Influence of atomic-scale defects on coherent phonon excitations by THz near fields in an STM

Abstract: Coherent phonons describe the collective, ultrafast motion of atoms and play a central role in light-induced structural dynamics. Here, we employ terahertz scanning tunneling microscopy (THz-STM) to excite and detect coherent phonons in semiconducting 2H-MoTe$_{2}$ and resolve how their excitation is influenced by atomic-scale defects. In a THz pump-probe scheme, we observe long-lived oscillatory signals that we assign to out-of-plane breathing and in-plane shear modes, which are both forbidden in the bulk. Remarkably, the relative excitation strength of these modes varies near defects, indicating that local band bending modulates the coupling to the THz field. This defect-tunable coupling offers new opportunities to control material properties via selective excitation of vibrational modes at the nanoscale.