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

Ultrafast manipulation of phases and phase domains in quantum materials is a key approach to unraveling and harnessing interwoven effects of charge and lattice degrees of freedom. In the intensely-studied charge-density-wave (CDW) material, 1T-TaS₂, static Rayleigh-phonon coupling to periodic lattice distortions (PLDs), as well as incommensurate (IC) domain growth and coarsening over the first 100 ps following femtosecond photoexcitation, suggests ultrafast, displacively-excited coherent acoustic phonons (CAPs) may strongly couple to PLDs. Here we find evidence for such coupling using 4D ultrafast electron microscopy (UEM). For ultrathin room-temperature crystals, photoinduced Bragg-peak dynamics spanning the first 75 ps are characterized by partial CAP coherence and localized low-amplitude c-axis dilations. These relatively weak, partially-coherent dynamics then give way to higher-amplitude, increasingly-coherent oscillations, the transition period of which is well-matched to timescales of photoinduced IC domain growth and stabilization from the nearly-commensurate (NC) phase. Diffraction experiments are correlated with nanoscale UEM imaging, where it is found that phonon wave trains emerge from nanoscale linear defects 100 ps after photoexcitation. The CAPs consist of coupled longitudinal and transverse character and propagate at an anomalously-high 4.6 nm/ps along wave vectors independent from NC-phase PLDs, instead being dictated by static defect orientation. Such behaviors illustrate a potential means to control phases in quantum materials using defect-engineered coherent-phonon seeding.