Wednesday, December 18th, 2013

Stanene — atom-thick tin — is a topological insulator:

While the interior of such a material is an electrical insulator, the outside edges and surfaces are electrically conductive. If exploited properly, this weird property of topological insulators can make it possible for electrons to flow without resistance.

To understand, an explanation of how electrons move in topological insulators is needed. Like tiny twirling bar magnets with north and south poles, electrons spin as they move around in a material. When electrons move around in the surfaces and edges of topological insulators, the direction of their spin becomes aligned with the direction of their flow. A consequence of this effect — known as the quantum spin Hall state — is that flowing electrons can’t easily reverse direction. That is true even if they hit an impurity within the material — an event that in normal conductors causes electrons to scatter backwards and dissipate energy.

When electrons travel along the surface of a three-dimensional topological insulator, they generally can’t bounce backwards, but they can still jostle each other sideways, wasting energy. But in two-dimensional topological insulators — surfaces that are just one atom thick — flowing electrons become restricted to a single lane, eliminating all interference. Recent experiments confirm that electrons can zip along the edges of flat topological insulators with 100 percent efficiency.

In the past decade, researchers have made topological insulators from compounds of electron-rich, heavy elements including mercury, bismuth, antimony, tellurium and selenium. None of them were perfect conductors of electricity at room temperature. Then Stanford University theoretical physicist Shoucheng Zhang and colleagues decided to investigate tin, a similarly electron-rich, heavy element. The team’s calculations suggest that single-atom layers of tin are topological insulators where electrons flow perfectly at and above room temperature.

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