Stacking Order Regulated Coherent Shear Phonons in Octahedral MoTe₂ Revealed by Ultrafast Electron Microscopy

Area of Science:

• Condensed Matter Physics
• Materials Science
• Ultrafast Science

Background:

• Two-dimensional layered van der Waals materials offer unique properties tunable by stacking order.
• Femtosecond lasers can modify lattice structures and stacking sequences in these materials.

Purpose of the Study:

• Investigate laser-induced lattice dynamics in T' and Td phases of octahedral MoTe2.
• Explore the relationship between stacking order and ultrafast structural dynamics.

Main Methods:

• Ultrafast electron microscopy (UEM) for real-time imaging.
• Ultrafast selected-area electron diffraction (SAED) for reciprocal space analysis.
• Temperature-dependent studies across phase transitions.

Main Results:

• Identified two crystal plane-dependent acoustic phonon modes (breathing and shear) in T'-MoTe2.
• Observed a switch from acoustic to optical shear mode upon changing stacking order via temperature.
• Proposed the inverse piezoelectric effect as a key mechanism for large-amplitude shear phonons.

Conclusions:

• Demonstrated stacking sequence identification using coherent phonons via UEM.
• Showcased the regulation of coherent shear phonons and topology switching through stacking order manipulation.