Optical study on topological superconductor candidate Sr-doped Bi₂Se₃

Utilizing infrared spectroscopy, we study the charge dynamics of the topological superconductor candidate Sr_xBi_2Se_3. The frequency-dependent reflectivity R(\omega) demonstrates metallic feature and the scattering rate of the free carriers decreases with temperature decreasing. The plasma edge shows a slight blue shift upon cooling, similar to the behavior of Cu_xBi_2Se_3. As the carrier concentration n obtained by Hall resistivity increases slightly with the decreasing temperature, the effective mass is proved to increase as well, which is in contrast with that of Cu_xBi_2Se_3.We also perform the ultrafast pump-probe study on the Sr_0.2Bi_2Se_3 compounds. Resembling its parent compound Bi_2Se_3, three distinct relaxation processes are found to contribute to the transient reflectivity. However, the deduced relaxation times are quite different. In addition, the electron-optical-phonon coupling constant is identified to be \lambda = 0.88.