Bismuth-based Nanoribbons Show 'Topological' Transport, Potential for New Technologies

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Researchers have created nanoribbons of an emerging class of materials called topological insulators and used a magnetic field to control their semiconductor properties, a step toward harnessing the technology to study exotic physics and building new spintronic devices or quantum computers.

Unlike ordinary materials that are either insulators or conductors, topological insulators are paradoxically both at the same time - they are insulators inside but conduct electricity on the surface, said Yong P. Chen, a Purdue University associate professor of physics and astronomy and electrical and computer engineering who worked with doctoral student Luis A. Jauregui and other researchers.

The materials might be used for "spintronic" devices and practical quantum computers far more powerful than today's technologies. In the new findings, the researchers used a magnetic field to induce a so-called “helical mode” of electrons, a capability that could make it possible to control the spin state of electrons.

The findings are detailed in a research paper that appeared in the advance online publication of the journal Nature Nanotechnology on Jan. 18 and showed that a magnetic field can be used to induce the nanoribbons to undergo a “topological transition,” switching between a material possessing a band gap on the surface and one that does not.

“Silicon is a semiconductor, meaning it has a band gap, a trait that is needed to switch on and off the conduction, the basis for silicon-based digital transistors to store and process information in binary code,” Chen said. “Copper is a metal, meaning it has no band gap and is always a good conductor. In both cases the presence or absence of a band gap is a fixed property. What is weird about the surface of these materials is that you can control whether it has a band gap or not just by applying a magnetic field, so it’s kind of tunable, and this transition is periodic in the magnetic field, so you can drive it through many ‘gapped’ and ‘gapless’ states.”  

The nanoribbons are made of bismuth telluride, the material behind solid-state cooling technologies such as commercial thermoelectric refrigerators.

“Bismuth telluride has been the workhorse material of thermoelectric cooling for decades, but just in the last few years people found this material and related materials have this amazing additional property of being topological insulators,” he said.

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