Illustrations of the Td-WTe₂ crystal structure

Case study: The study of the shear phonon mode in the 2D material

Here we investigate the structural response in Td-WTe₂ following ultrafast photoexcitation by time-resolved electron diffraction. Excitation of a coherent interlayer shear phonon at 0.24 THz frequency has been observed by intensive THz or IR laser pump and has been studied using time-resolved reflection and time- and angle-resolved photoemission spectroscopy (tr-ARPES). It is interesting to study such lattice dynamics with UEM technique, which combines high temporal and spatial resolution.

The thin flake of the Td-WTe₂ was prepared by the microtome (Leica Ultracut S) and placed on the Graphene supported copper TEM grid. JEOL JEM-2100 LaB₆ TEM (operated at 200kV) equipped with a fast Timepix hybrid pixel detector (512 x 512 pixels, Amsterdam Scientific Instruments) was used for UEM data collection. A double-tilt holder was used for the data collection under room temperature.

A 0.23 THz shear phonon, involving layer displacement along the b axis, was excited by a 515 nm laser pulse. Pump fluences in excess of a threshold of ∼1 mJ/cm² result in formation, with an ∼5 ps time constant, of a new stacking order by layer displacement along the b axis in the direction toward the centrosymmetric 1T* phase. The shear displacement of the layers increases with pump fluence until saturation at ∼8 pm. We demonstrate that the excitation of the shear phonon and the stabilization of the metastable phase are decoupled when using an optical pump as evidenced by observation of a transition also in samples with a pinned shear phonon.