Ultrafast carrier and phonon dynamics in Bi2Se3 under high pressure

The nonequilibrium carrier and phonon dynamics in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ was studied by a combination of Raman scattering and ultrafast optical pump-probe spectroscopy at high pressures up to 31 GPa. Detailed analysis reveals that the pressure dependence of the relaxation time and the damping rate of the ${A}_{1g}^{1}$ phonon mode exhibit an anomaly around 3 GPa supporting the occurrence of an electronic topological transition. At higher pressure, the transient reflectivity spectra manifest discontinuous changes around 11.5, 18.9, and 28 GPa, which can be assigned to the structure transitions as evidenced by the Raman spectra. Especially, low-frequency phonon modes, corresponding to the ${A}_{g}$ phonon modes in the monoclinic $C2/m$ phase, were identified in both Raman and transient reflectivity spectra in the pressure range of 11.5 to 18.9 GPa. These results not only provide crucial insights into carrier-phonon interaction of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ under high pressure, but also pave the way for investigating the pressure-induced phase transitions from transient reflection spectroscopy.