# Synopsis: Insulating behavior in topological insulators

A new topological insulator that approaches true insulating behavior in the bulk is synthesized.

3D topological insulators represent a unique quantum state for matter that is supposed to show insulating behavior in the bulk and spin-dependent metallic conduction on the surface. In practice, the best-known exemplars of materials that show a topologically protected metallic surface state, such as ${\text{Bi}}_{2}{\text{Se}}_{3}$ and ${\text{Bi}}_{2}{\text{Te}}_{3}$, are also conducting in the bulk due to the presence of vacancies. Significant efforts in trying to find a topological insulator that is truly insulating in the bulk have met with little success.

Presenting their results as a Rapid Communication in Physical Review B, Zhi Ren and colleagues from Osaka University, Japan, have synthesized a new topological insulator, ${\text{Bi}}_{2}{\text{Te}}_{2}\text{Se}$, that approaches insulating behavior in the bulk with a high resistivity. Ren et al. demonstrate variable-range hopping that is the hallmark of an insulator in high-quality single crystals of ${\text{Bi}}_{2}{\text{Te}}_{2}\text{Se}$ and Shubnikov-de Haas oscillations coming from the 2D surface metallic state. Surface contribution to the total conductance of the crystal at $6%$ is the largest ever achieved in a topological insulator. From a detailed study of the Hall effect, the authors also determine the transport mechanism in the bulk that reveals an impurity band in the band gap along with hopping conduction of localized electrons. These results pave the way for exploiting the unique surface conduction properties of topological insulators. – Sarma Kancharla

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