Anderson localization, the absence of diffusive transport in disordered systems, has been manifested as hopping transport in numerous electronic systems, whereas in recently discovered topological insulators it has not been directly observed. Here we report experimental demonstration of a crossover from diffusive transport in the weak antilocalization regime to variable range hopping transport in the Anderson localization regime with ultrathin (Bi1−xSbx)2Te3 films. As disorder becomes stronger, negative magnetoconductivity due to the weak antilocalization is gradually suppressed, and eventually positive magnetoconductivity emerges when the electron system becomes strongly localized. This works reveals the critical role of disorder in the quantum transport properties of ultrathin topological insulator films, in which theories have predicted rich physics related to topological phase transitions.The concept of Anderson localization has profoundly influenced our understanding of electron conductivity [1]. While examples for disorder driven metal-insulator transition are abundant in three-dimensions (3D), the question of whether Anderson transitions exist in 2D has posed a lot of theoretical and experimental challenges. Scaling theory proposed by Abrahams et al. predicts that there are no truly metallic states in non-interacting 2D electron systems [2,3]. It was, however, discovered later that extended electron states may exist when electronelectron (e-e) interaction, spin-orbit coupling (SOC) or magnetic field comes in to play [4,5]. The 3D topological insulators (TIs) discovered in recent years [6,7] provide novel types of 2D electron systems that are of particular interest for study of the localization-delocalization problem. The Dirac surface states of 3D TIs are believed to be topologically protected from localization due to its special symmetry class [8][9][10][11][12]. Moreover, when a 3D TI thin film is sufficiently thin, the hybridization between the top and the bottom surface states opens an energy gap near the Dirac point [13], and it is suggested theoretically that the hybridization gap would drive the electron system to topologically different phase, such as a quantum spin Hall insulator or a trivial band insulator [14][15][16]. Even though a lot of work has been carried out on electron transport properties of TI thin films [17][18][19][20], the fate of such electron systems under the condition of strong disorder (or in other words, whether Anderson localization could take place), still remains unclear.In this work, we have studied electron transport in a large number of highly gate-tunable TI thin films with various thicknesses and chemical compositions. We found that only in ultrathin TI films in which surface hybridization and disorder effects are significant, hopping transport, a hallmark of strong localization [21][22][23], can be observed. The observed temperature and magnetic field dependences of conductivity suggest that electron transport can be driven from the diffusive transport governed by we...

We report that the finite thickness of three-dimensional topological insulator (TI) thin films produces an observable magnetoresistance (MR) in phase coherent transport in parallel magnetic fields. The MR data of Bi2Se3 and (Bi,Sb)2Te3 thin films are compared with existing theoretical models of parallel field magnetotransport. We conclude that the TI thin films bring parallel field transport into a unique regime in which the coupling of surface states to bulk and to opposite surfaces is indispensable for understanding the observed MR. The {\beta} parameter extracted from parallel field MR can in principle provide a figure of merit for searching TI compounds with more insulating bulk than existing materials.Comment: 6 pages, 4 figure

The psychometric properties of the C-MMAS-8 are satisfactory.