Ultrafast surface Dirac fermion dynamics of Sb2Te3-based topological insulators

Topological insulators (TIs) characterized by gapless and spin-polarized conical band dispersion on their surfaces have been extensively studied over the last decade. This article reviews our recent works on ultrafast carrier dynamics of Sb₂Te₃-based nonmagnetic and magnetic TIs by utilizing state-of-the-art femtosecond time- and angle-resolved photoelectron spectroscopy. We have demonstrated that the electronic recovery time elongated from a few ps to $>$400 ps in case that the Dirac point was close to the Fermi energy in the series of (${\mathrm{Sb}}_{1-x}$${\mathrm{Bi}}_x$)₂Te₃. We also investigated how the magnetic-impurity affects the carrier dynamics in ferromagnetic ${\mathrm{Sb}}_{2-y}$$V_y$Te₃. It was found that the electronic recovery time drastically shortened from a few ps to $<$500 fs with increasing vanadium concentration. Since the lifetime of the nonequilibrated surface Dirac fermions can range from femto- to nano-second, Sb₂Te₃-based TIs would be promising for ultrafast spin switching and spin-polarized current generation device applications.