No abstract

Topological insulators are predicted to present interesting surface transport phenomena but their experimental studies have been hindered by a metallic bulk conduction that overwhelms the surface transport. We show that the topological insulator Bi 2 Te 2 Se presents a high resistivity exceeding 1 ⍀ cm and a variable-range hopping behavior, and yet presents Shubnikov-de Haas oscillations coming from the topological surface state. Furthermore, we have been able to clarify both the bulk and surface transport channels, establishing a comprehensive understanding of the transport in this material. Our results demonstrate that Bi 2 Te 2 Se is, to our knowledge, the best material to date for studying the surface quantum transport in a topological insulator.

We report a new strategy to induce superconductivity in iron-based oxyarsenide. Instead of F − substitution for O 2− , we employed Th 4+ doping in GdFeAsO with the consideration of "lattice match" between Gd2O2 layers and Fe2As2 ones. As a result, superconductivity with T onset c as high as 56 K was realized in a Gd0.8Th0.2FeAsO polycrystalline sample. This Tc value is among the highest ever discovered in the iron-based oxypnictides.