Stacking Order Regulated Coherent Shear Phonons in Octahedral MoTe Revealed by Ultrafast Electron Microscopy.

Manipulation of the stacking order between layers in two-dimensional layered van der Waals materials provides a fascinating platform for exploring exotic phenomena. Femtosecond laser offers the capability to instantaneously modulate the lattice structure and even stacking order of layered materials, and vice versa, changes in stacking order may affect ultrafast structural dynamics. Here, we use ultrafast electron microscopy (UEM) to investigate the laser-excited lattice dynamics in T' and Td phases of octahedral MoTe. Two crystal plane-dependent acoustic phonon modes in the room temperature T' phase are identified by ultrafast selected-area electron diffraction (SAED) in reciprocal space and confirmed by ultrafast real-space imaging, which are attributed to the breathing mode and shear mode, respectively. Temperature-dependent ultrafast SAED results directly indicate that the acoustic shear mode switches to the optical shear mode when the stacking order is changed by crossing the phase transition temperature. Based on simple model derivations, shear acoustic waves in principle can be excited by the thermal elastic effect in low-symmetric 2D layered materials. However, incorporating numerical calculations and model analysis, we speculate that the laser-induced inverse piezoelectric effect plays a key role in the large-amplitude shear phonons observed in T'-MoTe. Our research demonstrates examples of stacking sequence identification using coherent phonons revealed by UEM, as well as the regulation of coherent shear phonons and topology switching via stacking order.