Optical study on topological superconductor candidate Sr-doped Bi₂Se₃

Abstract Utilizing infrared spectroscopy, we studied the charge dynamics of the topological superconductor candidate Sr$_x$Bi$_2$Se$_3$. 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$_x$Bi$_2$Se$_3$. As the carrier concentration $n$ obtained by Hall resistivity increases slightly with the decrease of temperature, the effective mass is proved to increase as well, which is in contrast with that of Cu$_x$Bi$_2$Se$_3$. We also performed ultrafast pump-probe study on the Sr$_{0.2}$Bi$_2$Se$_3$ compounds. Resembling its parent compound Bi$_2$Se$_3$, 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.