Coherent Helix Vacancy Phonon and its Ultrafast Dynamics Waning in Topological Dirac Semimetal Cd3As2

We report an ultrafast lattice dynamics investigation of the topological Dirac semimetal Cd3As2. A coherent phonon beating among three evenly spaced A1g optical phonon modes (of frequencies 1.80, 1.96, and 2.11 THz, respectively) is unambiguously observed. The two side modes originate from the counter helixes composing Cd vacancies. Significantly, such helix vacancy-induced phonon (HVP) modes experience prominent extra waning in their ultrafast dynamics as temperature increases, which is immune to the central mode. Above 200 K, the HVP becomes inactive, which may potentially affect the topological properties. Our results in the lattice degree of freedom suggest the indispensable role of temperature in considering topological properties of such quantum materials. © 2017 American Physical Society.