Ultrafast Dynamics of Bi1.5Sb0.5Te1.8Se1.2 Topological Insulator*

Bi1.5Sb0.5Te1.8Se1.2~(BSTS) is a type of topological insulator, which is an insulator in bulk but surface states are gapless. In this work, we took optical pump-probe data on BSTS crystal to analyze the dynamics of phonons and charge carriers. The ultrafast dynamics were obtained as a function of temperature ranging from 10K to 300K, as well as fluence ranging from 1 $\mu $J/cm2 to 10 $\mu $J/cm2. In additional to the coherent optical phonon mode found in other topological insulators, acoustic phonon mode was observed in our experiment. We also observed phonon softening and the temperature dependence of carrier lifetime in BSTS.