Influence of local symmetry on lattice dynamics of topological surface states

We investigate coupled electron-lattice dynamics in the topological insulator Bi₂Te₃ with time- and angle-resolved photoemission spectroscopy (trARPES). It is well established that coherent phonons can be launched by optical excitation, but selection rules typically restrict these modes to specific wavevectors (q~0) and symmetries (Raman-active).  We find that the topological surface state in Bi₂Te₃ couples to coherent modes that are forbidden in the bulk, including infrared active phonons as well as those with non-zero wavevectors. Our calculations show that these behaviors naturally arise as a consequence of the translational and inversion symmetries broken at the surface. These effects are important to consider when interpreting ultrafast experiments, and may expand the phase space for tailoring material properties on-demand.