Mode-Selective and Anomalous Decoherence Mechanisms of Interlayer Phonons in a Topological Insulator.

Interlayer phonons in van der Waals (vdW) quantum materials offer promising opportunities for ultrafast control and engineering of material properties at the nanoscale. A fundamental understanding of their dephasing mechanisms is crucial for advanced phononic device applications and for probing buried interfaces. However, the interplay between interlayer phonons and quantum electronic phases remains mostly unexplored. Here, we investigate the mode-dependent dynamics of quantized interlayer breathing modes in a topological insulator using ultrafast coherent phonon spectroscopy. By varying the pump fluence and film thickness, we identify three discrete interlayer breathing modes governed by distinct dephasing mechanisms. The lowest-order interfacial mode undergoes dephasing through acoustic energy dissipation at the interface due to mode-independent acoustic mismatch, and exhibits photoinduced softening in agreement with Lifshitz vdW interaction theory. The lowest-frequency breathing mode shows strong fluence- and thickness-dependent dephasing, attributed to electron-phonon coupling with topological surface state electrons, as evidenced by a superlinear scaling of the dephasing rate. In contrast, the second lowest-frequency breathing mode exhibits a fluence-independent damping but a clear frequency-governed dephasing, indicative of anharmonic three-phonon scattering. These results demonstrate interlayer phonons as sensitive and selective probes of buried interfacial mechanics in vdW systems and provide a comprehensive basis for understanding phonon dynamics shaped by topological and anharmonic effects.