Magnetic impurity mediated ultrafast electron dynamics in the carrier-density-tuned topological insulator ${\mathrm{V}}_{0.04}{\left({\mathrm{Bi}}_x{\mathrm{Sb}}_{1-x}\right)}_2{\mathrm{Te}}_3$

We investigated the ultrafast Dirac-electron dynamics on the surface of a magnetically doped topological-insulator (TI) system, ${\mathrm{V}}_{0.04}{\left({\mathrm{Bi}}_x{\mathrm{Sb}}_{1-x}\right)}_2{\mathrm{Te}}_3$. 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.

Figure 1

TARPES images of (a) ${\mathrm{V}}_{0.04}{\mathrm{Sb}}_{2.03}{\mathrm{Te}}_3$ ($p=0.21$ mJ/${\mathrm{cm}}^2$) and (b) ${\mathrm{V}}_{0.04}{\left({\mathrm{Bi}}_{0.34}{\mathrm{Sb}}_{0.66}\right)}_{2.03}{\mathrm{Te}}_3$ ($p=0.19$ mJ/${\mathrm{cm}}^2$) taken along the $\overline{\mathrm{\Gamma }}\overline{M}$ direction at several pump-probe delay times $t$. White solid and black dashed lines indicate the levels of $E{}_{\mathrm{D}}$ and $E{}_{\mathrm{F}}$, respectively. The regions indicated by S1–S8 and B1–B4 are located in the TSSs and bulk states, respectively. (c, d) Normalized intensity variations of the frames in (a, b) as a function of $t$. The time resolution is indicated as a Gaussian function with FWHM of 0.31 ps in (c).

Figure 2

6 nm $\times$ 6 nm topographic images of (a) ${\mathrm{V}}_{0.04}{\mathrm{Sb}}_{2.03}{\mathrm{Te}}_3$ ($V{}_{\mathrm{s}}=-0.1$ V) and (b) ${\mathrm{V}}_{0.04}{\left({\mathrm{Bi}}_{0.34}{\mathrm{Sb}}_{0.66}\right)}_{2.03}{\mathrm{Te}}_3$ ($V{}_{\mathrm{s}}=+0.5$ V) taken at 4 K. (c, d) STS taken at 4 K at different points across the V site shown as colored circles in (a, b).

Figure 3

(a, f) Illustration of the ultrafast process in TIs (a) before and (f) after V doping. (b–e) Simulation of the ultrafast process in TIs without magnetic doping. The DOSs used in the simulation are shown in (b) and (c) and the calculated electron occupancy is shown in (d) and (e), respectively. (g–j) Simulation considering the $s-d$ exchange interactions with $P_{\mathrm{sd}}=0.5P_{\mathrm{ep}}$.

Figure 4

(a) ARPES image of ${\mathrm{V}}_{0.04}{\mathrm{Sb}}_{2.03}{\mathrm{Te}}_3$ at 0.4 ps. The energy positions of $E{}_{\mathrm{D}}$ and BCB are determined by Gaussian fitting of the angular distribution curves and are indicated by white solid and dashed lines, respectively. (b) Illustration of the decay process in ${\mathrm{V}}_{0.04}{\mathrm{Sb}}_{2.03}{\mathrm{Te}}_3$. (c, d) ARPES image and the illustration of the decay process in ${\mathrm{V}}_{0.04}{\left({\mathrm{Bi}}_{0.34}{\mathrm{Sb}}_{0.66}\right)}_{2.03}{\mathrm{Te}}_3$.