We report that ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ thin films can be epitaxially grown on ${\mathrm{SrTiO}}_3$ substrates, which allow for very large tunablity in carrier density with a back gate. The observed low field magnetoconductivity due to weak antilocalization (WAL) has a very weak gate-voltage dependence unless the electron density is reduced to very low values. Such a transition in WAL is correlated with unusual changes in longitudinal and Hall resistivities. Our results suggest a much suppressed bulk conductivity at large negative gate voltages and a possible role of surface states in the WAL phenomena.

A prominent feature of topological insulators (TIs) is the surface states comprising of spin-nondegenerate massless Dirac fermions. Recent technical advances have made it possible to address the surface transport properties of TI thin films by tuning the Fermi levels of both top and bottom surfaces. Here we report our discovery of a novel planar Hall effect (PHE) from the TI surface, which results from a hitherto-unknown resistivity anisotropy induced by an in-plane magnetic field. This effect is observed in dual-gated devices of bulk-insulating Bi_2−x Sb _x Te₃ thin films, where the field-induced anisotropy presents a strong dependence on the gate voltage with a characteristic two-peak structure near the Dirac point. The origin of PHE is the peculiar time-reversal-breaking effect of an in-plane magnetic field, which anisotropically lifts the protection of surface Dirac fermions from backscattering. The observed PHE provides a useful tool to analyze and manipulate the topological protection of the TI surface.

To study the interface between a conventional superconductor and a topological insulator, we fabricated Pb-Bi₂Te₃-Pb lateral and sandwiched junctions, and performed electron transport measurements down to low temperatures. The results show that there is a strong superconducting proximity effect between Bi₂Te₃ and Pb, as that a supercurrent can be established along the thickness direction of the Bi₂Te₃ flakes (100~300 nm thick) at a temperature very close to the superconducting T_c of Pb. Moreover, a Josephson current can be established over several microns in the lateral direction between two Pb electrodes on the Bi₂Te₃ surface. We have further demonstrated that superconducting quantum interference devices can be constructed based on the proximity-effect-induced superconductivity. The critical current of the devices exhibits s-wave-like interference and Fraunhofer diffraction patterns. With improved designs, Josephson devices of this type would provide a test-bed for exploring novel phenomena such as Majorana fermions in the future.

The topological crystalline insulator SnTe has been grown epitaxially on a Bi${}_{2}$Te${}_{3}$ buffer layer by molecular beam epitaxy. In a 30-nm-thick SnTe film, $p$- and $n$-type carriers are found to coexist, and Shubnikov\char21{}de Haas oscillation data suggest that the $n$-type carriers are Dirac fermions residing on the SnTe (111) surface. This transport observation of the topological surface state in a $p$-type topological crystalline insulator became possible due to a downward band bending on the free SnTe surface, which appears to be of intrinsic origin.

The charge-current-induced spin polarization is a key property of topological insulators for their applications in spintronics. However, topological surface states are expected to give rise to only one type of spin polarization for a given current direction, which has been a limiting factor for spin manipulations. Here, we report that in devices based on the bulk-insulating topological insulator ${\mathrm{BiSbTeSe}}_2$, an unexpected switching of spin polarization was observed upon changing the chemical potential. The spin polarization expected from the topological surface states was detected in a heavily electron-doped device, whereas the opposite polarization was reproducibly observed in devices with low carrier densities. We propose that the latter type of spin polarization stems from topologically trivial two-dimensional states with a large Rashba spin splitting, which are caused by a strong band bending at the surface of ${\mathrm{BiSbTeSe}}_2$ beneath the ferromagnetic electrode used as a spin detector. This finding paves the way for realizing the “spin transistor” operation in future topological spintronic devices.