Anomalous transport in an $n$-type topological insulator ultrathin ${\text{Bi}}_2{\text{Se}}_3$ film

We have performed in situ spin- and angle-resolved photoemission and ex situ magnetotransport measurements on ultrathin topological insulator ${\text{Bi}}_2{\text{Se}}_3$ films. The surface states deviate from a simple isotropic Dirac fermion due to hexagonal warping effects and their spin helical nature has been verified. In addition, the bulk states also cross the Fermi level, showing that the films are actually $n$ doped. However, the temperature dependence of the film resistivity was insulating and saturated below 35 K. Furthermore, the magnetoresistivity changed drastically as the sample was cooled down and showed an anomaly near zero field below 20 K. The possible origin of this peculiar behavior and the nature of the carriers involved are discussed in comparison with the results for nonmetallic bulk samples.

Figure 1

(Color online) (a) $E\text{−}k$ band dispersion image of an 8 QL ultarthin ${\text{Bi}}_2{\text{Se}}_3$ film along the $\overline{\Gamma }\text{−}\overline{\text{K}}$ direction near the $\overline{\Gamma }$ point. [(b)–(i)] Constant energy contour mapping of an 8 QL ultarthin ${\text{Bi}}_2{\text{Se}}_3$ film for respective energies.

Figure 2

(Color online) Spin-resolved ARPES spectra of an 8 QL ultrathin ${\text{Bi}}_2{\text{Se}}_3$ film along the $\overline{\Gamma }\text{−}\overline{\text{M}}$ direction [(a) and (c)] and the spin-polarization curves deduced from them [(b) and (d)]. The spin orientation is along the $\overline{\Gamma }\text{−}\overline{\text{K}}$ direction [$y$ direction, red (light gray) for $+y$ and blue (dark gray) is for $-y$, respectively].

Figure 3

(Color online) The temperature dependence of the (a) film resistivity $\rho$ and (b) the Hall coefficient $R_H$ for the 8 QL ultrathin ${\text{Bi}}_2{\text{Se}}_3$ film.

Figure 4

(Color online) The magnetic field dependence of the resistivity $\rho$ for the 8 QL ultrathin ${\text{Bi}}_2{\text{Se}}_3$ film with the magnetic field applied in the (a) out-of-plane direction and (b) in-plane direction at respective temperatures. The inset in (a) shows the enlarged curves at the measurement temperatures of 5, 10, and 20 K.

Figure 5

(Color online) (a) The magnetic field dependence of the resistivity $\rho$ for the 8 QL ultrathin ${\text{Bi}}_2{\text{Se}}_3$ film plotted as a function of $\text{log}B$ for the 5, 10, and 20 K curves of Fig. 4a. (b) The magnetic field dependence of the conductivity $\sigma$ as a function of $\text{log}B$ for the 5, 10, and 20 K curves of Fig. 4a. The dashed lines near the curve of 5 K show a fitting to a linear function.