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Coherent Phonon Disruption and Lock-In During a Photoinduced Charge-Density-Wave Phase Transition
preprintsubmitted on 01.04.2021, 23:03 and posted on 05.04.2021, 04:34 by Spencer A. Reisbick, Yichao Zhang, Jialiang Chen, Paige Engen, David Flannigan
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, phonon coupling to periodic lattice distortions (PLDs) and atomically-incoherent picosecond structural phase transitions suggest transitional periods could exist for delayed onset of mode coherence. Here we find evidence for such a connection between displacively-excited coherent acoustic phonons and PLDs using 4D ultrafast electron microscopy. Following femtosecond optical excitation of an ultrathin crystal, a propagating hybridized mode is imaged emerging from linear defects within a 1-μm region. Partial coherence and low amplitudes during onset of the incommensurate phase convert to higher-amplitude, increasingly-coherent oscillations as phase-growth stabilizes. The hybrid mode consists of large out-of-plane distortions coupled to basal-plane bond oscillations propagating at anomalously high velocities. The strongly-correlated behaviors observed here represent a potential means to control phase behaviors in quantum materials using defect-engineered coherent-phonon seeding.