In 2005, physicists of condensed matter, Charles Kane and Eugene Mele, considered the fate of graphene at low temperatures. Their work led to the discovery of a new state of matter called the "topological insulator" that would usher in a new era of materials science.
"A topological insulator is a material that is an insulator inside but has high conductivity on its surface," said Andrea Young, assistant professor of physics at UC Santa Barbara. In two dimensions, an ideal topological insulator would have "ballistic" conductivity at its edges, Young said, meaning that electrons traveling through the region would encounter zero resistance.
Ironically, Kane and Meles' work would lead to the discovery of topological isolation behavior in a variety of materials whose original prediction ̵
At the center of the problem is the spin-orbit coupling – a weak effect in which the spin of the electron interacts with its orbital motion around the nucleus. Critically for all topological insulators, the spin-orbit coupling in graphene is extremely weak, so that the topological isolation behavior is superimposed by other effects emanating from the surface on which the graphene is carried.
"Graphene is a pity," said postdoctoral researcher Joshua Island, because topological insulators in two dimensions did not work that well in practice. "The previously known two-dimensional topological insulators are disordered and not very easy to handle," Iceland said. The conductivity at the edges decreases rapidly with the distance the electrons travel, suggesting that it is not ballistic. The realization of a topological insulator in graphene, an otherwise very perfect two-dimensional material, could provide a basis for low-dissipation ballistic electrical circuits or form the material substrate for topologically protected quantum bits.
Published in the journal Nature Iceland, Young and their collaborators have found a way to turn graphene into a topological insulator (TI). "The goal of the project was to increase or improve the spin-orbit coupling in graphene," said lead author Iceland, adding that attempts have been made with limited success over the years. "One possibility is to bring a material with a very large spin-orbit coupling close to the graphene, and the hope was that your graphene electrons would take on the property of the underlying material," he explained.
The material of choice? After exploring various options, the researchers chose a transition metal dichalcogenide (TMD) composed of the transition metal tungsten and the chalcogen selenium. Similar to graphene, tungsten diselenide occurs in two-dimensional monolayers held together by van der Waals forces, which are relatively weak and depend on the distance between atoms or molecules. However, in contrast to graphene, the heavier atoms of the TMD lead to a stronger spin-orbit coupling. The ballistic electron conductivity of the graphene of the resulting building block was influenced by the strong spin-orbit coupling from the nearby TMD layer.
"We have seen a very significant improvement in this spin-orbit coupling," Iceland said.
"Adding Joshua found that a spin-orbit coupling of the right type actually leads to a new phase that is nearly topologically insulating," said Young. In the original idea, the topological insulator consisted of a monolayer of graphene with a strong spin-orbit coupling.
"We had to use a trick that was only available in graphene multilayers to achieve the right kind of spin-orbit coupling." Young explained her experiment using a graphene bilayer. "And so you get something that is about two topological insulators stacked on top of each other." Functionally, however, the device from Iceland works just as well as other well-known topological 2-D insulators – the important edge states extend at least a few micrometers longer than other known TI materials.
Moreover, Young's work is one step closer to building an actual topological insulator with graphene. "Theoretical work has since shown that a graphene trilayer prepared in the same way would lead to a true topological insulator."
Most importantly, the devices realized by Iceland and Young can be easily tuned between a topological isolation phase and a tuned phase. Regular insulator that does not have conductive edge states.
"You can lead these perfect ladders wherever you want," he said. "Nobody could do that with other materials."
Investigation of new spintronic functions in graphene heterostructures
Spin Orbit-driven Band Inversion in Two-Layer Graphene by the Van der Waals Proximity Effect, Nature (2019). DOI: 10.1038 / s41586-019-1304-2, https://www.nature.com/articles/s41586-019-1304-2
Physicists move closer to building a topological graphene-based insulator (2019, June 12)
retrieved on June 13, 2019
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