![]() When the external field is perpendicular to the film plane, linear MR is observed above 16 T. The MR is measured in pulsed high magnetic field (PHMF) up to 50 Tesla (T). The in situ angle-resolved photoemission spectroscopy (ARPES) result shows that two quantum well states of the conduction band, as well as the surface states, contribute to the density of states near the Fermi level. Here we comprehensively study the transport properties of 5 quintuple layers (QLs) Bi 2Se 3 films grown on epi-ready α-Al 2O 3 (0001) substrates by molecular beam epitaxy (MBE). In comparison, the parallel field magneto-resistance (MR) properties of TI films reveal more bulk-state information 25 26, and therefore, the parallel magnetic field transport measurements provide a powerful tool in investigating the quantum interference effects in the TI bulk states. However, it is difficult to distinguish the bulk state features from the surface state behaviors by the perpendicular field transport measurements. Recently, the WL effect of bulk states has been studied in the ultrathin Bi 2Se 3 films under perpendicular magnetic field 24. In order to develop a comprehensive understanding on the exotic properties of TIs, it is important to systematically study the transport of TIs taking into account the effects from the bulk states.Ĭontrasting to the symmetry protected surface states, the TI bulk states with strong SOC can lead to either WAL or weak localization (WL) depending on the experimental conditions 5 6 23. Whereas, it is known that the bulk states of the existing three-dimensional (3D) TIs are not perfectly insulating even at low temperatures, and as a direct consequence, the bulk states can also influence the transport behaviors of TIs. Plentiful interesting behaviors of TIs, particularly in the topologically protected surface states, have been explored through electrical transport experiments 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22. As a result of the strong spin-orbit coupling (SOC), the spin-momentum locked surface states always show weak antilocalization (WAL) effect 4 5 6 due to the acquired π Berry phase when completing a time-reversed self-crossing path. Topological insulators (TIs), a new type of quantum materials characterized by the existence of a gap in the energy spectrum and the symmetry-protected surface states within the bulk gap, have attracted significant interest in recent years 1 2 3. It presents an important step toward a better understanding of the existing three-dimensional topological insulators and the potential applications of nano-scale topological insulator devices. Our work directly shows the crossover of quantum interference effect in the bulk states from weak antilocalization to weak localization. The measured magneto-resistance exhibits interesting anisotropy with respect to the orientation of parallel magnetic field B // and the current I, signifying intrinsic spin-orbit coupling in the Bi 2Se 3 films. In particular, the crossover from weak antilocalization to weak localization in the bulk states is observed in the parallel magnetic field measurements up to 50 Tesla. The angle-resolved photoemission spectroscopy data show that the Fermi level of the system lies in the bulk conduction band above the Dirac point, suggesting important contribution of bulk states to the transport results. We report transport studies on the 5 nm thick Bi 2Se 3 topological insulator films which are grown via molecular beam epitaxy technique.
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