Topological insulators: Oscillations in the ribbons

Quantum transport properties of single-crystalline Ag2Se0.5Te0.5 nanowires as a new topological material

We report a ternary silver chalcogenide, Ag2Se0.5Te0.5, as a new topological material with improved quantum transport properties. Single-crystalline nanostructures of ternary silver chalcogenides Ag2SexTe1-x are synthesized with a tunable chemical composition via the chemical vapor transport method. Quantum transport studies reveal that Ag2Se0.5Te0.5 nanowires present topological surface states with higher electron mobility and longer mean free path compared to binary Ag-chalcogenides. First-principles calculations also indicate that Ag2Se0.5Te0.5 is a topological insulator, and the observed enhancement in transport properties could imply reduced bulk carrier contribution in the new ternary silver chalcogenide.

Dual evidence of surface Dirac states in thin cylindrical topological insulator Bi₂Te₃ nanowires

How the surface state (SS) develops and how the spin transport in a curved cylindrical topological insulator nanowire have attracted theoretical attention recently. However, experimental confirmation for the SS in such a real modeling system still remains insufficient. Here we carried out a systematic comparative study on the cylindrical single-crystal Bi₂Te₃ nanowires of various diameters, and report unambiguously dual evidence for the Dirac SS. Both the predicted anomalous Aharonov-Bohm (AB) quantum oscillations with a period of h/e in H_// and the 1/2-shifted Shubnikov-de Haas (SdH) oscillations (i.e., γ = −1/2) in H_⊥ were indentified below 1.4 K. In addition, Altshuler-Aronov-Spivak (AAS)-like oscillations with a period of h/2e and ordinary SdH oscillations with γ = 0 were also resolved. These data provide clear evidence of coexistence of the nontrivial topological Dirac state and trivial electron state on the surface of topological insulator nanowire.

Nanoelectronics: A topological twist for transistors

A nanoribbon of a material with topological surface states has been used as the channel in a field-effect transistor.

Manipulating surface states in topological insulator nanoribbons

Topological insulators display unique properties, such as the quantum spin Hall effect, because time-reversal symmetry allows charges and spins to propagate along the edge or surface of the topological insulator without scattering. However, the direct manipulation of these edge/surface states is difficult because they are significantly outnumbered by bulk carriers. Here, we report experimental evidence for the modulation of these surface states by using a gate voltage to control quantum oscillations in Bi(2)Te(3) nanoribbons. Surface conduction can be significantly enhanced by the gate voltage, with the mobility and Fermi velocity reaching values as high as ~5,800 cm(2) V(-1) s(-1) and ~3.7 × 10(5) m s(-1), respectively, with up to ~51% of the total conductance being due to the surface states. We also report the first observation of h/2e periodic oscillations, suggesting the presence of time-reversed paths with the same relative zero phase at the interference point. The high surface conduction and ability to manipulate the surface states demonstrated here could lead to new applications in nanoelectronics and spintronics.

Few-layer nanoplates of Bi 2 Se 3 and Bi 2 Te 3 with highly tunable chemical potential

A topological insulator (TI) represents an unconventional quantum phase of matter with insulating bulk band gap and metallic surface states. Recent theoretical calculations and photoemission spectroscopy measurements show that group V-VI materials Bi(2)Se(3), Bi(2)Te(3), and Sb(2)Te(3) are TIs with a single Dirac cone on the surface. These materials have anisotropic, layered structures, in which five atomic layers are covalently bonded to form a quintuple layer, and quintuple layers interact weakly through van der Waals interaction to form the crystal. A few quintuple layers of these materials are predicted to exhibit interesting surface properties. Different from our previous nanoribbon study, here we report the synthesis and characterizations of ultrathin Bi(2)Te(3) and Bi(2)Se(3) nanoplates with thickness down to 3 nm (3 quintuple layers), via catalyst-free vapor-solid (VS) growth mechanism. Optical images reveal thickness-dependent color and contrast for nanoplates grown on oxidized silicon (300 nm SiO(2)/Si). As a new member of TI nanomaterials, ultrathin TI nanoplates have an extremely large surface-to-volume ratio and can be electrically gated more effectively than the bulk form, potentially enhancing surface state effects in transport measurements. Low-temperature transport measurements of a single nanoplate device, with a high-k dielectric top gate, show decrease in carrier concentration by several times and large tuning of chemical potential.