Anomalous transport in an n-type topological insulator ultrathin Bi2Se3 film

We have performed in situ spin- and angle-resolved photoemission and ex situ magnetotransport measurements on ultrathin topological insulator Bi2Se3 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.