Magnetic impurity mediated ultrafast electron dynamics in the carrier-density-tuned topological insulator V0.04(BixSb1-x)2Te3

We investigated the ultrafast Dirac-electron dynamics on the surface of a magnetically doped topological-insulator (TI) system, V0.04(BixSb1-x)2Te3. Even when the Fermi level is located around the Dirac point of the topological surface states (TSSs), the recovery was accomplished shortly, within 3 ps; namely, a bottleneck-type slowing of the recovery was not observed. Besides, the recovery was independent of energy. The unique shortness and independence can be ascribed to the s-d interaction between the TSSs and the impurity states of V formed near the Fermi level. Magnetic doping thus provides a new route to control the nonequilibrium dynamics on a TI surface.