Daniel X. Du, and David J. Flannigan, Structural Dynamics (2020). The article can be accessed at https://doi.org/10.1063/1.5144682 .