Physicists are approaching the construction of a topological insulator based on graphene

In 2005, condensed matter physicists Charles Kane and Eugene Mele examined the fate of low-temperature graphene. Their work led to the discovery of a new state of matter called "topological insulator", which ushered in a new era of materials science.

"A topological insulator is an insulating material inside but highly conductive on its surface," said Andrea Young, professor of adjunct physics at the University of Santa Barbara. In two dimensions, an ideal topological insulator would have a "ballistic" conductance on its edges, explained Young, which means that the electrons moving in the region would meet no resistance.

Ironically, while the work of Kane and Mele would lead to the discovery of a topological insulating behavior in a wide variety of materials, their original prediction – a topological insulator in graphene – remained unrealized.

Spin-orbit coupling, a weak effect in which the spin of the electron interacts with its orbital motion around the nucleus, is at the heart of the problem. Spin-orbit coupling is extremely weak in graphene, a critical element for all topological insulators, so that any topological insulating behavior is drowned out by other effects from the surface on which graphene is supported.

"The weak spin-orbit coupling in graphene is really a shame," said Joshua Island, a postdoctoral researcher, because in practice things did not really work out well for two-dimensional topological insulators. "The two-dimensional topological insulators known to date are messy and difficult to handle," said Island. Edge conductance tends to decrease rapidly with the distance traveled by electrons, suggesting that it is far from being ballistic. Realizing a graphene topological insulator, a two-dimensional material that is otherwise extremely perfect, could serve as a basis for low dissipation ballistic electrical circuits or constitute the material substrate for topologically protected quantum bits.

Now, in the work published in the journal Nature, Island, Young, and their collaborators have found a way to transform 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 Island, adding that attempts have been made over the past decade. years with limited success. "One way to do this is to place a material with a very large spin-orbit coupling near graphene, and we hope that your graphene electrons will take this property of the underlying material," he explained. .

The material of choice? After studying several possibilities, the researchers opted for a transition metal dichalcogenide (TMD), composed of tungsten and chalcogen selenium. Similar to graphene, tungsten diselenide occurs as two-dimensional monolayers, linked together by van der Waals forces, which are relatively weak interactions between atoms or molecules and which depend on distance. Unlike graphene, however, heavier TMD atoms result in stronger spin-orbit coupling. The ballistic electronic conductance of the graphene of the resulting device is impregnated with the strong spin-orbit coupling of the neighboring TMD layer.

"We have seen a marked improvement in this spin-orbit coupling," said Island.

"By adding the spin-orbit coupling of the right kind, Joshua found that it actually led to a new, almost topologically insulating phase," said Young. In the original idea, he explained, the topological insulator consisted of a monolayer of graphene with a strong spin-orbit coupling.

"We had to use a trick only available in graphene multilayers to create the right kind of spin-orbit coupling," Young said of their experiment, which used a graphene bilayer. "And so you get something that is close to two topological insulators stacked one on the other." Functionally, however, the Island device works just as well as the other topological insulators known in 2D: the edge states, which are very important, propagate over at least several microns, much longer than in the past. Other known TI materials.

In addition, according to Young, these works are one more step towards the construction of a graphene-based topological insulator. "The theoretical works have since shown that a graphene trilayer, manufactured in the same way, would lead to a true topological insulator."

More importantly, the devices made by Island and Young can be easily tuned between a topological insulating phase and a regular insulator, which does not exhibit conductive edge conditions.

"You can get those perfect drivers wherever you want," he said. "And it's something that no one has been able to do with other materials."