Cross-Scale Lattice Distortions and Vibration Spectra in Bi 2 Se 3 Revealed by Ultrafast Transmission Electron Microscopy

Abstract Layered topological insulators such as Bi 2 Se 3 exhibit rich phonon dynamics, which are crucial for understanding their thermal transport mechanisms and electronic properties. In this work, we investigate the multiscale nonequilibrium lattice dynamics in Bi 2 Se 3 thin films using ultrafast transmission electron microscopy (UTEM). By exploiting the complementarity between real-space ultrafast electron microscopy (UEM) and ultrafast electron diffraction (UED), we show that the characteristic frequencies of the lattice dynamics are consistent in real and reciprocal space. This cross-validation supports the interpretation that the transient anti-phase oscillation of Friedel diffraction pairs originates from reciprocal-space geometric evolution induced by the breathing mode: under large-angle tilting, reciprocal rods with nonzero out-ofplane Miller indices undergo periodic stretching and displacement along the c * axis, thereby asymmetrically changing their intersections with the Ewald sphere. This process corresponds directly to the coherent motion of lattice bend contours observed in real space. The study further reveals that phonon propagation exhibits pronounced scale dependence. On the picosecond timescale, the dynamics are dominated by a longitudinal standing-wave breathing mode confined by the film thickness, with the oscillation frequency following an inverse dependence on film thickness. Under the [001] zone-axis condition, defect-mediated in-plane traveling waves propagating at the speed of sound are observed. Furthermore, on the nanosecond timescale, the suspended thin film exhibits mechanical resonance with a high quality factor, accompanied by a rich spectrum of higher-order harmonics. These findings provide direct visual evidence for the multilevel dynamical responses of coherent phonons under the same pump excitation, manifesting at different timescales and boundary conditions from the gigahertz (GHz) to megahertz (MHz) regime, highlighting the potential of Bi 2 Se 3 for high-frequency nanomechanical applications.