Nanoscale Imaging of Unusual Photoacoustic Waves in Thin Flake VTe₂

Abstract

The control of acoustic phonons, which are the carriers of sound and heat, has become the focus of increasing attention because of a demand for manipulating the sonic and thermal properties of nanometric devices. In particular, the photoacoustic effect using ultrafast optical pulses has a promising potential for the optical manipulation of phonons in the picosecond time regime. So far, its mechanism has been mostly based on the commonplace thermoelastic expansion in isotropic media, which has limited applicability. In this study, we investigate a conceptually new mechanism of the photoacoustic effect involving a structural instability that utilizes a transition-metal dichalcogenide VTe2 with a ribbon-type charge-density-wave (CDW). Ultrafast electron microscope imaging and diffraction measurements reveal the generation and propagation of unusual acoustic waves in a nanometric thin plate associated with optically induced instantaneous CDW dissolution. Our results highlight the capability of photoinduced structural instabilities as a source of coherent acoustic waves.

Coherent Phonon Disruption and Lock-In During a Photoinduced Charge-Density-Wave Phase Transition

“…Interestingly, observation of CAP d-spacing oscillations (Fig. 3c) is uncommon despite PLDs having large-amplitude LA-mode character consisting of significant basal-plane atomic modulations (8,40,(61)(62)(63)(64)(65). As seen in Figure 3c, both Bragg-scattering parameters display anomalous behavior for t < ~75 ps.…”

“…For example, Nakamura et al found good agreement between calculated and UEM-measured plate modes in VTe2, but they did observe an unexpected symmetry they ascribed to transient disruption of the CDW ordering (i.e., a photoinduced melting or "dissolution" of the CDW phase) (63). It is interesting to note that abrupt structural changes at the C-to-NC critical temperature result in acoustic emission, much like what occurs during diffusionless phase transitions and magnetoacoustic responses (76).…”

Coherent Phonon Disruption and Lock-In During a Photoinduced Charge-Density-Wave Phase Transition

“…Importantly, contrast dynamics resulting from CAP excitations are easily distinguished from large-scale specimen bending and bend-contour motion. [40][41][42][43][51][52][53][54][55] Further, correlating CAP contrast dynamics with strong electron scattering by defects enables nanometer-femtosecond resolved imaging of nucleation points at atomic-scale discontinuities. 42,43 Note that Bragg spots were monitored here in order to directly relate real-space and reciprocallattice CAP dynamics; with respect to constitutive relations at critical points, the much weaker CDW satellite spots display the same behaviors of interest.…”

“…For example, Nakamura et al found good agreement between calculated and UEM-measured plate modes in VTe2, though they did observe an unexpected symmetry they ascribed to transient disruption of the CDW ordering (i.e., a photoinduced melting or "dissolution" of the CDW phase). 55 It is interesting to note that abrupt structural changes at the C-to-NC critical temperature result in acoustic emission, much like what occurs during diffusionless phase transitions and magnetoacoustic responses. 78 Were similar emission to occur at the NC-to-IC transition, one wonders if the associated pulse might also interfere with CAP formation and launch.…”

“…3c) is uncommon despite PLDs having large-amplitude LA-mode character consisting of significant basal-plane atomic modulations. 8,40,55,[63][64][65][66] Despite this, subpicometer d-spacing changes are indeed generally observable with UEM. 40,67,68 Most interestingly, as seen in Figure 3c, both Bragg-scattering parameters display anomalous behaviors for t < ~75 ps.…”