Surface optical and bulk acoustic phonons in the topological insulator, Bi2Se2Te

We explore the phonon dynamics in thin films of the topological insulator material Bi2Se2Te using ultrafast pump-probe spectroscopy. The time resolved differential reflectivity in these films exhibit fast and slow oscillations. We have given a careful analysis of variation of phonon frequency as a function of film thickness, which we attribute to the existence of standing acoustic modes. However, no variation in the frequency of the optical phonon modes was found with film thickness. This indicates that the optical phonons intrinsically belong to the surface of topological insulators. The controllability of acoustic phonons by way of varying the film thickness will have tremendous implications in the application of these materials in low power spintronic device operating at room temperature.