We report the strong experimental evidence of the existence of topological surface states with large electric field tunability and mobility in beta-Ag2Te. Pronounced 2D SdH oscillations have been observed in beta-Ag2Te nanoplates. A Berry phase is de
termined to be near pi using the Landau level fan diagram for a relatively wide nanoplate while the largest electric field ambipolar effect in topological insulator so far (~ 2500%) in a narrow nanoplate. The pi Berry phase and the evolution of quantum oscillations with gate voltage (Vg) in the nanoplates strongly indicate the presence of topological surface states in beta-Ag2Te. Moreover, the mobility of the narrow Ag2Te nanoplate is ~ 3x10^4 cm^2s^-1V^-1 when the Fermi level is near the Dirac point. The realization of topological surface states with large electrical tunability and high mobility indicates that beta-Ag2Te is a promising topological insulator for fundamental studies.
We present evidence of topological surface states in beta-Ag2Te through first-principles calculations and periodic quantum interference effect in single crystalline nanoribbon. Our first-principles calculations show that beta-Ag2Te is a topological i
nsulator with a gapless Dirac cone with strong anisotropy. To experimentally probe the topological surface state, we synthesized high quality beta-Ag2Te nanoribbons and performed electron transport measurements. The coexistence of pronounced Aharonov-Bohm oscillations and weak Altshuler-Aronov-Spivak oscillations clearly demonstrates coherent electron transport around the perimeter of beta-Ag2Te nanoribbon and therefore the existence of metallic surface states, which is further supported by the temperature dependence of resistivity for beta-Ag2Te nanoribbons with different cross section areas. Highly anisotropic topological surface state of beta-Ag2Te suggests that the material is a promising material for fundamental study and future spintronic devices.
We report experimental evidence of surface dominated transport in single crystalline nanoflake devices of topological insulator Bi1.5Sb0.5Te1.8Se1.2. The resistivity measurements show dramatic difference between the nanoflake devices and bulk single
crystal. The resistivity and Hall analysis based on a two-channel model indicates that ~99% surface transport contribution can be realized in 200 nm thick BSTS nanoflake devices. Using standard bottom gate with SiO2 as a dielectric layer, pronounced ambipolar electric field effect was observed in devices fabricated with flakes of 100 - 200 nm thick. Moreover, angle-dependent magneto-resistances of a nanoflake device with thickness of 596 nm are fitted to a universal curve for the perpendicular component of the applied magnetic field. The value of phase coherence length obtained from 2D weak antilocalization fitting further confirmed the surface dominated transport. Our results open a path for realization of novel electric and spintronic devices based on the topological helical surface states.
Topological insulator is composed of an insulating bulk state and time reversal symmetry protected two-dimensional surface states. One of the characteristics of the surface states is the locking between electron momentum and spin orientation. Here, w
e report a novel in-plane anisotropic magnetoresistance in topological insulator Bi1.5Sb0.5Te1.8Se1.2/CoFe heterostructures. To explain the novel effect, we propose that the Bi1.5Sb0.5Te1.8Se1.2/CoFe heterostructure forms a spin-valve or Giant magnetoresistance device due to spin-momentum locking. The novel in-plane anisotropic magnetoresistance can be explained as a Giant magnetoresistance effect of the Bi1.5Sb0.5Te1.8Se1.2/CoFe heterostructures.
